need help reducing chronic understeer

Gday folks,

On the weekend just gone, I competed in a round of the ASBK (national superbikes). normally Im just a club racer, but being around a higher caliber of rider meant i was able to lift my game and drop my lap times, however, Im running into issues with the bikes handling.

I seem to be able to comfortabley make the initial turn in ok, but when i then try to get the bike down on the apex it pushes wide, pushing on the inside bar only results in the front pushing more. the problem gets worse with higher fuel loads and new slicks. the bike finishes the corners ok.

the zip tie on the fork tells me i have no travel left, so i've been chasing that, gone from 1.05 to 1.1kg with 18 turns of preload. running 25mm carts with 155 air gap.

A suspension tech from one of the teams i was racing against kindly gave me some time over the weekend to help me, (mind you his rider was breaking lap records whilst winning by massive margins, so he had a bit of time lol)
he suggested i go to 1.2kg springs and reduce oil height to 165. his rider is 55kg and using 1.15kg springs. im 90kg.

I dont have reason to doubt him, but it flies in the face of all i've known before. so im unsure of what to do? add oil height to what i got? change to stiffer springs? change geometry?

My gut says i need to add oil height. But im just a hack lol! The bike seems to work ok in the faster more flowing corners compared to the stop start corners.

Currently the bike is an 08 gsxr 1000 25mm ohlins carts with 1.1kg springs 155 air gap, caps showing, rear is a ttx mk1 with 115 spring 317 eye to eye

cheers.joe.

 

i glazed over a lot of this post, but in most cases oil height controls bottoming (if the valving is in ballpark and compression/reb is adjusted in a typical range)... however, what is the min air gap you can run ? you may be fairly close already ?

 

ohlins install manual says 240-140 is the range

 

i assume that is measured with the spacer that has the rubber bump stops at each end out ?

 

yes, thats correct

 

in these situations, i added 5mm (5ml) of oil at a time till i got it where i wanted.. it's way easier to add, than take out.

 

Always a gamble trying to cure a handling problem by keyboard.

That said, it almost sounds like the bike is squatting in the rear and causing the problem.
If you add oil height in the fork, it will reduce fork travel. I don't know how that is supposed to help turn in?

Forget about his riders settings, maybe he needs that much spring since he's such a monster on the brakes.

Seems backwards, but make sure your shock isn't the problem.

 

I think if he puts michelins on the bike.. all the issues will go away.

 

Well shit, everyone knows that!

 

is the problem worse on or off the brakes?

less airgap increases the total springrate a little bit, mostly towards the bottom of the stroke. if u are cornering at the bottom of the stroke, more oil will raise that up. but its hard to tell if that is actually your problem.

 

Its very hard to figure this stuff out online, but I'll take a stab. The thing about using a zip tie on the fork is that you have no idea when or where it hits bottom, only that it does.
After turning in and while you're trying to hit the apex the fork better not be bottomed out or your helmet will likely be filling with dirt shortly after.

If it turns in ok and finishes the corner ok, there are a lot of possible things to adjust and knobs that could be twiddled, so finding the right one or combination is almost impossible without being there.
Or it may be as simple as you adjusting your body position and or riding style to hit the apex better. Lucky for you this is a very cheap and reasonably easy thing to try.

That said, if you have 18 turns of preload and are still bottoming the forks, you may want to get stiffer springs and or add some oil. What do you have for sag Fr and Rr?

 

55 kg (121 lbs) rider running 1.15 kg springs? Damn, that seems quite stiff. I remember reading the test on Mladin's K9 SBK and they said he was running 1.1 kg/mm springs and I believe he weighed in the 160 lbs range.

I'm assuming you weigh decently more than 55 kg, you hadn't mentioned that above. Also how is the front and rear ride height looking. You mentioned switching to slicks. Most guys I recall on the 07 and later GSXR 1000's were using fork extenders and raising the front and rear on taller slicks to get the proper turn in. Also how does the bike feel past the apex when getting back on the gas? Are you able to finish the corner properly or are you normally having to deal with it pushing wide on exit too?

 

good point Tortuga about the preload.

18 turns is 18mm right? with some unknown amount of installed preload, id venture to say thats too much preload. long topout springs could make it really hard to tell over the internet. IMO, 10mm total preload is a good starting spot for stiff topout springs.

 

Worked for me this weekend. :up: Thanks Greg.

 

you are welcome Sir

 

Understood Re: zip tie.

pics show me about mid travel on the fork in most corners, but the pics are also in corners where generally the front is not loaded as much (still need massive inputs to the bars to steer the bike --- ive torn the grip putting that much force into them!)

currently im seeing just 26mm of sag on front and 24mm of sag on rear, this is why im concerned about going to stiffer springs, as im normally used to seeing 34mm sag on front 23mm sag rear on previous bikes.

I understand sag is just a number, but I thought it was a usefull tool to put you in the ball park. maybe i'll get more sag running stiffer springs with bugger all preload installed. oh and im 90kg btw.

thanks for the input

 

why did u keep adding preload until u only had 26mm of front sag? what issue did that extra preload fix?

 

the track has big braking sectors teamed with big bumps.
T1 is 240 entry down to 160 entry, constant deceleration to 90 for turn 2
T4 is 220kmph on aproach down to 80 entry.
T6 is 160kmph down to 70
turn 9 is 270 down to 150 entry over lots of bumps

Ive always run slicks for the past 5 years now. the problems are more apparent when a new rear slick, as it pushes the front harder.

the bike is manageable past the apex, but I do have to wait longer compared to others to really get back on the throttle, so that suggests im still dealing with getting the bike to finish the corner

 

I didn't have any stiffer springs in the box to put in the bike, and so based on the advice of the suspension guy the only option I had was to either add oil or add preload.

he said for the track I should only use extra oil height as a last resort as it will reduce travel on corner entry bumps. so try adding preload.

it did help the settle the bike on braking over the big bumps, but the understeer still remained.

 

these are a couple snaps, hope it helps.

I appreciate the feedback guys. good suspension guys are few and far between around here and up until now ive been self taught via people like youselves willing to help out
I understand its hard to cure issues over the net, but im not looking for the magic bullet. just more ideas and difference of opinion. Im willing and happy to try things to see what does what, but I don't want to be chasing my tail not making any further improvements over the next few race meets. I don't really do trackdays, so I kinda do all my adjustments and tests throughout sprint races at racemeets


from left to right..
turn 3 entry, accelerating up hill - *should* be tighter and on the throttle here, but I have to close the throttle and wait till im on a better line, I lose lots of time here
mid turn 3
exit of turn 2
turn 3 entry