XBR engine transplant into CB250RS

[guest564]

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.

[tevie54]

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

[Propellor]

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.

[Propellor]

........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.

[tevie54]

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.

[Propellor]

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?

[Propellor]

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.

[Propellor]

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.

[Propellor]

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.

[tevie54]

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.

[Propellor]

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.

[Steve H]

• 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.

[Propellor]

"..........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.

[Propellor]

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.

[Propellor]

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