Lightweight wheels
 

Alert the suspension experts.

I've just installed Dymags - 3 spoke front/5spoke rear - which give a major weight saving over the standard Marchesini/Brembos.

Having got the suspension set up nicely with the old wheels (using Section 8's very useful guide), I am now completely confused as to what direction I should go in to accomadate the reduced unsprung mass.

I start thinking about it, and reckon I've got it sussed, and then my head begins to hurt and the voices start.

Does anyone out there have a baisc rule of thumb bit of advice as to the approach I should take?

If none of the DSC gurus can help, I'll have to ask a grown-up at the next track day, and that would just be embarrassing.

Yours in a complete kerfuffle,

Spinoli

Well, since they're lighter wheels, logically you would expect that the springs wouldn't need to push as hard to keep them pinned to the floor, that the suspension would have less momentum to deal with as it moved up and down and therefore need less damping.

So I'd say start with the settings that you've currently got (which worked for you with heavier wheels) then take a little bit of everything off a bit at a time

I'm going through the same thing at the moment (from 3 spoke brembos to Mag Marchesini's, and lighter front discs), and am pretty happy at the moment with a lot less compression damping, and slightly more rebound damping. (on the road)

I'll be leaving it alone for a few weeks to see how I settle into it, then probably get some expert assistance to fine tune it, sort it for trackdays etc.

Jason

The springs should not be affected, Jools. You use the springs to hold up the bike. Just make sure the static sag is set correctly, which won't be affected by a change of wheels.

Dampings may be affected, but can only be sorted out be riding/feedback/adjustment/riding/.....

I have several sets of wheels that I use on my bike ranging from standard 3 spokes (heaviest) standard 5 spokes (lighter) 5 spoke Bucci's (lighter still) to 3 Spoke Dymags (lightest) I don't make any alterations to the suspension when switching between the wheels.

The most noticeable difference is how easy the bike is to turn with the light wheels in.

I am sure Troy Bayliss might benefit from fine adjustments when using different weight wheels, but for me - a reasonably competent rider who hasn't got much of an idea about suspension twiddling I leave it alone.

Probably a sensible starting point assuming you were happy with the settings before is leave it as it is and try it and see what you think.

If you do feel you want to change anything do one thing at a time, this way you'll know what works.

Jools is on the right track,
ideally a lighter spring would be the starting point. when selecting a spring for a shock the stroke of the shock is factored into the calculation as is a inertia value which in racing changes from track to track and is why in any top race truck you will see a container full of springs.
looking back at some of my old Spring calcs I find I used inertia values between 1.6g and 2.0g. with Shock strokes ranging from 43mm to 54mm.
When the lighter wheel hits a bump it exerts less force on the suspension than a heavier wheel would over the same bump at the same speed and hence uses less of the shocks stroke. In race track conditions this would mean some of the shock stroke was unused and to get the best from your suspension you would fit a lighter spring that the damping would be easier to control. In racing its not the wheel weight that changes but the size of the bumps and the speed at which they are hit, so with the use of data logging a spring selection is made.
All a bit pointless on the road as you never know what size of bump will be encountered so having a bit of shock travel in hand can prove quite useful.

Kev, maybe I am completely off my rocker, but I don't agree. Clearly the inertia of the wheel has an effect on how much the spring gets compressed. However, the contribution from bumps towards the eventual deflection of the spring is minor. The much larger determining factor is the heavy braking period and preventing the forks from reacing the end of travel. That's why the carry such an array of springs.

My thoughts (and responses I got when I asked a similar question) were that there would be no need to change the suspension settings for lighter wheels, the springs hold the bike up (support the bike's weight) and not push the wheels down (which would require less pressure if the wheels were lighter).

The strength required to hold the bike up is the same irrespective of the weight of the wheels I reckon..... its got me thinking though!!

Tim

All ok providing there are no bumps in the braking area, which was the down fall of most of the Antidive systems. Where the object was to seperate the suspension from the braking forces allowing it to do its job of soaking up the bumps
How much fork travel is left after heavy braking, why leave any at all unless you intend to cope with the odd bump. say you leave 5mm and the wheel hits a bump that uses a futher 4mm of travel, if the wheel weight was then doubled so that 8 mm of travel was used you may find a spring change may be in order.
MotoX bikes have far more suspension travel than Road bikes not because they brake harder or are heavier but because they have to deal with bigger bumps the contribution from bumps towards the eventual deflection of the spring is major,
keeping the rear wheel in contact with the ground and constantly driving the bike forward is the main aim of the rear suspension, the more reactive the suspension can be the better.

Agree with the bumps in the braking area, but the forces acting on the spring under heavy braking are dominated by the sprung weight, not the unsprung weight. That is when the springs are that far compressed, essentially identical to a huge amount of spring preload, it makes little difference if the unsprung weight is slightly less. The reduction of unsprung weight is most noticable when the bike is balanced on its suspension, i.e. not fully compressed on either end. The reduction of unsprung weight allows the tyre to follow the road contour better, that is is doesn't jump over the bumps.

Still can't see spring changes due to slightly less unsprung weight.

[Edited on 2-4-2005 by Felix]

Thank you Guys,

Well, I'm glad we got that cleared up.

To give some idea of the path of my confusion -

The wheel has less mass and inertia on the compression stroke - so I would have thought more compression damping is needed to regulate the faster stroke.

Likewise, on the rebound stroke, less inertia would need more damping to regulate the stroke.

However, the as the wheels are lighter, it would seem natural that the suspension is not being "worked" so hard and would not need to be so "stiff".

I tried taking the concept to extremes - wheels with zero weight need infinite damping. No that makes no sense.

Wheels with infinite weight need no damping. This actually makes a little sense, as they would never move. Accelaration would suffer a bit, though.

I agree with the comments regard preload - but then again, I'm probably wrong.

It will surely be a case of see how it goes, and adjust by feel. At the moment, and on the road, the suspension feels like it has a little too much
compression damping.

I've been caught out before by terminal chatter under braking from high speeds on the track, and the result was not pretty.

My first track day this year is at Nogaro (not been there before) in 3 and a bit weeks. Have you seen the length of the main straight, and the hairpin that follows it? My buddies stock 03 R1 touches 170mph before braking.

So I'm a bit paranoid about about chatter, hence the questions.

Thanks to everyone for their input.

Spinoli

Sorry Skids I`m in the Felix/Rattler camp, the springs do not need to be altered to suit lighter wheels.

The spring and preload rate are affected by bike and rider weight.

This season lighter wheels will be mostly affectin rebound and compression.

You've got that the wrong way round, and Skidlids is right there. Less inertia produces less deflection over bumps, therefore you need less damping on both compression and damping, i.e. softer suspension as expressed in terms of damping.

Keep in mind, that this is pretty much a theoretical point because suspension setting are influenced by many factor. That's why most suspension related adivce is preceeded with the word "generally".

Just go ride the thing and adjust one thing at a time based on your feedback. Start with the current setting. The worst thing to do, most likely, would be to change a bunch of settings based on theoretical physics.

Thanks Felix,

You are, of course, completely correct. I'm pretty sure you are anyway.

Fully adjustable suspension should not be made accessible to muppets like myself. Still, at least I've stopped myself from random twiddling. I'll just ride the thing and try to deal with any wayward symptoms as they arise.

Hmm, there's nice hardtail XS 650 in the rag this week. If only it had shaft-drive and an automatic transmission.....