Suspension Question - Instability Under Hard Braking

[khp]

Ever since I got my 'new' YZF-R6 2008, I've been experience a bit of rear end instability while braking - something that I didn't experience with my old R6 (a 2001 model).

 

The old bike had the original Yamaha shock and front fork, but with slightly harder springs (Öhlins, 0.95kg/mm) to accommodate the 87kg of fleshy bits on top of it. And it didn't have a steering damper either. In short, pretty much stock conditions.

 

The new bike, on the other hand, is decked out with Öhlins everywhere: Öhlins YA789 TTX36 rear shock, Öhlins FGK137 Front fork kit, and Öhlins steering (Yes, the swede I bought the bike from was very patriotic ).

 

What I experience is that if I brake medium hard to very hard, the rear end feels like it is squirming around. Enough to distract me, but not in any way that it feels dangerous. If I brake hard enough to lift the rear tyre, the squirming feeling goes away (had to do that when a guy cut across my front tyre a one point).

 

So far I have put zip ties around the front fork legs to verify that the forks don't bottom out (check - there's about an inch of travel to spare) and tried to turn out the rebound damping on the rear end two clicks (from 8 to 6 out from minimum) to see if it made a difference - It didn't.

 

The rear shock is length adjustable, but it's at the standard (~minimum) length. What I can see is that the front fork has been lowered 10mm through the triple-clamps (the service manual says "flush mounted"). I checked out the bike of a guy who's racing at the national level here in Denmark, and they have also lowered the front by 10mm on his bike (but he's probably 6" shorter and maybe 15kgs lighter than me ).

 

I'm starting to wonder if the lowered front is making all the trouble, but since I'm quite a newbie when it comes to dialling in suspensions, I'd appreciate any input.

 

As I'm going to a local track that I know well for 3 days this weekend, so I should have plenty of time to experiment with any suggestions.

 

Thanks,

 

 

Kai

[Bullet]

Ever since I got my 'new' YZF-R6 2008, I've been experience a bit of rear end instability while braking - something that I didn't experience with my old R6 (a 2001 model).

 

The old bike had the original Yamaha shock and front fork, but with slightly harder springs (Öhlins, 0.95kg/mm) to accommodate the 87kg of fleshy bits on top of it. And it didn't have a steering damper either. In short, pretty much stock conditions.

 

The new bike, on the other hand, is decked out with Öhlins everywhere: Öhlins YA789 TTX36 rear shock, Öhlins FGK137 Front fork kit, and Öhlins steering (Yes, the swede I bought the bike from was very patriotic ).

 

What I experience is that if I brake medium hard to very hard, the rear end feels like it is squirming around. Enough to distract me, but not in any way that it feels dangerous. If I brake hard enough to lift the rear tyre, the squirming feeling goes away (had to do that when a guy cut across my front tyre a one point).

 

So far I have put zip ties around the front fork legs to verify that the forks don't bottom out (check - there's about an inch of travel to spare) and tried to turn out the rebound damping on the rear end two clicks (from 8 to 6 out from minimum) to see if it made a difference - It didn't.

 

The rear shock is length adjustable, but it's at the standard (~minimum) length. What I can see is that the front fork has been lowered 10mm through the triple-clamps (the service manual says "flush mounted"). I checked out the bike of a guy who's racing at the national level here in Denmark, and they have also lowered the front by 10mm on his bike (but he's probably 6" shorter and maybe 15kgs lighter than me ).

 

I'm starting to wonder if the lowered front is making all the trouble, but since I'm quite a newbie when it comes to dialling in suspensions, I'd appreciate any input.

 

As I'm going to a local track that I know well for 3 days this weekend, so I should have plenty of time to experiment with any suggestions.

 

Thanks,

 

 

Kai

 

HI Kai,

 

Do you have your sag set correctly? Static and with you on the bike, are you getting much compliance?

 

Bullet

[khp]

Do you have your sag set correctly? Static and with you on the bike, are you getting much compliance?

Hi Bullet,

 

Yes, I had help to measure out and adjust the static sag last year, but didn't write down the details and frankly .... I can't remember the numbers :-|

Are you thinking too little sag on the rear?

 

Kai

[Bullet]

Do you have your sag set correctly? Static and with you on the bike, are you getting much compliance?

Hi Bullet,

 

Yes, I had help to measure out and adjust the static sag last year, but didn't write down the details and frankly .... I can't remember the numbers :-|

Are you thinking too little sag on the rear?

 

Kai

 

Its definitely possible mate, yeah. I found on my Aprilia that with too much pre-load in the rear, it caused a few problems, one of which was this. As it's so easy (relatively), to check, I'd start there, and make sure you've got some good sag numbers, then, if not that, check you've not got compression wound on to much (in the rear).

 

Let us know, though others may well have other ideas.

 

Bullet

[fossilfuel]

Do you have your sag set correctly? Static and with you on the bike, are you getting much compliance?

Hi Bullet,

 

Yes, I had help to measure out and adjust the static sag last year, but didn't write down the details and frankly .... I can't remember the numbers :-|

Are you thinking too little sag on the rear?

 

Kai

 

Its definitely possible mate, yeah. I found on my Aprilia that with too much pre-load in the rear, it caused a few problems, one of which was this. As it's so easy (relatively), to check, I'd start there, and make sure you've got some good sag numbers, then, if not that, check you've not got compression wound on to much (in the rear).

 

Let us know, though others may well have other ideas.

 

Bullet

 

I think you should do two things if everything else is exceptable. First, do what Bullet says. I would increase rebound damping in the rear. you went with less from 8 to 6 clicks. Did you ever think that you might be raising the rear to fast? I would try going to 10 to 12 clicks on the rear to keep it from raising up to fast. Then I would increase compression damping in the front about 2 to 4 clicks to slow down the rate of travel under breaking. You never mentioned what your front fork settings were?

[khp]

I think you should do two things if everything else is exceptable. First, do what Bullet says. I would increase rebound damping in the rear. you went with less from 8 to 6 clicks. Did you ever think that you might be raising the rear to fast? I would try going to 10 to 12 clicks on the rear to keep it from raising up to fast. Then I would increase compression damping in the front about 2 to 4 clicks to slow down the rate of travel under breaking. You never mentioned what your front fork settings were?

 

OK, first I need to correct myself: I have 7mm travel left on the front, not 25 (I confused the numbers from by roadbike, which seems to be heavily oversprung on the front. But that's a separate topic).

 

Here's the hard data:

Front: 12 clicks compression / 8 clicks rebound / 10 turns of preload. There are 20 clicks of compression and rebound, and 16 turns of preload.

Rear: 10 clicks compression / 6 clicks rebound / 5 turns of preload. There are 22 clicks of compression and rebound, and 16 turns of preload.

 

The reasoning for reducing the rebound damping was to allow the rear to extend quicker and thereby keeping a better contact with the road. Bullet's point is that it might be that the rear shock is topping out, in which case rebound has no effect at all.

 

Fossil, what is the logic for increasing the rebound damping on the rear in this case?

 

So far my plan says: 1) check sag, 2) reduce preload on the rear (-2), 3) increase the preload (+2) and/or compression damping in the front (I'm thinking preload as 3a, compression as 3b).

 

Thanks,

 

 

Kai

[fossilfuel]

I think you should do two things if everything else is exceptable. First, do what Bullet says. I would increase rebound damping in the rear. you went with less from 8 to 6 clicks. Did you ever think that you might be raising the rear to fast? I would try going to 10 to 12 clicks on the rear to keep it from raising up to fast. Then I would increase compression damping in the front about 2 to 4 clicks to slow down the rate of travel under breaking. You never mentioned what your front fork settings were?

 

OK, first I need to correct myself: I have 7mm travel left on the front, not 25 (I confused the numbers from by roadbike, which seems to be heavily oversprung on the front. But that's a separate topic).

 

Here's the hard data:

Front: 12 clicks compression / 8 clicks rebound / 10 turns of preload. There are 20 clicks of compression and rebound, and 16 turns of preload.

Rear: 10 clicks compression / 6 clicks rebound / 5 turns of preload. There are 22 clicks of compression and rebound, and 16 turns of preload.

 

The reasoning for reducing the rebound damping was to allow the rear to extend quicker and thereby keeping a better contact with the road. Bullet's point is that it might be that the rear shock is topping out, in which case rebound has no effect at all.

 

Fossil, what is the logic for increasing the rebound damping on the rear in this case?

 

So far my plan says: 1) check sag, 2) reduce preload on the rear (-2), 3) increase the preload (+2) and/or compression damping in the front (I'm thinking preload as 3a, compression as 3b).

 

Thanks,

 

 

Kai

Kai,

I was thinking of what they call the "pogo" effect. This is when you do not have enough damping for the shock to do its job. The spring is compressed while you are riding but as you apply brakes to the front, the down force on the front and shift of weight snaps the back up. If you dampen the compression on the front another 2 clicks and slow the rate of the forks compressing while damping the rate of rebound or pogo in the rear to 10 clicks, the transfer of weight to the front might be reduced....just a thought. You said you reduced rebound another two clicks and nothing happened? You are way down on the amount of rebound damping which makes me believe you don't have enough...

has the rear end felt a little bouncy or squirmy while riding over bumps or on the throttle on exit?

[Kevin Kane]

I was thinking of what they call the "pogo" effect. This is when you do not have enough damping for the shock to do its job. The spring is compressed while you are riding but as you apply brakes to the front, the down force on the front and shift of weight snaps the back up.

If you dampen the compression on the front another 2 clicks and slow the rate of the forks compressing while damping the rate of rebound or pogo in the rear to 10 clicks, the transfer of weight to the front might be reduced....just a thought. You said you reduced rebound another two clicks and nothing happened? You are way down on the amount of rebound damping which makes me believe you don't have enough...

has the rear end felt a little bouncy or squirmy while riding over bumps or on the throttle on exit?

Fossil;

When I started to experience the "pogo" effect about six or seven years ago, I assumed that the springs were too stiff for me (previous owner had 50+ lbs on me) so I swapped out the stiffer springs for the OEM and the problem was gone. Too bad I didn't realize that if I adjusted the settings I might have saved a few bucks and taken advantage of these springs as my experience and track confidence grew.

 

...You learn something everyday.

Thanks Fossil;

 

Rain

[ozfireblade]

I think you should do two things if everything else is exceptable. First, do what Bullet says. I would increase rebound damping in the rear. you went with less from 8 to 6 clicks. Did you ever think that you might be raising the rear to fast? I would try going to 10 to 12 clicks on the rear to keep it from raising up to fast. Then I would increase compression damping in the front about 2 to 4 clicks to slow down the rate of travel under breaking. You never mentioned what your front fork settings were?

 

OK, first I need to correct myself: I have 7mm travel left on the front, not 25 (I confused the numbers from by roadbike, which seems to be heavily oversprung on the front. But that's a separate topic).

 

Here's the hard data:

Front: 12 clicks compression / 8 clicks rebound / 10 turns of preload. There are 20 clicks of compression and rebound, and 16 turns of preload.

Rear: 10 clicks compression / 6 clicks rebound / 5 turns of preload. There are 22 clicks of compression and rebound, and 16 turns of preload.

 

The reasoning for reducing the rebound damping was to allow the rear to extend quicker and thereby keeping a better contact with the road. Bullet's point is that it might be that the rear shock is topping out, in which case rebound has no effect at all.

 

Fossil, what is the logic for increasing the rebound damping on the rear in this case?

 

So far my plan says: 1) check sag, 2) reduce preload on the rear (-2), 3) increase the preload (+2) and/or compression damping in the front (I'm thinking preload as 3a, compression as 3b).

 

Thanks,

 

 

Kai

Kai,

I was thinking of what they call the "pogo" effect. This is when you do not have enough damping for the shock to do its job. The spring is compressed while you are riding but as you apply brakes to the front, the down force on the front and shift of weight snaps the back up. If you dampen the compression on the front another 2 clicks and slow the rate of the forks compressing while damping the rate of rebound or pogo in the rear to 10 clicks, the transfer of weight to the front might be reduced....just a thought. You said you reduced rebound another two clicks and nothing happened? You are way down on the amount of rebound damping which makes me believe you don't have enough...

has the rear end felt a little bouncy or squirmy while riding over bumps or on the throttle on exit?

 

 

I get the exact problem in one particular turn every single time and it feels like the arse end is just like a pogo stick...now i know how to fix it

 

thanks Foss

[khp]

Fossil, what is the logic for increasing the rebound damping on the rear in this case?

I was thinking of what they call the "pogo" effect. This is when you do not have enough damping for the shock to do its job. The spring is compressed while you are riding but as you apply brakes to the front, the down force on the front and shift of weight snaps the back up. If you dampen the compression on the front another 2 clicks and slow the rate of the forks compressing while damping the rate of rebound or pogo in the rear to 10 clicks, the transfer of weight to the front might be reduced....just a thought. You said you reduced rebound another two clicks and nothing happened? You are way down on the amount of rebound damping which makes me believe you don't have enough...

has the rear end felt a little bouncy or squirmy while riding over bumps or on the throttle on exit?

 

Well, not bouncy and squirmy enough for me to notice it. I'll watch out for it.

 

 

Kai

[johnnyrod]

Has the new bike got a slipper clutch?

[khp]

Has the new bike got a slipper clutch?

Officially, yes. But I've never felt it kicking in, and when I asked a Yamaha dealer about this, his reply was that it didn't really work and if I wanted a slipper clutch, I should get an aftermarket unit.

[Gorecki]

From my limited experience and learning some cold hard facts the hard way, as Dave Moss would say, "If it's a new to you bike, you MUST go trough the suspension completely, you have no idea what's actually going on in there!".

 

Even with the very cool Ohlins bits, I would tend to believe no one can say they know it's functioning properly for certain. Some things may give hints like adjusting compression and rebound and having zero effect suggests the valving is hosed. The previous owner may believe their's a .95kg set of springs in the front when they're really a 1.05kg and if the new rider ways 50lbs less, that will be a serious difference. 50lbs rider weight difference alone is ~0.075kg fork spring and ~0.75 shock spring difference as a baseline.

 

I just this last weekend was breaking down my forks to do the seals when to my shock discovered the springs were completely shot! I mean they were ~40mm below minimal tolerance, meaning there was absolutely NO preload on them at all or could be based on their length. So now, I'm waiting for springs.

 

Just trying to share what I've learned in my misadventures.

[fossilfuel]

From my limited experience and learning some cold hard facts the hard way, as Dave Moss would say, "If it's a new to you bike, you MUST go trough the suspension completely, you have no idea what's actually going on in there!".

 

Even with the very cool Ohlins bits, I would tend to believe no one can say they know it's functioning properly for certain. Some things may give hints like adjusting compression and rebound and having zero effect suggests the valving is hosed. The previous owner may believe their's a .95kg set of springs in the front when they're really a 1.05kg and if the new rider ways 50lbs less, that will be a serious difference. 50lbs rider weight difference alone is ~0.075kg fork spring and ~0.75 shock spring difference as a baseline.

 

I just this last weekend was breaking down my forks to do the seals when to my shock discovered the springs were completely shot! I mean they were ~40mm below minimal tolerance, meaning there was absolutely NO preload on them at all or could be based on their length. So now, I'm waiting for springs.

 

Just trying to share what I've learned in my misadventures.

Gorecki is correct...What if you don't have the correct amount of nitrogen in the shock or not enough or any fork oil...Opening everything up and knowing what you have with fresh oil and seals is a great start... I race so I change my oil and replace the seals every season.

[khp]

Gorecki is correct...What if you don't have the correct amount of nitrogen in the shock or not enough or any fork oil...Opening everything up and knowing what you have with fresh oil and seals is a great start... I race so I change my oil and replace the seals every season.

I appreciate your concerns and hard-earned experiences. I already have checked the oils levels in the front fork last year (had to fix a leaking seal in the left leg anyway), so that end should be OK. I looked have verified that the shock spring is the standard stiffness. Wait, let me rephrase that: I have verified that the NUMBERS on the spring at the same as on the standard spring. YMMV.

 

I've found a local suspension guy who is getting rave reviews from racers. As soon as he's back from vacation (should be next week), I'll call him up to have him go over the rear shock and give some recommendations for the setting up the suspension in general (including my next little issue).

 

As for the grand scheme and plan to fix the squirming rear: I found I had bigger fish to fry, than a squirming rear

 

Until two weeks ago, I was running Pirelli Diablo Supercorsa SC2's on both the front and the rear. As my final SC2 rear was shot, I replaced it with a Dragon Supercorsa Pro SC0, which I got a good deal on. Notice the differences: Diablo <> Dragon and SC2 <> SC0. SC0 is a soft, while an SC2 is a hard tyre. The Diablo's are also supposedly a better tyre (it's definitely more expensive).

"OK, so the SC0 is softer, so it'll just go a little quicker, but that's OK since they're a lot cheaper" was my thinking (the rear SC2's have lasted 5-6 trackdays each). - Bzzzzzt! wrong, but thank you for playing!

 

As it turned out, I was tearing through the SC0 within 1 full trackday, to the point where it would slide from applying the power (kind of cool to feel it very controlled, but a tad bit expensive in tyres!).

 

I had a bit of help from a very friendly Swede, who suggested that I decreased the compression on the rear since he had previously had the same kind of tyre-shredding problem while riding motocross. I tried it and found that if I went from 8 to 6 and then to 4 clicks of compression, the tyre would stop tearing up (actually, the wear pattern started looking quite nice again at the end of the 2nd day), but it would still slide when pinning the throttle out on the straight

 

It dawned on me that previously the suspension was set up relatively hard, for a medium/hard rear tyre. But with a soft rear tyre and a suspension with hard compression, the tyre was trying to handle the full load, instead of "sharing" the compression load under acceleration. So in order to make the best with a soft tyre, I had to soften up the suspension too, to make then work better together. Or at least that was my take-away from the problem & the solution to it.

 

Oh, I did try to loosen up the preload on the rear and adding a bit of compression in the front, but the rear still squirms under braking (maybe not as much now, but I didn't pay too much attention to it). I'll take a bit of squirming over a sliding rear tyre any day.

[jasonbw]

Hey KHP, you sure the SC0 is just 'soft' ? Our SC0's (Pir SBK slicks in Oz - diff tyre but Pirelli would prob maintain a standard rating?) are qualifiers only.. on a warm day they'll give you one or two sessions only

[khp]

Hey KHP, you sure the SC0 is just 'soft' ? Our SC0's (Pir SBK slicks in Oz - diff tyre but Pirelli would prob maintain a standard rating?) are qualifiers only.. on a warm day they'll give you one or two sessions only

Hi Jason,

 

I actually tried to find that data on Pirelli's webpage (read: glitz-marketing-###-only-containing-superficial-information-page) before posting, but couldn't find anything about the SCx/SPx terminology. So, yes it could be "supersoft" as well. YMMV.

 

But at least I wont do that mistake again

[acebobby]

I use dragon supercorsa pro's, an SC1 front and an SC2 compound on the rear, as far as I'm led to believe the SC1 is soft and SC2 is medium which would lead me to believe that a SC0 would be supersoft and a sc3 would be hard!

Anyway thanks for bringing this question to the forum, discussions about suspension setup are rarely talked about and its good to hear peoples opinions on setup, what I would say is start taking notes of your settings and the feelings your experiencing, and change only one thing at a time, noting what you are feeling etc.

When you see the suspension guys at the track, understand and know why their making certain changes, they can be helpfull but in alot of cases their charging the uneducated £xxx to set their sag and set the bike to settle in less than a second, a good base setting really, but you want to personalise it from there, this is where you need to educate yourself, no two guys are looking for the same thing!

Another thing worth thinking about is you can end up chasing a setup issue, waste alot of track time on something that might not even gain you any time or even make the bike react in a different way in another corner!

[khp]

OK, I've fixed the squirming rear end!

I noticed at one point while braking deep into huge a 180 degree left-hander, that the front end was shaking a bit from the braking. This led me take off the forks and recheck everything - oil levels, preload, compression and rebound. Seems like that there was a slight difference in oil, and the turd who set the preload last time (read: me) had set one to 10 turns and the other to 12 turns!

 

So, the oil levels were reset to the same level, the preload was set to 10 turns on both legs: lo and behold, the squirming has gone (I did have to reduce the compression 4 clicks on the front, but that's because of the higher oil level caused it to be very harsh).

 

I've taken off the rear shocks off both the track and the street bikes and will have them refreshed with new oil & nitrogen next week. Hopefully this should also get me closer to understanding why the new SC2 rear tyre is getting worn from just 1½ days of riding, when much faster riders don't get the same wear marks with 2 x 20min races on the tyres.

[Gorecki]

OK, I've fixed the squirming rear end!

 

So, the oil levels were reset to the same level, the preload was set to 10 turns on both legs: lo and behold, the squirming has gone (I did have to reduce the compression 4 clicks on the front, but that's because of the higher oil level caused it to be very harsh).

 

snip...why the new SC2 rear tyre is getting worn from just 1½ days of riding, when much faster riders don't get the same wear marks with 2 x 20min races on the tyres.

 

OUTSTANDING! You found it!

 

That tire ware is a tell tale of the rear suspension's not giving in to the tarmac and the tire is taking the beating, so sounds like you've got everything in total control...cool!