Thumper Club Forum
Technical => Bike Problems/Questions => Topic started by: xbally on June 21, 2013, 09:21:09 PM
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Does anyone know how easy or difficult this conversion is and what are the potential difficulties such as ignition,electrics carb,airbox,exhaust and chain alignment etc?
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The XBR engine will fit but as its a dry sump engine it needs quite a bit of work whereas the XL or FT500 fits a lot easier.
Check out youtube Honda CB250/500RS
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Will the XBR engine fit? its a fair bit bigger than the XL engine and that was a tight squeeze. There used to be a lot of XL/FT engines in RS chassis, I'm sure if the XBR engine fitted it would have been done then.
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I wondered if the extra torque from the bigger motor might overwhelm the RS frame and swingarm?
I suppose, at the end of the day, it is gearbox output to rear wheel torque that affects the frame? Perhaps the rear wheel torque from an xl500/RS hybrid might not be so much different from a 305cc tuned RS motor. With appropriate gearing.
Torque induced chain tension is presumably the thing which might affect the chassis? Aside from inputs from the road/cornering, which would increase with road speed?
I'm interested in making my RS a little perkier, but wouldn't really want to go much above 305cc. It would loose the essence of what (I think) I'm after.
How does a 305cc RS stack up against an xl500 horsepower wise? How much heavier is the xl500 motor?
Cheers.
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The 500RS I built handled much the same as the 250, the main change was that I no longer needed to wring its neck to make good progress. The 500RS is more about mid-range and less gear changing which suits my riding style.
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The RS chassis handles the extra power without any real issues. It seems from the other posts that the XBR motor does fit, I would guess that it wasn't a combination that was used in racing because the XBR chassis was good enough and many people were moving to custom built chassis at the time.
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I agree, my 500RS is so much more torquey than the 250 although on paper the 250 produces much the same BHP, about 27 albeit at the crank on the 250 and at the wheel on the 500, the torque is double, mine puts out 31 ftlbs at the rear wheel. It changes the bike into a bit of a mad wheelie bike. It does vibrate a bit more as the mass of the frame is lighter and so doesn't absorb the vibrations as well.
Ive seen a video of an XBR engine in an RS frame but its a bit of a lash up. the engine mountings are different and it needs a place for the oil tank to go, exhausts are always fabricated units as well.
I also have a 305cc engine that i'm building for my XL250S, if anyone's got any experience of these big bore RS/XL engines let me know.
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Sounds very low for rear wheel torque?
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Well according to the Honda manual, the torque figure of the XL500 at the crank should be 30ftlbs; my RS500 put out 31 at the back wheel on a dyno but i have done a little bit of tuning so that would account for the increase. Standard 250RS's put out roughly 15ftlbs of torque at the crank so therefore that would be less by the time it got to the back wheel.
To put it into perspective Yamaha 350LCs put out 30ftlbs of torque and 47 BHP, both figures taken at the crank, a mildly tuned CB500RS should put out more torque but traditionally big singles produce less BHP than multi cylinder bikes because of the lower rev ceiling.
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Well according to the Honda manual, the torque figure of the XL500 at the crank should be 30ftlbs; my RS500 put out 31 at the back wheel on a dyno but i have done a little bit of tuning so that would account for the increase. Standard 250RS's put out roughly 15ftlbs of torque at the crank so therefore that would be less by the time it got to the back wheel.
To put it into perspective Yamaha 350LCs put out 30ftlbs of torque and 47 BHP, both figures taken at the crank, a mildly tuned CB500RS should put out more torque but traditionally big singles produce less BHP than multi cylinder bikes because of the lower rev ceiling.
Yes, sorry I misunderstood what you meant. With respect though, I'm still not sure the figure you mention is the torque at the rear wheel. This is a confusing aspect of what dynamometers actually measure. Again with respect, I don't believe your figure of 31ft lbs probably actually exists. I believe I can explain why I think that and what I mean by that in more detail if you want me to. But consider this:-
Unless the overall transmission ratio is 1:1 or lower, then torque will increase as measured at the rear wheel compared to crank. Each reduction ratio multiplies the torque, minus a small loss at each stage. So, in most cases, the torque at the rear wheel will have been multiplied a significant amount by the time it reaches the rear wheel. That is the whole point of reduction gears (The reduction refers to the speed).
Power is a constant, minus the losses at each stage of the transmission (most sources I've seen reckon on an overall loss of 10 to 12%). So it is entirely conceivable that the two engines in question, putting out pretty much the same horsepower, will have the potential to achieve the same top speed and produce the same rear wheel torque.
Am I right in saying that the link between power, torque and rpm is directly proportional? If so, then an engine producing the same power as another, but with half the torque, will have to rev twice as fast. But that is very little to do with rear wheel torque. In those two cases you'll find that the overall transmission ratio(s) are vastly different (or should be!).
Please don't take offence. I'm not out to offend, just to discuss and of course put forward my own thoughts.
Cheers.
Anyone agree?
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Well, as a general rule more cc equals more torque. Torque is a measurement of turning force, a dynomometer actually measures torque first then converts it to BHP, because bikes produce their power in a turning action whether at the end of a crank or at the back wheel the dyno is the only way it can be measured, once the highest torque or turning force is measured it multiplied by the RPM then divided by 5252 to get the horsepower at that RPM level. As my dad says 'torque gets you there, horsepower keeps you there'
The shove in the back you feel when you accelerate is torque not horspower
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Quoting tevie54.
Quote: "Well, as a general rule more cc equals more torque."
As a general rule, you'd have to say yes. Engine torque, that is. And probably, but by no means certainly, rear wheel torque too.
Quote: "Torque is a measurement of turning force,"
Agreed. Force x radius.
Quote: "a dynomometer actually measures torque first then converts it to BHP, because bikes produce their power in a turning action whether at the end of a crank or at the back wheel the dyno is the only way it can be measured, once the highest torque or turning force is measured it multiplied by the RPM then divided by 5252 to get the horsepower at that RPM level"
I need to be a little careful because there are a variety of different types of dynamometer. Though we are almost certain to be concerned with the rolling road type. The inertial rolling road type? If so, then doesn't this measure the rate of acceleration of a mass and express the amount of power involved? If I'm right there, then this will give a readout of rear wheel power.
But here's where I think the confusion sets in. A lot of graphs also show a torque curve based on the rear wheel power curve, but expressed at engine rpm. This is what I think is your 31ft lbs. I don't think this is rear wheel torque though. In fact, is it ANY actual torque? Isn't it engine crankshaft torque, but taking account of transmission losses which have not yet occurred? Now that is confusing. Which is what I said at the beginning. Assuming I'm right, that is.
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A dyno can only measure torque at the rear wheel or gearbox sprocket if its an engine dyno. A revcounter will be attached to the bike so the measured torque can be corrected to the equivalent of 1:1 gearing. The power and torque graphs show the corrected torque figure at the rear wheel and the rear wheel bhp calculated from the corrected torque curve.
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A dyno can only measure torque at the rear wheel or gearbox sprocket if its an engine dyno. A revcounter will be attached to the bike so the measured torque can be corrected to the equivalent of 1:1 gearing. The power and torque graphs show the corrected torque figure at the rear wheel and the rear wheel bhp calculated from the corrected torque curve.
Well, whatever else I said, I did say it was confusing!
At the end of the day, there will be as many rear wheel torque curves for the bike as there are gearbox ratios. And for the lower gears the rear wheel torque figure will be very much higher than that at the crankshaft. But there will be only one power curve for the rear wheel regardless of the gear selected. This curve will be around 10 to 12 percent less than the same power curve at the crankshaft.
Going back to tevie54's 31ft lb figure. If this is a rear wheel torque figure, which gear was the bike in? What was the torque figure at the crankshaft at the same time? What rpm was the rear wheel doing at this time? Using the formula described by tevie54 we should be able to reconcile (or not) all the above.
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Of course the other thing to consider is the power to weight ratio, an XBR engined RS will feel quicker than a standard XBR because the RS is lighter so acceleration will be better and i suppose pushing a lighter frame along will mean the running gear has an easier time.
I know with my 500RS the chain and sprockets seem to be lasting just as long as when it was just a 250.
Either way if you did manage to fit the XBR engine into your RS frame you would have a lightweight torquey thumper that will surprise a few big bikes at the lights.
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At the end of the day, there will be as many rear wheel torque curves for the bike as there are gearbox ratios. And for the lower gears the rear wheel torque figure will be very much higher than that at the crankshaft.
There will be but the figure that comes out of the dyno will be calculated on a 1:1 ratio so it doesn't matter what gear you are in. Its a simple calculation because the dyno measures the rear wheel speed and compares that to engine RPM to get the correction factor.
When I was racing I wrote a programme that would graph torque at the rear wheel and plot it against load, it was very useful for seeing where the power dropped out between gears and to determine top speed. It was very accurate and was very useful when modifying the engine as you had to keep the torque spread relatively wide because of the stock gearbox ratios. One day I will have to sit down and rewrite it.
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My favourite comparison between torque vs horsepower is the Bentley Mulsanne turbo, how could a car that weighed two and a half tons sprint from 0 to 60 in six and a half seconds with only 265 BHP,
Its because it also put out over 450 ftlbs of torque
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At the end of the day, there will be as many rear wheel torque curves for the bike as there are gearbox ratios. And for the lower gears the rear wheel torque figure will be very much higher than that at the crankshaft.
There will be but the figure that comes out of the dyno will be calculated on a 1:1 ratio so it doesn't matter what gear you are in. Its a simple calculation because the dyno measures the rear wheel speed and compares that to engine RPM to get the correction factor...........
That's confusing. I apologise, but you may have to explain that bit in more detail.
What I think you're saying is that, whatever data is extracted by the rolling road drum, it is plotted at engine speed. Have I interpreted that right?
If so, then it will have accounted for the losses in the transmission? Have I interpreted that right?
If so, then as far as the power curve is concerned, that is ok. But when it comes to the torque curve it will be "abstract". It won't actually exist. Torque wise, it seems to be expressing something which never actually happened. And certainly not rear wheel torque. Not even crankshaft torque.
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........Standard 250RS's put out roughly 15ftlbs of torque at the crank so therefore that would be less by the time it got to the back wheel......
Just done a quick calc. Ok, I could have crunched it wrong and if anyone wants to double check that'd be great.
Taking the figure of 15ftlb crank torque for the RS. In first gear the rear wheel torque is 292ftlb and in top gear it is 97 ftlb. I've used the data in the Haynes manual for transmission ratios and I've allowed for 10 percent loss through the transmission.
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What is it you're confused about? Modern rolling road dynamometers measure the torque at the rear wheel, the power produced by the engine in a turning action against a rotating drum, it then converts that turning force into a BHP figure using known mathematical formula involving RPM and wheel speed. The BHP figure is obtained FROM the torque figure. Vehicle manufacturers will always give the power figures taken at the crank as these are always higher than the rear wheel figures therefore making their product look more appealing to the customer.
The power and torque figures I gave were taken from the workshop manual so I'm pretty sure they were taken at the crank. When I had bike tested on the dyno the operator took it up to 5th then changed down once and held the throttle wide open, its quite scary watching your bike being revved that high. When its over you get a sheet showing two graphs, one showing rear wheel torque, one showing rear wheel HP.
At the end of the day if the figures are high you're happy, if they're low you're not.
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What is it you're confused about? Modern rolling road dynamometers measure the torque at the rear wheel, the power produced by the engine in a turning action against a rotating drum, it then converts that turning force into a BHP figure using known mathematical formula involving RPM and wheel speed. The BHP figure is obtained FROM the torque figure. Vehicle manufacturers will always give the power figures taken at the crank as these are always higher than the rear wheel figures therefore making their product look more appealing to the customer.
The power and torque figures I gave were taken from the workshop manual so I'm pretty sure they were taken at the crank. When I had bike tested on the dyno the operator took it up to 5th then changed down once and held the throttle wide open, its quite scary watching your bike being revved that high. When its over you get a sheet showing two graphs, one showing rear wheel torque, one showing rear wheel HP.
At the end of the day if the figures are high you're happy, if they're low you're not.
Hee Hee. Yes, I suppose that is the point.
What am I confused about? What has happened to my rear wheel torque? That's what. The method of getting us our figures, I believe, is not as straightforward as you're making out. Not when it comes to the torque figure at any rate. It's easy to forget that torque gets multiplied by the transmission, but the power does not. Any chance I can have a look at your printout? If you're truly seeing the torque produced at the rear wheel then you'll need a curve for each gear.
Earlier you gave me a figure of 31 ftlb as your rear wheel torque. If you can give me the gear involved, the engine rpm and the three ratios involved in the transmission, we can explore the figures and see if they can be reconciled using the hp formula you mentioned earlier.
Regarding manufacturers published figures. I don't think we should blame them for using crankshaft figures. It's not their fault that the power coming out the far end is lower. That's a fact of physics. At least, being upstream of the gearbox, we are less likely to be confused with torque figures. However, we should be very critical if they are found to be not entirely truthful. It's good that we all now have access to rolling road dyno's, should we want to use them. I wonder if the figures derived sometimes only confuse the issue though?
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There is a power/torque graph posted on the "chatter" section which perfectly illustrates what I've been saying.
There are three curves each for the power and torque. Crank/gearbox/wheel rim being the three stages. I fully understand doing that for the power curve, but not for the torque curve. It just seems plain inaccurate. By a large margin. This is not the first example I've seen so I guess it is usual practice. But you ask why I'm confused. There's my answer. Ask yourself what speed the rear wheel is doing at the time.
If it is common practice to express torque that way, with respect to dyno curves, can anyone explain why?
Cheers.
Edit: Just realised the graph has the wheel rim speed as 2959rpm. Can't be right.
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Sounds very low for rear wheel torque?
Eureka!
The answer is obvious, if a little unlikely.
The rear wheel torque is 31ftlb, as long as the rear wheel is spinning the same speed as the engine crank. At the same time.
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A dyno can only measure torque at the rear wheel or gearbox sprocket if its an engine dyno. A revcounter will be attached to the bike so the measured torque can be corrected to the equivalent of 1:1 gearing. The power and torque graphs show the corrected torque figure at the rear wheel and the rear wheel bhp calculated from the corrected torque curve.
I've spent some time pondering on this, trying to convince myself where I might be going wrong. But I can't.
I think I understand what is happening right enough, but I think it seems wrong to refer to the curve in question as rear wheel torque. Adjusting the measured figures to align with a fictitious 1:1 ratio isn't sufficient to qualify the resulting figures as actual rear wheel torque. Whatever was measured or "felt" at the dyno drum doesn't seem to find its way onto dyno curves. Not talking about power here, but torque. It seems as if they are trying to " fudge" torque into behaving like power. The figures on the torque curves supposedly relating to the rear wheel don't seem to stack up.
The best description I can think of to describe the curve I'm referring to would be "effective crankshaft torque". I other words, the portion of crankshaft torque left after all subsequent losses have been pre removed.
I'm more than ready to accept my error in understanding but, as yet, I've not had it convincingly explained. Apologies for my persistence, but it would be great if I could get this straight in my head, either way.
Cheers.
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I'd give up if I were you Propellor, you're the only one who doesn't understand it, I think this subject has been exhausted.
In conclusion, I think the answer to the original question is yes you can fit an XBR engine to a Honda CB250RS but it's a lot of work and probably not worth it, you'd be better off buying a complete XBR.
You can however fit an XL or FT engine much easier and this is a more common conversion and one the bike is capable of handling.
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I'd give up if I were you Propellor, you're the only one who doesn't understand it, I think this subject has been exhausted.
In conclusion, I think the answer to the original question is yes you can fit an XBR engine to a Honda CB250RS but it's a lot of work and probably not worth it, you'd be better off buying a complete XBR.
You can however fit an XL or FT engine much easier and this is a more common conversion and one the bike is capable of handling.
Aw c'mon, I'm just getting warmed up. If you can't have a bit of fun with some power, torque and a few gears then it's a poor do.
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- The back wheel turns a big roller
- They rev the tits of the bike and measure how fast the big roller accelerates
- From the acceleration and mass of the roller it possible to calculate the force applied by the back wheel
- They shut the throttle, the roller slows down because it now has to spin the engine
- From the deceleration rate they can determine the force applied by the back wheel and get a measure of the losses in the drive train
- They add the two together to get crankshaft torque.
- Simples
Not all Dynos work this way but a lot do.
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"..........measure how fast the big roller accelerates"
That's what I said earlier.
".........From the acceleration and mass of the roller it possible to calculate the force applied by the back wheel"
Yes. And they tell us that in power terms, but not torque.
"........From the deceleration rate they can determine the force applied by the back wheel and get a measure of the losses in the drive train......They add the two together to get crankshaft torque."
Can't fault that. You said CRANKSHAFT torque yes?
"Simples"
Should be. But they complicate it don't they.
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Perhaps this is a misunderstanding of definition. Or terminology use? To see if the misunderstanding lies with me, I'll try to explain what I mean.
I'm referring to the torque at the rear wheel. With the wheel spindle as the axis about which the moment is applied. The same torque as applied to the final drive "driven" sprocket. The point at which all the effort initiated by the piston is multiplied and "resolved" into actually propelling the machine forwards. The torque which has a tendency, sometimes successfully, to wheelie the bike. The figure involved will be substantially higher than the torque at the crankshaft because it will have been multiplied by the various ratios involved in the transmission.
Am I right to call that "rear wheel torque"?
Most dyno curves don't seem to show this curve, or more accurately "curves". They usually seem to show a torque figure which is abstract. The graph posted on the "chatter" section perfectly illustrates what I'm trying to say.
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Done some trawling. I am not alone:-
http://xlforum.net/vbportal/forums/showthread.php?t=627660
And a couple of interesting articles:
http://www.nrhsperformance.com/tech_power.shtml
http://www.vmaxoutlaw.com/tech/torque_hp.htm
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Is your point that there is little point is displaying a rear wheel torque value as this will be entirely dependent upon gearing. In which I would agree that the actually values are not that useful on there own but as a side by side comparison between runs they do have a value and the shape of the torque curve gives an indication of volumetric efficiency
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Is your point that there is little point is displaying a rear wheel torque value as this will be entirely dependent upon gearing. In which I would agree that the actually values are not that useful on there own but as a side by side comparison between runs they do have a value and the shape of the torque curve gives an indication of volumetric efficiency
Mainly I'm probing for explanations. At this point I think I understand what is happening a lot better and I don't think it changes anything I've said at all. It substantiates what I've been saying, I believe. The torque curve relating to rear wheel power, but expressed at engine speed is the issue, regarding a cause of confusion. It's abstract. It never happened. It's not rear wheel torque, but people frequently say it is. I assume that the "crankshaft torque as measured at the rear wheel" value is shown as a check on the value of the losses? Or why else is it shown? People seem to frequently misuse it.
As regards using the dyno to check on gains from modifications, then yes, we are interested in what is happening at the crankshaft. But people seem to treat the crankshaft figures with disdain. As if it doesn't really count.
But if we want to compare different machines then surely actual rear wheel torque values are the most relevant? Initially I picked up on something said by tevie54 but I also made a comment regarding an engine with half the cc but the same power value. In that context actual rear wheel values are very relevant indeed.
I'm grateful for your reply. It's appreciated. I apologise for my persistence, but i don't give in too easily! At the same time, I'm ready to acknowledge my mistakes, if I'm proved to have made them and most important, to learn from the experience.
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Bloody hell propellor bet you,ve only persisted with it so you kud become a "FULL MEMBER" 100 post n counting now eh
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Jeez guys all this technical stuff is making my head hurt! :o
Getting back to the original question, I think the answer is Yes, you probably can fit an XBR engine in a 250RS and if you did it will probably be not that much faster but may well accelerate like a scalded cat! However it'll probably be a bit of a bugger to do so unless your RS engine is knackered or you relish an extensive winter project and are mechanically proficient you may well be better off sticking with what you have!.....I think :o :o ........going to lie down now.....
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Bloody hell propellor bet you,ve only persisted with it so you kud become a "FULL MEMBER" 100 post n counting now eh
Full member?
Er indoors is nodding.
Chortle.
Edit: Hee Hee. I managed to get a bit of smut past the filter. Through the back door so to speak. Oops, there went another one. Oo er missus.
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Jeez guys all this technical stuff is making my head hurt! :o
Getting back to the original question, I think the answer is Yes, you probably can fit an XBR engine in a 250RS and if you did it will probably be not that much faster but may well accelerate like a scalded cat! However it'll probably be a bit of a bugger to do so unless your RS engine is knackered or you relish an extensive winter project and are mechanically proficient you may well be better off sticking with what you have!.....I think :o :o ........going to lie down now.....
To be fair, I did start out my "quest" saying it was confusing. And I think I proved it is! Ha ha. I apologise if I caused any innocent bystanders to vomit.
Look on the bright side, it got me out of the short trousers category, as observed by moto63, who has some serious posting to do if he's to catch me up.
The thing which would concern me about dropping a 500 in there, especially an xbr, is that the RS frame is so lightweight. It is VERY skimpy. To say nothing of the wheel rim width, yoke proportions, fork proportions, swingarm. It looks fine for maybe 30 hp, but I'm not sure I'd want to put more through it. Not without mods. Xbr size rims. Xbr size yokes. Xbr size forks. Add material to the frame..... Doh!
Then again, that's trying to apply logic. Sometimes you just says bo ll ocks I'm going for it. I'm not unfamiliar with that approach either.
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I,ll say. I,ve seen your Beemer oo er more tea vicar
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The RS chassis is fine with a 500 engine. What you end up with is a 250 with a 500 engine and the corresponding advantages and compromises that it brings. I loved mine and only sold because my wife was sick of me spending so much time on it. The Dominator I replaced it with was dull by comparison.
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Bloody hell propellor bet you,ve only persisted with it so you kud become a "FULL MEMBER" 100 post n counting now eh
99 of which were on this thread!
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Don,t worry I,ll just keep posting back nice easy meaningless replies until I get my "FULL MEMBER" or shud that av read till I become a full member either way i,ll some more tea vicar thank you, oh and I,ll catch that propellor you just wait n see.
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Bloody hell propellor bet you,ve only persisted with it so you kud become a "FULL MEMBER" 100 post n counting now eh
99 of which were on this thread!
Party pooper. Ha ha.
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The RS chassis is fine with a 500 engine. What you end up with is a 250 with a 500 engine and the corresponding advantages and compromises that it brings. I loved mine and only sold because my wife was sick of me spending so much time on it. The Dominator I replaced it with was dull by comparison.
Yes I realize it's been done. Just that I had "concerns" over the light duty components. But I duly note your comments for future reference. I wouldn't put it past myself to do something like that. Besides, if someone told me it couldn't, or even better, shouldn't be done, I'd do it just to spite em. Hee hee.
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Sounds very low for rear wheel torque?
I'll offer an apology as the fault may possibly lie with me, in misinterpreting what you meant. I can see that your line above is not necessarily an explicit reference to transmitted rear wheel torque. I now assume that you meant "my effective crankshaft torque measures 31ftlb at the rear wheel."
In my defence, I believe it is open to misinterpretation making reference to the rear wheel when the figure refers to the crankshaft. The only relevance to the rear wheel, as far as I can see, is that it is the place where the measurement (of crankshaft torque) took place. I told you it was confusing!
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Blimey, I wish my bike put out as 'torque' as you Propellor
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I converted my RS250 to a 500 about four years ago and I'm still doing little bits to it to improve it even now, Ive had all sorts of bikes over the years Fireblades, NC30 etc but the RS gives me the most fun. It has CBX 250 wheels a Mikuni 40mm flatslide home built exhaust plus various detail changes to frame , footrests, handlebars etc.
Power wise It'll do 100mph and still lift the front wheel in third so realistically I don't think it needs any more 'tuning'.
I might change the carb for a 34mm roundslide to give it a bit more of a progressive power delivery as its a bit abrupt in the lower gears.
I need earplugs on a run as the 'blat blat blat' starts to get tiring although I have been to France on it twice. I think Propellor is right about the frame though, it could do with a bit of extra bracing. Over the winter I'm taking it off the road to do some cosmetic restoration and a few mechanical jobs so I'll be using my XL250 for work were I'm building a 305cc version of the XL/RS engine.
Electrics are swapped from the RS, airbox has to go though, use the camshaft from the RS as this has the tacho drive in it. Not sure whether to get '500RS' stickers made for the sidepanels or leave the 250 ones just to surprise people at the lights.
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A number of people seemed to think the 250 cam would work well in the 500 I found it seemed to restrict it, I ended up with an XR500 cam and fitted a pin to drive the tacho drive.
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I must admit I nearly just drilled the XL500 camshaft but when I read you could just use the 250 one I did that. I didn't notice any difference at the time but maybe I'll try it again when I take the engine apart this winter.
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Blimey, I wish my bike put out as 'torque' as you Propellor
It's a forum. That's all we can do.
Look how much talking this person needs:-
http://charming.awardspace.com/otto_diesel/power-vs-torque.html
I draw your attention to the final paragraph.
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Is your point that there is little point is displaying a rear wheel torque value as this will be entirely dependent upon gearing. In which I would agree that the actually values are not that useful on there own but as a side by side comparison between runs they do have a value and the shape of the torque curve gives an indication of volumetric efficiency
I have no experience of engine tuning so please forgive (and correct) any errors. Trying to place myself in the position of someone making changes to the engine with expectation of improvements in power, wouldn't I be most interested in looking at curves relating to the crankshaft output?
Thinking about the dyno in the context of comparing the engine performance of different machines, I'm not sure rear wheel power figures are the best way, and definitely not the rather abstract torque figure seen on a lot of dyno sheets. In this context wouldn't the best way to show power/torque figures be based on crankshaft output? When comparing engines isn't that the ONLY way? Not only that, but the resulting information would very easily expose any cloak and dagger marketing by manufacturers. A lot better than it does now? The only sure way of comparing, in figures, the performance difference between complete machines is a tractive effort graph?
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STEVE H, you might be on to something there, I checked the specs of both camshafts and although lift is the same for both, the XL500 cam exhaust timing opens 10 degrees earlier than the RS, don't know if it makes much difference to power but it might.
The XR cam might be even more radical.
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STEVE H, you might be on to something there, I checked the specs of both camshafts and although lift is the same for both, the XL500 cam exhaust timing opens 10 degrees earlier than the RS, don't know if it makes much difference to power but it might.
The XR cam might be even more radical.
The extra duration will give more power. The XR camshaft has even more duration and will give a little more power, I don't recall the exact timing specs though.
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The thing which would concern me about dropping a 500 in there, especially an xbr, is that the RS frame is so lightweight. It is VERY skimpy. To say nothing of the wheel rim width, yoke proportions, fork proportions, swingarm. It looks fine for maybe 30 hp, but I'm not sure I'd want to put more through it. Not without mods. Xbr size rims. Xbr size yokes. Xbr size forks. Add material to the frame..... Doh!
The race bikes used to put down around 45bhp at the rear wheel without issues. There was probably just as much stress put through the chassis and forks by the race tyres than by the engine.
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The thing which would concern me about dropping a 500 in there, especially an xbr, is that the RS frame is so lightweight. It is VERY skimpy. To say nothing of the wheel rim width, yoke proportions, fork proportions, swingarm. It looks fine for maybe 30 hp, but I'm not sure I'd want to put more through it. Not without mods. Xbr size rims. Xbr size yokes. Xbr size forks. Add material to the frame..... Doh!
The race bikes used to put down around 45bhp at the rear wheel without issues. There was probably just as much stress put through the chassis and forks by the race tyres than by the engine.
Thank you. That's very interesting information. The race bikes ran the stock forks, yokes and wheel rim size? Xbr engine?
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STEVE H, you might be on to something there, I checked the specs of both camshafts and although lift is the same for both, the XL500 cam exhaust timing opens 10 degrees earlier than the RS, don't know if it makes much difference to power but it might.
The XR cam might be even more radical.
If you plot a graph of lift against crankshaft rotation , then my understanding is that power output is related to the area under the graph. There is also a point above which the lift has minimal effect so rather than have a peak on the graph you have a flat line capping the top of the peak off. If you are looking to increase power then increasing the area under the graph will help. The most effective way to do this is increase the duration, the other factor is to get the valve open as soon as possible so that it is open for as long as possible at the maximum flow rate, if you increase the lift (and keep the duration the same) then the valve has to open correspondingly faster, increasing the area under the graph.
In summary duration has a bigger part to play than lift.
I did run a megacycle cam for a short period of time and that was very quick, unfortunately it also ate the rockers very quickly. Hence the XR cam.
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STEVE H, you might be on to something there, I checked the specs of both camshafts and although lift is the same for both, the XL500 cam exhaust timing opens 10 degrees earlier than the RS, don't know if it makes much difference to power but it might.
The XR cam might be even more radical.
If you plot a graph of lift against crankshaft rotation , then my understanding is that power output is related to the area under the graph. There is also a point above which the lift has minimal effect so rather than have a peak on the graph you have a flat line capping the top of the peak off. If you are looking to increase power then increasing the area under the graph will help. The most effective way to do this is increase the duration, the other factor is to get the valve open as soon as possible so that it is open for as long as possible at the maximum flow rate, if you increase the lift (and keep the duration the same) then the valve has to open correspondingly faster, increasing the area under the graph.
In summary duration has a bigger part to play than lift.
I did run a megacycle cam for a short period of time and that was very quick, unfortunately it also ate the rockers very quickly. Hence the XR cam.
Now that makes a lot of sense. Even to a tuning dummy like me. Would using lighter valves and springs allow steeper ramps? That, in a naive way, would seem to gain effective duration without increasing lift.
Do I get brownie points or custard pie?
What do you know about torques?......
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- The back wheel turns a big roller
- They rev the tits of the bike and measure how fast the big roller accelerates
- From the acceleration and mass of the roller it possible to calculate the force applied by the back wheel
- They shut the throttle, the roller slows down because it now has to spin the engine
- From the deceleration rate they can determine the force applied by the back wheel and get a measure of the losses in the drive train
- They add the two together to get crankshaft torque.
- Simples
Not all Dynos work this way but a lot do.
I hear what you're saying. But they don't seem to do that. Nearly all curves I've looked at as examples seem to simply take the rear wheel horsepower and plot a torque curve against it at engine speed.
This is not the same as you said.
What you said, in principle is great. In practice they don't seem to 100% trust the "coastdown run". Not enough to categorically say "here, this is your crankshaft figure".
Simples? Should be, but it ain't.