Effect of wider rear wheel

[Beau]

I've ridden plenty of sportsbikes with 180's and a few with 190's on the rear and there's no way in hell I'll be putting either on my TRX.

I run 120/70 front and 160/60 rear Pirelli Diablo Rossos and the suspension has been fully rebuilt and resprung for my weight and it is the sweetest turning bike I've ever owned. I run emulators in the forks and an extra valve in the rebound damping to help stop the bike squatting under acceleration as well as heavier fluid in the rear shock to keep it nice and funky when it gets hot. You may find that the suspension is part of your problem as well as the tyre...

The wider tyre will be pushing the front, but most TRX's will also squat at the rear when you apply the power in a corner. This reduces the steepness of the steering head and encourages the bike to push wide. Adding the additional valve to my rear shock stopped the squatting, and now you can cane it just before the apex and it doesn't push, it just hunkers down and squirts at the apex and then really boogies on exit while holding as tight a line as you like. It also helps the back hook up really well and means you're upright earlier on exit and therefore faster.

The wider tyre you're running will only slow the turn in and that is one of the big advantages the TRX has over many bikes... why take it away?

As for the ground clearance thing, the wider tyre will by rights actually increase your ground clearance, but then that is negated by having to lean more for a given turn because the tyre doesn't steer as well. With the 160 on the rear of mine there is plenty of ground clearance and I find myself running less lean angle than mates on other sportsbikes for a given corner at the same speed. With the narrower tyres the bike will turn more for a given lean angle than a wide-tyred bike. That's not to say the extra lean angle isn't there with the 160 if you want it... I've managed to scrape my waterpump housing on the road, so she goes over a long way with a good 160 fitted. I reckon with a 190 or a 180 you'd have crashed before you touched the waterpump down, but the 160 Pirelli has a pretty steep profile and plenty of meat on the road at big lean.

[slow codger]

the 180 rear on mine didnt make any difference to the rear height!, the rear tyre still fits under the hugger. Only the YZF dogbones jacked it up.

Dave E

[Tarwetijger]

Cheers Beau for your extensive reply.
I think I will try other dogbones first. The height is (unlike the rest of the bike) still standard, and the rear looks a bit low. I've YZF dogbones laying around somewhere so its easily done.
If it won't work, I'll use the TRX wheel / 160 tyre again, at least for track use. So in most ways I agree with you: the 190 tyre is not likely to make things better...

Btw, what do you exactly mean with squatting? (my dictionairy didn't help me out...)

[slow codger]

rear end sits down on accelleration therefore taking some of the power from the drive.

Dave E

[Beau]

the 180 rear on mine didnt make any difference to the rear height!, the rear tyre still fits under the hugger. Only the YZF dogbones jacked it up.

Dave E

I never suggested the 180 would raise the rear... I suggested it gives better ground clearance when leant right over. It does this by virtue of being a wider tyre... the side of the tyre is further from the bike's centre line, therefore getting everything little further off the ground at a given lean angle. It doesn't make a massive difference, but it is there.

[Beau]

rear end sits down on accelleration therefore taking some of the power from the drive.

Dave E

Yup... but it's not so much any power loss that is the problem, it's the change in front end geometry it causes... and given that this happens when you apply the throttle mid-corner, it is the reason many TRX's want to lift their nose a little and push wide under power on exit.

I see lots of guys jacking up the rear ride height with new dog bones to try and get the steering head angle steeper thinking this is the problem, but if you haven't set up the rear to stop it compressing under power, then as you apply the throttle the front end geometry will pretty much go straight back to the standard numbers anyway as the front lifts (or should i say as the rear drops). It will be a bit better than standard if you've raised the ride height, but you haven't actually solved what caused the problem in the first place. It was never the steering head angle or ride height... it was always the rear end. Most people agree the bikes turn in exceptionally well in at the standard ride height, so the steering head angle is not the problem... it is what happens to it under power that is the problem.

Having said all that, I am only discussing my bike here (and a couple of others I have ridden over the years). They all tend to want to push under power when cranked over... and it has less to do with the rear ride height than it has to do with the suspension. But your own bikes may be different.

[honkdawillydahonk]

Surely the rear suspension extends under power (because of the chain pull).... and compresses due to cornering force...

[BigAl-TC]

Surely the rear suspension extends under power (because of the chain pull).... and compresses due to cornering force...

That depends on your swing arm angle, relative to the centerline between countershaft sprocket and rear axle and wether the swingarm pivot is below or above that line.
In drag racing circles, that can either make the back end "squat" causing the front to lift or "lift" helping keep the front down.
If you draw two diagrams showing these two scenario's, you should be able to see how it works.
Will try to find the article i got this info from.

[Tarwetijger]

I'm just reading the chapter about chain pull angle etc. in the book from Cocco.
I better read the complete book then, before I decide what to do

[Beau]

In the case of the TRX it will squat... even with YZF dogbones.

Don't forget, it's not just chain pull that determines the effect... weight transference plays a huge part as well.

To be honest the revalving of the shock to stop the squatting wasn't a mod I had thought of. I just accepted the tendency to push wide under power to be a trait of the bike. It was my suspension tuner who suggested it... and he was spot on. It's like a whole new bike.

[Tarwetijger]

Beau, so you revalved for more damping in the rear shock, right?
I fitted a YZF öhlins rear shock, it has been serviced and is completely adjustable.
It is just that I get the feeling that the suspension-specialist does not take me - with my 'old' TRX - seriously enough to provide some REAL advise. Will look for someone else.

[HansJ]

Beau, interesting findings! I thought the same as Honk, that the rear extends while accelerating. I'll check next time when the bike's up on a test bench (I know this is not real life, as the weight transfer won't be there, but still)

Just a thought, would re-valving really help this phenomena? I mean, what you've accvhieved is more re-bound damping, right? This will only slow down the extension of the suspension, but will it really stop it? The extension will be delayed, but it will still extend, or? And too much re-bound damping might pachk the rear as well, haven't you any problems with this?

[Beau]

Good question... and here's where my technical knowledge gets a bit shakey...

If you rode my bike you would not consider it to be overdamped at all... it's actually reasonably plush in terms of damping, so it's not just a case of winding the damping up harder. That would be counterproductive. I'm a big fan of fitting the right springs and then running the least amount of damping possible to get the thing to ride well. IMO overdamping is often done by people who are trying to make the bike feel "stiffer". But using extra damping to overcome too soft springs just stops the suspension from doing what it's supposed to do... move. So I started with fitting the right springs and then gradually winding up the damping (both compression and rebound) until it reached a point where it was controlling the movement of the suspension the way I want. Afterall, that is the job of the damping function... to control the movement of the suspension... not to override it.

As for the changes that were made to my shock to stop it squatting, I'm not totally up on the technical terms, but it was a bit more than just revalving the rebound damping circuit (I think). The way my tuner described it was to think of it like an extra rebound damping circuit, which doesn't affect the suspension at all over normal bumps, but under long slow continuous compression (such as you get when it squats under power) it will come into play and stop the suspension from getting bogged down. I don't understand the mechanical side of what he actually did to the shock internals, but he definitely described it as though it operated as another rebound circuit. Having said that, it's not like I have an extra rebound control or anything, so it may just be a case of revalving, but he described it as an extra valve or circuit. Perhaps those on here more technically minded than I might have a better idea of what I'm describing than me. It's also worth remembering that my suspension guy is one of the best in Oz, so he may be a little more advanced in his knowledge than most people (and fair enough too).

All I know is that whatever he did worked a treat. I can tighten the line of the bike easily midcorner if necessary and it's even capable of a totally different riding style now. TRX's are known for good turn in and mid corner speed, but mine can also be ridden hard by dragging it to the apex still heavy on the brakes, turning it hard and sharp at a lower speed, then getting on the gas as hard as you please and firing it off the exit on a tight exit line... it will hold a tight line on exit pretty much no matter what you do. Works even better on switchbacks or "S" bends where you want a tight exit from the first bend to set up your line for a fast and wide exit out of the second bend.

Anyway... I have a track day coming up early december, so I'll report back on how it goes on a racetrack with the changes. I think I will find it still a bit soft for track work, but it is mostly a road bike 98% of the time, so I want it to remain road biased.

[Beau]

It is just that I get the feeling that the suspension-specialist does not take me - with my 'old' TRX - seriously enough to provide some REAL advise. Will look for someone else.

You could be right there... but also there's plenty of suspension "tuners" who don't actually really understand what they're talking about. If they did they'd tell 80% of their customers they needed new springs before they bothered with damping setting changes.

[phuk72]

Beau, interesting findings! I thought the same as Honk, that the rear extends while accelerating.

I think you'll find it does

the simplest way of illustrating this is to push your bike up against a brick wall (or apply the front brake), wind on some revs and relese the clutch lever - as if strating a burn out.

Have a look at what the rear of the bike does