Suspension Guru required!

[Berlin]

I'm throwing this one open but its a pretty specific question.

Rear Suspension chatter when hard on the brakes! How to get rid of it?

I've set the bike up so it rides superbly (GSXR K1 Ktek fork internals, WP rear) and the settings are about as nice as I'll ever get. But under hard breaking to slow bends (hairpins) the rear chatters.

Is there a way of reducing the chatter without adjusting the riding parameters? High speed compression/rebound damping?

The Hornet chatters like a gibbon and I presume thats due to lack of rebound damping. But the Suzuki's rebound is fab as it is. I don't want to increase it any more if I don't have to.

and I'll not be fitting a slipper clutch! :D

Any ideas?

C

[yorkie_chris]

Maybe you have too much rear rebound?

[Berlin]

Strange, I was thinking too little!

C

[yorkie_chris]

So if back gets kicked up a little, too much rebound would hold it off the floor rather than it springing directly back into contact.

Remember rebound works both ways so if you back off rebound you will need to add compression to keep same low speed on that circuit.

[ThEGr33k]

Well too little rebound and it will mean that the wheel when compressed fast will extend back out too fast causing a hammer effect, or at least I visualise it like that, or with a lightened back end simply extend fast enough to have a pogo effect, throwing the back end in the air.

Think about having too much rebound and you will get it not extending fast enough which shouldn't really effect braking, in that it would simply mean the wheel loses contact with the floor momentarily. Though that happening when the brakes arnt strong enough to keep the bike endoing could cause it to crash back down. Though this would also probably mean you had issues when on the power as it would compress and then not get back in contact with the ground fast enough over bumps and would cause you to lose grip, again that's how Id visualise it.

See which one may fit what you feel... then my suggestion, play with the setting on it, but of course make sure you know how you got it at the moment so if you muck it up more you can go back to how it is now. :)

I set mine up how I like by thinking about what sensation I didnt like and visualising what might cause it, and then what might fix it. Often worked out! :)

Suspension is often hard to get right, especially riding fast like you on a track. Different people like different sensations, so testing is the only way to be certain imo.

Hope that helps some mate!

[yorkie_chris]

Only solution will be to try more rebound and see if it improves, then try less, see same.

If no improvement found anywhere then there must be some problem in valving, or maybe it is perfect and you need to buy a slipper clutch, which will break :-P

[ThEGr33k]

Only solution will be to try more rebound and see if it improves, then try less, see same.

If no improvement found anywhere then there must be some problem in valving, or maybe it is perfect and you need to buy a slipper clutch, which will break :-P


Exactly how I see it. Though too much compression over bumps could also cause issues lol :p

[Berlin]

Well then, there's an excuse to book another track day :D

As I visualise it, if something is occilating between two points (chatter), the way to stop it doing so it to dampen it (more). If it can't occilate as quickly then the frequency will change and lessen the chatter.

But on these new fangled shocks with high and low speed damping changing the high speed damping would seem the way to cancel it as chatter is a high speed movement (all be it over a small distance)

Fiddlin' time!

C

[Sid Squid]

Suspension terms are so subjective and my suggestion is based upon my understanding of your description.
But, assuming I correctly understand the problem you're having:
Is the problem apparent on all surfaces while braking hard? if so then I'd not look to damping first, I'd suspect the rear preload is too high and the suspension is topping out and unloading the tyre.

Maybe.

[rictus01]

Suspension terms are so subjective and my suggestion is based upon my understanding of your description.
But, assuming I correctly understand the problem you're having:
Is the problem apparent on all surfaces while braking hard? if so then I'd not look to damping first, I'd suspect the rear preload is too high and the suspension is topping out and unloading the tyre.

Maybe.

the solution would be ....... a pillion then.....:smt044


Cheers Mark.

[Red Herring]

I take it we're talking track riding here? If so when braking hard the rear is gong to be completely unloaded (often off the ground completely, especially into the slower corners when we're braver) so playing with the preload won't stop it topping out however it will influence the initial suspension movement which may in turn help with the chatter. Engine braking can be another cause of rear end chatter, have you tried just changing down slightly later?

[Berlin]

Mr Herring, you're very close!

I started doing that half way through the day and reduced the amount of engine braking and it cured it... and I got trounced on the brakes every time with riders coming past each time. :)

The *cause* is engine breaking

But that's a symptom that surely can be dialled out with suspension settings? chatter is chatter, whatever causes it.

and the 750, with it's Ohlins rear shock, doesn't do it.
Just the 600 (with a WP shock).

C

[Red Herring]

Are they coming past you because you're not braking so hard (perhaps because you are concentrating on the down changes) or were you really relying on the engine braking to slow you down that extra little bit, in which case are you overshooting or braking earlier?
You can try replacing engine braking with a little rear brake but I was never able to do that accurately, even when Supermoto racing where sideways is the order of the day... although some sports bike riders have found thumb brakes easier to use.
I'd be tempted to just keep trying subtle adjustments to the bike, you're bound to get somewhere eventually...

[yorkie_chris]

How long since the shock had fresh oil and gas pressure checked?

[skidmarx]

Mr Herring, you're very close!

I started doing that half way through the day and reduced the amount of engine braking and it cured it... and I got trounced on the brakes every time with riders coming past each time. :)

The *cause* is engine breaking

But that's a symptom that surely can be dialled out with suspension settings? chatter is chatter, whatever causes it.

and the 750, with it's Ohlins rear shock, doesn't do it.
Just the 600 (with a WP shock).
If you think engine breaking is important, then maybe you should try feathering the clutch when this happens to give you a bit more control over that aspect of the problem. Sounds pretty tricky to me though!
CNot sure that helps much tho.

[toadie]

I used to suffer with rear end chatter when I first started racing, tried all sorts of suspension settings to sort but nevcer really got there. The best I found was running a touch more sag than normal on the rear, If I remember correctly about 15mm of static, but to be honest the trade of on corner exit understeer wasn't worth it.

i then brought a gixxer thou from a british superstock rider and remember suffering even more so on it, when I spoke to him about it, he simply said "yeah just slip the clutch a little till it's all in line!!" eh? I thought!!!

Well I tried it on a track day and just persisted till I got it right, i have to say it works and works well......that was six years ago now and I still slip clutches going into turns, especially so on my SV.

It's simply down to the road speed and the rear wheel speed not being sychronised as you hammer down through the box, changing down later does the same trick....

Just my two penneth worth

Paul T

[ThEGr33k]

You can reduce the engine braking by putting the tick over up a bit/lot... :)

[Red Herring]

You can reduce the engine braking by putting the tick over up a bit/lot... :)

Good tip, I should have remembered that. That's one of the things we did to my Supermoto bike to stop it chattering.

[ThEGr33k]

Good tip, I should have remembered that. That's one of the things we did to my Supermoto bike to stop it chattering.


Im not just a pretty face.... Oh wait :(

lol

[Berlin]

Excellent replies!

On the Bimota 650 single, I did raise the tickover to stop that stepping out as the engine breaking that has is ridiculous But I doubt it would help as much on the IL4's

I've been using the "pulled in clutch technique" but only when things are getting seriously chattery and the bike is out of line to get it back to some sort of chance of making it round the corner :) This is effectively a manual slipper clutch and I think I need a lot more practice at it. :) I'm only using it in emergencies at the moment.

Due to my generaous proportions the sag I run on the bikes is more than it should be anyway and both technically need a stiffer rear springs. I measured both bikes last night and they are double what they should be, strangely static sag was over too so it looks like whoever bought the shocks for the bikes got them wrong in the first place. The static sag on the 750 is 3cm (should be 0.5 -1.5) Rider sag is 7.5 cm (should be 2.0-3.0). And the results are the same for the 600. This is also with them wearing race tail units with less weight than standard. However, the ride height adjusters are banked right up so the high sag is bringing the nose position back to normal when sagged this much.

Bizzarly, the Ohlins spring preload unit on the 750 is locked in a fixed position with a pin and is only adjustable by remote hydraulic unit. This, from max to min, made a "whopping" 5mm difference which is bassically useless! I need to get the shock out, remove the pin and wind it up a good way then replace it.

I don't know the shock number codes for either shock so finding out what the standard setting are is proving troublesome.

The 600 and 750 forks are bang on the money sag wise and damping wise.

C

[yorkie_chris]

Falkner make some pleasantly cheap rear springs.

[SV650Racer]

Rear sag should be set static with 8mm minimum through to 16mm if your running a race spec TTX Ohlins. Not enough sag will cause the shock to top out and create a "judder" type feeling on the brakes from the rear.

[Berlin]

Hi Had 3cm static and 7.5 rider on the day so actually about twice what it should have been.

Maybe It's my thing about braking points? I keep a list of where the top boys in whatever class of racing brake for particular corners then try and get as close to that as possible. If they can brake from there and still get around the corner... etc.

:)

C

[SV650Racer]

Woh! 30mm static sag on the rear?. How did you measure it?. Thats way too much.

[chakraist]

I would suggest a teensy bit more compression on the front coupled with what the other guys have said on the rear. Is the rear wheel coming up when you brake? Make sure your body positioning is right as well, you want to be up against the back of the seat.

[ThEGr33k]

Woh! 30mm static sag on the rear?. How did you measure it?. Thats way too much.


Its about right with someone sat on it... With rider its supposed to be between 30-40mm.

[Berlin]

Static sag measured after three bounces at a marked point on the tain unit. Unloaded sag measured with rear tyre just starting to compress with bike on side stand at same point

Rider sag measured with feet up after three bounces at same point.

From a susopension forum, it looks like the ohlins preload unit is full of air and needs bleeding. You have to take the shock off, spring off the shock and then increase the preload manually if this happens so its a big job and it'll be done over the winter.

C

[SV650Racer]

Rider sag is pretty much useless. We never ever use that to set the rear up. Static sag measured from a fixed point with the bike sat under its own weight compared to the measurement to the same fixed point with the rear shock fully unloaded - ie with the bike pulling the rear wheel off the floor - to measure the about of free movement at the top of the shock.

If the preload canister has air in it then it needs bleeding and re-gassing.

[ThEGr33k]

Rider sag is pretty much useless. We never ever use that to set the rear up. Static sag measured from a fixed point with the bike sat under its own weight compared to the measurement to the same fixed point with the rear shock fully unloaded - ie with the bike pulling the rear wheel off the floor - to measure the about of free movement at the top of the shock.



That sounds a little silly to me... Not trying to say you are wrong, from the way you talk it sounds like you know a thing or two, but here is how I see what you put;

The way you set it must mean that spring rate simply doesn't matter, (or that it is already known to be right, which lets face it isnt common). You'd effectively use your method set it up the same for me and Berlin (or any other rider using the same bike)... I can tell you that me and Berlin certainly would not want the same preload!

Also its rare id say that someone has the right spring rate and so using your method would have something that would handle less than perfect.

Rider sag takes all that problem out as you set sag to suit the whole weight of the unit (rider and bike).

Cheers

[SV650Racer]

Of course spring rate matters. BUT you cant postively tell correct spring rate from just measuring rider sag. Ok it gives you a ball park but you dont ride the bike sitting stationary on it. This particulary applies to race applications.

If you set my supersport bike using just rider sag it would show me i require a .8kg spring whereas my perfect setup in terms of my pace, riding style and general bike chassis setup requires a .9kg spring.

Ohlins work with their non ttx shock on a 8-12mm static sag range whereas the TTX works to a greater 10-18mm range.

Im not the techy geek here, this is only what ive learnt watching Steve work for the past six years. So excuse my lack of technical terms!..

[ThEGr33k]

Of course spring rate matters. BUT you cant postively tell correct spring rate from just measuring rider sag. Ok it gives you a ball park but you dont ride the bike sitting stationary on it. This particulary applies to race applications.

If you set my supersport bike using just rider sag it would show me i require a .8kg spring whereas my perfect setup in terms of my pace, riding style and general bike chassis setup requires a .9kg spring.

Ohlins work with their non ttx shock on a 8-12mm static sag range whereas the TTX works to a greater 10-18mm range.

Im not the techy geek here, this is only what ive learnt watching Steve work for the past six years. So excuse my lack of technical terms!..


Aye, I can see where it works. I think its probably a better track application when you do indeed have the right spring rate. So fair enough there :D Suppose that means ill have to check my static sag, as I have the right spring rate in there :)

By using rider on the bike sag I mean to say that you do make up a little for the spring rate being off, which would work better for a road bike where the spring is very likely to be wrong... :(

[SV650Racer]

Trouble is we see alot for the road is when a rider comes in suggesting the spring rate is wrong, unless they are either end of the weight scale its often more just plainly down to a poorly setup bike overall or the fact the shock isnt working quite as it should.

Setting up especially spring rates is often a massive compromise for the road.

[ThEGr33k]

Trouble is we see alot for the road is when a rider comes in suggesting the spring rate is wrong, unless they are either end of the weight scale its often more just plainly down to a poorly setup bike overall or the fact the shock isnt working quite as it should.

Setting up especially spring rates is often a massive compromise for the road.


Aye, fair enough there :)

Does your shop by any chance rebuild Penske race shocks?

[SV650Racer]

Aye, fair enough there :)

Does your shop by any chance rebuild Penske race shocks?

Yes we can:cool:

[ThEGr33k]

Yes we can:cool:


Cool, ill certainly ask you (for a price at least :p) then when it comes to the time :D

[yorkie_chris]

Gr33k, you not fancy doing it yourself?

Re. spring rates and stuff, surely the spring rate is the deciding factor in whether you can get the ratio of static sag and rider sag correct?

Spring Rate OK - Both free and rider sag within acceptable range.
Spring Rate too soft - Rider Sag OK, but too little or no Free Sag
Spring Rate too firm - Rider Sag OK, but too much Free Sag

Then again, I can see that with very stiff setups on a front-biased bike like a 600 that it would be hard to accurately set and measure bike sag.

[ThEGr33k]

Gr33k, you not fancy doing it yourself?



I suppose I could. But getting it re gassed may prove a problem. Mostly because I dont know where I could get it done. :rolleyes:

Not to worry, I got a few more miles before that :eek:

[Berlin]

I need it de-gassed not regassed :) The hydraulics shouldn't have any air in it.

I'd also argue that rider sag is more important that static sag as bikes don't ride themselves.

Given the choice (which I have been with my Bimota) I set the correct rider sag even though the static sag was non existant. The tail unit is too light to get any sag.

Any disparity between what rider and static sag should be and are, tells you whether the spring rate is right.

If static sag is OK and rider sag is too high (large) then the spring rate is too low. If the static sag is ok and rider sag is too low (small), then the spring rate is too high. This is where static sag is useful. But in the real world it's rider sag that should be set correctly if you have to choose between the two due to incorrect spring rate.


C

[SV650Racer]

If you have no static sag then your shock will top out as soon as it extends, ie under hard braking and will then lift the rear wheel off the floor hence causing chatter under braking.

Ask how many race teams measure rider sag!. Spring rate isnt 100% down to rider weight. Overall bike geometry, rider pace and style all are just as important.

[ThEGr33k]

If you have no static sag then your shock will top out as soon as it extends, ie under hard braking and will then lift the rear wheel off the floor hence causing chatter under braking.

Ask how many race teams measure rider sag!. Spring rate isnt 100% down to rider weight. Overall bike geometry, rider pace and style all are just as important.


But to be fair, surely anything will top out with the weight of the wheel and swing arm pulling it down and nothing to push it back. Id guess how hardh to top out depends on rebound damping. :confused:

[Berlin]

If you have no static sag then your shock will top out as soon as it extends, ie under hard braking and will then lift the rear wheel off the floor hence causing chatter under braking.

Ask how many race teams measure rider sag!. Spring rate isnt 100% down to rider weight. Overall bike geometry, rider pace and style all are just as important. I'd imagine race teels know exactly what rider sag their are running.. in wet dry and all other contitions for every track.

It won't top out if I'm running 80mm of rider sag instead of 40. It has 80mm to rise before topping out. My Bimota runs 50mm of rider sag and zero static sag because the bike only weighs 149kg and can't sag the rear with the rider sag set and that's with a spring thats too soft for me.

Some bikes run a very weight forward weight distribution compared to others. Take away the weight that the bike used to have in its tail end (tool kit, plate, indicators, all excess metal and bolts etc) when turning it into a race bike and the back end is then lighter and won't sag as much in stratic form. Therefore weight distribution, rear ride height, fork drop In stanchions) change in weight distribution etc have to be taken into account. Static sag is only a small part of the overall equasion.

I've now set up my 600 with 12mm static and 40mm rider sag so we'll see what that does to the chatter on the 28th. The 750 will be running 20mm and 75mm because I can't change the preload until stripdown.

C

[yorkie_chris]

If the preload canister has air in it then it needs bleeding and re-gassing.

Preload unit has gas in? Thought they were filled up under no pressure with just hydraulic fluid? Haven't had one apart, just curious.

[zadar]

Preload unit has gas in? Thought they were filled up under no pressure with just hydraulic fluid? Haven't had one apart, just curious.
It does not, just fluid. Remote reservoir is gas charged.

[zadar]

It won't top out if I'm running 80mm of rider sag instead of 40. It has 80mm to rise before topping out. My Bimota runs 50mm of rider sag and zero static sag because the bike only weighs 149kg and can't sag the rear with the rider sag set and that's with a spring thats too soft for me.

Well, if spring is to soft for you it will have to be preloaded to get 50mm rider sag which would make you run out of static sag. Stiffer spring with less preload would give you better numbers.
Static sag is important, more than rider sag, simply because suspension works in two ways, compression and extension.

[Red Herring]

Forgive me for being a little bit ignorant here but under the circumstances originally given by the OP surely the rear suspension is going to top out regardless of how much preload/static sag it has. In my experience when braking hard into slow corners and hairpins how hard you can brake, at least in the dry, is governed simply by the need not to loop the bike over the handlebars, in other words the rear end is skipping off the tarmac and completely unloaded. The static sag/preload is important because it will effect how the suspension will react when some weight is returned to it (which is why we thought it could cause the chatter) and certainly if you start to turn in whilst still braking it will help you control the rear, but I don't see how it will stop it topping out under braking.

[ThEGr33k]

Forgive me for being a little bit ignorant here but under the circumstances originally given by the OP surely the rear suspension is going to top out regardless of how much preload/static sag it has. In my experience when braking hard into slow corners and hairpins how hard you can brake, at least in the dry, is governed simply by the need not to loop the bike over the handlebars, in other words the rear end is skipping off the tarmac and completely unloaded. The static sag/preload is important because it will effect how the suspension will react when some weight is returned to it (which is why we thought it could cause the chatter) and certainly if you start to turn in whilst still braking it will help you control the rear, but I don't see how it will stop it topping out under braking.


Id say your right tbh Red. :)

Zadar; If you have a static sag of 8mm on a spring meant for a heavy person then surely rider sag will be stupid? I dont understand why people would set a bike for its own weight and not the riders as well... :confused:

[zadar]

you do set it up for both, static and rider but static is more important. rider can be +- more than static. ie. 5mm on rider sag will not make as big difference as 5mm on static.
if you have 5mm static and preload spring to get 0 mm static you will have no extension and top out.