Thumper Club Forum
Club House => Chatter => Topic started by: casper on February 03, 2007, 02:03:47 PM
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Hi, I have just purchased a Sachs 650 Roadster. It uses the Suzuki 650 engine (as used on the DR Suzuki trailie). I find it seems to need plenty of revs to jeep it moving along and it won't rumble along in top gear at a low speed.
Is this cos it has a twin carbs?
For low speed work you have to be in a lower gear. Is this a characteristic of just this bike or all thumpers?
Does anyone have one of these unusual beast and I'd appreciate advice?
I have not ridden a large single before!
Casper (Dave)
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I've not ridden the Sachs but it seems a lot of modern big singles don't like to work below about 3,000 rpm, certainly my MZ Mastiff was like that and it uses a rebadged XTZ660 engine. It has become better with use, it now tolerates about 2500 revs and even sometimes 2000 if I'm very gentle!
Anyone else...?
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skorpion 660cc didn't like low revs. xt 600 was better at lower revs (less power, smaller carb, cam timing). Older stuff like xt500 better still at low revs.
i've never found a jap single that can 'chug' like a brit single, but on the other hand any brit single over 40bhp is a pig to ride at anything other than flat out.
its all swings and roundabouts.
bullet350
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Thanks guys, I think it is just an art and a bit of accrued experience to ride it. The CCM (R30 I think) uses the same Suzuki engine, so its a well tried power train unit.
My guess is the twin carbs feed it plenty of fuel, so it doesn't 'plod' like a good old Brit bike.
You have to drop it down a gear or two at lower speed, so at 20 mph its in 2nd, 30 mph in 3rd and 40 mph in 4th and 50 mph plus in 5th. I use this as a rule of thumb guide but it does seem to work on this bike.
So far its been reliable. But because there are so few around you can't really share experiences with other riders. So if anybody has one, I'd be really interested to know how another rider gets on with it.
Casper
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You have to drop it down a gear or two at lower speed, so at 20 mph its in 2nd, 30 mph in 3rd and 40 mph in 4th and 50 mph plus in 5th. I use this as a rule of thumb guide but it does seem to work on this bike.
Casper
Sounds about right matey :-) Least thats about where my Dommie's happiest. She'll only pull sub 3k revs in the first three gears, after that you need to keep her working between 4 and 5.5k Makes for a more pleasant ride if you ask me as you have to put more in to get the best out. Can't get lazy on a jap single :-)
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Thanks Squirrel.
This is my first big single and thats reassuring to know you are in the same situation. Yes it does like to work up and down the gears. Once you get the knack it seems to work quite well indeed.
The main thing is to make sure it doesn't go into a high gear at low speed. You just have to change down and this seems fine as long as you remember to do the change down.
This is a new riding experience for me (although I have been riding bikes for years). Riding a 'thumper' is definately different!
Casper.
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I think all 4 valve per cylinder engines are "more revvy" than 2 valves per cyl, also over square (larger bore than stroke(like jap vs old brit)) are also more revvy ?
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J hop
many thanks. I have found out that you do need to use the gearbox quite frequently on this bike. Speaking to a guy with the CCM (R30 I think)that has exactly the same engine (and carbs I thinks as well) and he agreed working up and down the gears was needed.
Thanks for your comments.
I am much more used to it now and coming from twins and fours, a single like this is a very different experience.
Cheers Casper (Dave) - West Sussex
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Where in West Sussex Dave? I'm in Shoreham-by-Sea and there is/was Gary Louth in Midhurst (anybody heard from him lately?). Oh, and Terry Lee is, I think, still in Brighton. Blimey, we could well have a group ride from the South coast to theAnnualRally this year. :-)
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Bill, I am in sunny Littlehampton (or LA as locals sometimes call it!)
I am (as you can see by the thread) new to single's. Although I have been riding bikes more years than I care to remember!!!! (33 years actually when i started on a Yamaha step through 50 cc as a 16 year old).
I am a member of the Bury Hill Bikers (BHB) group, who welcome all sorts of bikers, sports, classics, you name it. Sponsored ride in July????
http://www.buryhillbikers.co.uk/phpBB2/index.php
So like today (usually Sundays) I was up at the Hill.
Always happy to go for a ride locally and meet up at Bury. Just drop me a line:
jmssax@tiscali.co.uk
Safe riding,
Casper (Dave)
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Hi, I think you will find the lack of chugability at low revs is due to the short stroke, big bore and high compression. I run DR800's and they are worse, you wouldn't dare give it full throttle at less than 4000revs, but they will rev to 7500.
My daughter runs an LS650 which uses a longer stroke, smaller bore and lower compression and chugs along nicely and so only needs a four speed box.
Dave
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Dave#22 , yup, I have now got (or am getting) the knack of riding this particular 'thumper'.
It sure does like its revs and if you get it right, its just fine. I just feel like I'm revving the nuts off it as i go up and down the gearbox. But it does seem to be OK with it.
I had to get used to it and as I only got it in late December, I haven't really had a lot of riding time on it. Yesterday I managed to get it out and do a half decent ride on it.
This rule of thumb seems to work 20 mph = 2nd gear, 30 mph = 3rd gear, 40 mph = 4th gear, 50 plus mph = 5th gear.
Cheers
Casper (Dave)
(http://i134.photobucket.com/albums/q119/casperdog777/P7260016.jpg)
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Aye Casper,
That a Triumph in the background there with the reverse meggas or something else? Handlebars look similar to a Duke single scrambler, but I think the 'C' reg makes it to early unless its an American import.
Nosey old git here, and please tell me to 'bog off' if you don't wish to say.
As for thumping, after my '53 AJS 500S and an Aerial 500HT, none of the Jap singles I've tried do. Can't retard the ignition, oversquare bore, bigger carbs, lumpier cams = more revs. Duke 450 dies if you try to move off with less than 2.0K rpm and thats with the standard head, 29mm carb and soft cam. You'll also knock the big end out in 500 miles. Sally the SRX600 needs 1.5 - 2.0K to get underway and then gets taken to 3.0 - 3.5K through the lower gears and 4.0 - 5.0K for 4th to 5th. However, if you want to get a shift on take it to 5.0K through the gears!
The Blagger or SteveL would probably suggest 6.5K in every gear with at least 7.0K in top, but thats at Snetts!
I am sure I can say, "We hope you enjoy the alternative way" and you'll make of it what you will.
Cheers, Bill
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Make that 9k with red mist at cadwell, (not recommended)OOOPs, good thing I put stiffer valve springs in.
Any way if it says 7k on the rev counter before the red than 7 k is safe as far as I am concerned.
I don't think modern thumpers are good low down as old british iron. My theory is that a lack of mass ie no large flywheel etc. is the reason they need more revs. This translates to much better acceleration from todays bikes.
Like I say it's my theory and Im sticking to it.
Ignorance is bliss.
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Thanks for the advice guys. Its all making more sense now.
The picture Bill, is of my bike is from the dealer I bought the bike from (GB Motorcycles in Wiltshire). So thats his garden I presume. So to be honest I don't know what the bike is in the background. He does mainly Brit bikes though.
The stroke and flywheel etc etc. all slots into place. Basically, the Jap singles (like the DR 650) work at higher compression, on a shorter stroke then? This presumably makes them faster and more 'revvy' etc. This would account for the fact it needs to be in lower gears at lower speeds, unlike the old Brit 'chuggers'?
Cheers, Casper (Dave)
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This is primarily guesswork but here goes
- Two vales per cylinder generate better filing/swirl at lower revs so torque is improved, however they restrict flow at high revs so four/five valve head is needed.
- Singles only have one power pulse every 720 degrees, four cylinder bikes have four. Therefore on a single after the initial power pulse everything is slowing down, until the next power pulse. So the engine is constantly accelerating and slowing down and 'pulsing'. My guess is that at lower speeds this 'pulsing' sets up resonance in the drive train and causes the snatching.
- To overcome the problem of one power pulse per 720 degrees is to have a BIG flywheel. Because this has a lot of rotational inertia the engine slows down less than with a light flywheel. Since the 'pulsing' is of lower amplitude the drive train snatches less. Problem is a heavy flywheel means that the engine is very slow to pickup speed and is reluctant to rev-out, therefore reducing power.
- Modern singles tend to have much shorter strokes, to allow higher revs without breaking. Ive seen an argument that this increases torque at low revs but I'm not convinced, as a short stroke single with a an equivalent capacity would have to have a bigger piston (to get the same capacity) thereby increasing the force for the same combustion pressure, and hence the same torque. (I haven't thought about this too hard so could be wrong)
Basically its a compromise, if you want high revving singles then you have to live with the low speed snatching. Ive no doubt people have loads of examples of fast British singles to shoot me down with the ABSAF and Goldstar singles (interestingly ABSAF don't quote any power figures), but what were are they actually like to ride on the road ?
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Seem to remember years ago John Robinson addressed this in Performance Bikes or Mortocycle Mech - and his conclusion - and who am I to argue with the late JR, was that old singles lugged like a good un because they had massive flywheels. Ditto for twins that lug well - look at late Bonnie cranks and how much in demand they are compared to light flywheel cranks
R
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John Robinson was the only reason I bought Performance Bikes. Dont know what but he always reminded me of John Peel and vice versa. I've yet to find another bike magazine with a similiar level of useful technical content.
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John,
that makes a lot of sense, thanks for that. I appreciate the detail and I now know why this particular Suzuki single behaves like it does. Other club members may have this engine in their Suzuki DR650 and indeed the CCM R30 uses the same lump too. Both are trailie bikes, mine is a roadster.
The stroke must be indeed shorter and the flywheel lighter than say the LS Suzuki 650, which seems to be of similar ilk to the older Brit bikes.
Its reassuring to know it should be like that. Once I got used to it, it seems perfectly sensible. The trick seems to be to keep it in the right gear at the right speed.
Casper (Dave)
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Sorry Steve H, I called you John. That was a really helpful reply. Many thanks.
Its all making sense, slowly!!!!
Casper (AKA Dave) :)
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Performance bikes is probably solely responsible for my love of singles, NWS or Harris racing singles, John Laker and Over Racing SRXs etc etc :)