Help to ride a thumper!

[Steve H]

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 ?

[guest27]

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

[Steve H]

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.

[casper]

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)

[casper]

Sorry Steve H, I called you John. That was a really helpful reply. Many thanks.

Its all making sense, slowly!!!!

Casper (AKA Dave)

[J Hop]

Performance bikes is probably solely responsible for my love of singles, NWS or Harris racing singles, John Laker and Over Racing SRXs etc etc