Finding grip & longer swingarms

[jc2]

Interesting comments but too many are off the track. Perhaps I didn't state clearly in the original post what I meant.

I'm not referring to extending a swingarm on an existing design. That obviously affects wheelbase, weight distribution & sometimes suspension travel all of which are going to affect grip.

I mean, all other things being equal - weight distribution, wheelbase, power characteristics, wheel travel, damping & spring rates - why is a longer swingarm design said to give more grip?

I've heard/read it repeatedly applied to trials bikes, MX bikes & motoGP bikes, but perhaps no more so than regarding trials bikes which are my main interest now.

Can anyone shed light on it?

[htrdoug]

Adding swingarm length helps greatest when added at the front of the swingarm,keeping the ideal 52inch wheelbase means moving the engine forward,but I think the issue that causes the increase in traction is that your footpeg position is then farther behind the pivot point of the swingarm,increasing the leverage of your body weight against the action of the suspension.This is why I brought up the example of modern bikes having more grip everywhere,regardless of whether the suspension is acting or not, than a vintage twinshock bike,the back tire doesn't know how many shocks are attached to it,it only know contact pressure. The relationship of the countershaft sprocket to the swingarm position only matters when there is enough traction to transfer that force into the suspension,in low traction,light throttle situations I don't believe there's enough influence to effect traction as much as weight distribution does. Look at the early vintage bikes with super short swingarms,even though the footpegs were back at the leading edge of the rear tire they were at best in line with the swingarm pivot,sometimes in front of it. Look at a modern bike,footpegs will be several inches in front of the leading edge of the back tire but also several inches behind the swingarm pivot,I think this amplifies your weight enough to more than offset the weight of moving the engine forward(I weigh quite a bit more than my entire modern bike) giving greater contact pressure at the tire.

Ok,how can we utilze this in vintage twinshock? Well lets take the Suzuki RL for example,I have never ridden one but I read on the intermanet that they have less than optimum performance as a vintage trials bike,look how short the swingarm is,yet there is room to move the swingarm pivot forward in relation to the engine without moving the engine forward if one were to swap to a "h" shaped swingarm as opposed to the current "u" shaped swingarm.This would straddle the backbone design of the frame and would accomplish several positive improvements(stiffer swingarm mounting in addition to the weight transfer benefits) If you constructed the new swingarm to retain the same wheelbase you would still increase the effect of the footpeg placement acting onto the contact pressure,resulting in increased traction in trials riding situations,I'd bet this would even apply in braking situations also,I'm no engineer but I'd speculate that it would also not decrease steering traction as much as just moving the footpegs rearward alone.

Again,I'm no engineer but I received a private email that sparked my understanding on this matter plus as I was letting my non trials riding buddies ride my gasgas one of them commented that as soon as you touched a foot down while climbing a slick rock you lost all traction at the tire(effectively unweighting the footpegs) it occured to me how much influence the footpegs have on trials bikes(I knew this,but it was surprising to see a first timer notice it also)

WARNING!FIRESTARTER! This is what makes a Airmono to Twinshock conversion very interesting to me

(Please do not comment about AM to TS conversions in this thread,P.M. me with insults if you must,I canz take it!)

[jc2]

I've done a bit of research on this. As I suspected, it seems its all to do with chain-pull torque/effect under power, which extends the rear suspension & stiffens it up, making it less supple over undulations potentially causing loss of grip. A longer swingarm reduces that effect so potentially offers more grip - most noticeable under acceleration/full power on rough ground.

It also means less tendency to wheelstand, since it reduces the effect of raising centre of gravity from the extended suspension. And it means more power is available for drive since less is lost extending the swingarm & raising the C of G.

This of course is what Vesty was after when he modified his Bul in the early 80's.

I hasten to add that a longer swingarm is not the only thing that affects chain-pull effect on grip & Vesty's bike used several mods to achieve his aim.