Understeering issues...
10-31-2010, 11:58 AM
#1
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Understeering issues...
So i went down this weekend and messed the bike up a little, but not mechanically at all. Im fine just scratched my elbow and tore up my one piece suit. I was doin about 35mph and was hanging off to the left in a tight curve and felt the front end tuck real bad, then caught traction and did a tank slapper and thats when for some reason I let go in fear of a highside and tumbled off the road while the bike stayed upright and slowed down until it fell over. Ghost riding I like to call it lol. I have felt the front end tuck like that a few times when I push it hard and lean in deep. Just put the 1000rr front end on and have a steering damper on it and had the bikes suspension setup for me. What could cause it to want to dive like that when it is leaned in so far?
10-31-2010, 02:57 PM
#3
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Well the 1000rr forks would not need a fork brace.
I can't speak for your situation, but I know with mine I'd get some wheel chatter when deeply leaned because ~1/4 turn too much damping was dialed in by Ted Rich's tech guy when I had the aztrackday people setup the suspension for me.
I can't speak for your situation, but I know with mine I'd get some wheel chatter when deeply leaned because ~1/4 turn too much damping was dialed in by Ted Rich's tech guy when I had the aztrackday people setup the suspension for me.
11-01-2010, 07:23 AM
#6
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having a damper only works if you have it set tight enough, it does/should make you work at tip in but the reward is having the bike track straight.
tim
tim
Last edited by trinc; 11-01-2010 at 11:02 AM.
11-01-2010, 07:53 AM
#7
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Understeer is the tendency to run wide in a corner.
How is the bike set up?
Rear ride height (compared to stock)?
Front ride height (compared to stock)?
How does your fork length compare to stock?
Are you usig a gull wing top triple clamp?
Are you braking in the corner?
Did you have any heat in the tires (ie had you been in the twisties or was this a single corner)?
What was the temperature?
How is the bike set up?
Rear ride height (compared to stock)?
Front ride height (compared to stock)?
How does your fork length compare to stock?
Are you usig a gull wing top triple clamp?
Are you braking in the corner?
Did you have any heat in the tires (ie had you been in the twisties or was this a single corner)?
What was the temperature?
11-01-2010, 10:05 AM
#8
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No braking used in the corner at all, just engine braking. Using little throttle in the apex and that is where it happened. I never really set the damper very tight as it feels harder to turn in initially. Tires fully warmed up as I had been riding twisties all day. Ride height is lower than stock. 1000rr forks are shorter and took 10mm out of rear to match close to it. Non gullwing type upper tripple. Temperature was about 65 degrees and sunny. The front end tucked three times that day and caused it to get real shakey in the front. Tire pressure was 31 front and 30 rear on Pilot powers. I haven't had it on the track since I did the front end and braced the swingarm, but I didnt exect all these to be a factor in riding on the street in average weather.
11-01-2010, 10:58 AM
#9
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I'm guessing these will likely be your problem. Loose steering head bearings might do it too. Old tires as well. I've never liked the profile of Pilot Powers, they tip the bike in for you and sometimes that can mask a problem. Personally from what you've described it seems like your front is too high.
11-01-2010, 11:05 AM
#10
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To get it safe you need a gullwinged upper if you are using 1000RR forks.
Even with my 954 forks I have to have them to get it to ride right. (but my forks are pushed up a few mm over the gullwing, I'd expect that yours wouldn't be)
11-01-2010, 11:06 AM
#11
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I'm guessing these will likely be your problem. Loose steering head bearings might do it too. Old tires as well. I've never liked the profile of Pilot Powers, they tip the bike in for you and sometimes that can mask a problem. Personally from what you've described it seems like your front is too high.
11-01-2010, 11:19 AM
#12
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Understeer is the tendency to run wide in a corner.
How is the bike set up?
Rear ride height (compared to stock)?
Front ride height (compared to stock)?
How does your fork length compare to stock?
Are you usig a gull wing top triple clamp?
Are you braking in the corner?
Did you have any heat in the tires (ie had you been in the twisties or was this a single corner)?
What was the temperature?
How is the bike set up?
Rear ride height (compared to stock)?
Front ride height (compared to stock)?
How does your fork length compare to stock?
Are you usig a gull wing top triple clamp?
Are you braking in the corner?
Did you have any heat in the tires (ie had you been in the twisties or was this a single corner)?
What was the temperature?
all good questions above i would add how the front & rear suspension is matched as far as proper spring rates & valving ?
the little things like having more input not just to initiate the turn but to keep the bike running wide and adding throttle at the apex may be enough for a street tire like the PP. + i think the PP works better at 34-36psi COLD for street ride.
tim
11-01-2010, 12:16 PM
#13
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The ride height using CBR 1000RR forks (the 954/929 forks are a few mm longer) with a gullwing 954/929 top triple is 24 mm with them set so that the line for the top cap is level with the top of the triple... 20 mm shorter if flush... I'm not exactly sure how big the difference is with a non gullwing, but I'd say roughly another 20-25 mm... That's 40-45 mm shorter than the stock fork... I'd say that's borderline dangerous in terms of geometry...
Taking 10 mm of the rear... Well... Is that 10 mm from stock? Is it 10 mm at the shock or overall rideheight?
I'm not sure it's actually helping or making things worse... But I'd say take it very, very easy and get yourself a 929/954 upper gullwinged triple...
I'm fairly certain you can stop looking at the other options... No suspension setup in the world is going to make that geometrical problem go away...
Taking 10 mm of the rear... Well... Is that 10 mm from stock? Is it 10 mm at the shock or overall rideheight?
I'm not sure it's actually helping or making things worse... But I'd say take it very, very easy and get yourself a 929/954 upper gullwinged triple...
I'm fairly certain you can stop looking at the other options... No suspension setup in the world is going to make that geometrical problem go away...
Last edited by Tweety; 11-01-2010 at 12:21 PM.
11-01-2010, 12:55 PM
#16
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A gullwing triple is stepped in height making it not completely flat... Moves the clamping place lower gaining fork "length"... Have look at pictures and it will be clear... Ny eplanation leaves a bit to desire...
11-01-2010, 02:03 PM
#17
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Not a big expert here on geometry but by my experience I would agree the front is too low. Dropping my front 10mm on stock forks hits the sweet spot perfectly. Might be better to only go 8mm. Any further and tucking seems certain! While the bike does not need a damper in stock form, drop the front and it becomes downright necessary. Right now riding on loose steering head bearings and unmodified suspension (6' 200lbs) with the springs cranked down as far as they will go and a fork brace on. The Scotts damper is sitting on the other bike along with race-tech front end and Dougherty upgraded rear shock. It is kind of fun flopping the bike around at it's limits as set-up, but will surely be better with good steering head bearings and suspension. Diablo Rosso Corsa tires don't look as cool as PPs, but they work a lot better on the sides and are far more neutral at all angles! Seems to me going to inverted forks and braced swingarm might just take some of the fun out of twisting this thing around the tight back roads here along the RI/CT border!
11-01-2010, 03:09 PM
#18
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i understand jumping on the geometry but it's odd that his complaint would be understeering... pushing.
if it's off that far, & i did play around with it, it would be twitchy. i wouldn't expect someone to say that it's understeering.
tim
if it's off that far, & i did play around with it, it would be twitchy. i wouldn't expect someone to say that it's understeering.
tim
Last edited by trinc; 11-01-2010 at 04:25 PM.
11-01-2010, 03:34 PM
#19
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I agree. It sounds like it is just dangerously unstable because of too little rake and trail. The fact that he said it felt like it was going to tuck several times before it actually did, in addition to serving as a warning for him to stop riding (which was ignored, apparently), confirms it.
11-01-2010, 04:20 PM
#20
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Wow didnt expect to have so many answers in every direction. Im also not thrilled about peoples assumtions about how they think that my setup is dangerous. The bike is not dangerously unstable as I have been riding the bike hard for the last month and have only had this happen a few times. Only under extreme lean angles have I felt the front end tuck like that. I have an rc51 upper tripple with the top of the forks flush with top of tripple . So by gaining that little in fork length with a gullwing upper tripple will cause it to be more geometrically stable?
11-01-2010, 05:04 PM
#21
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Look at how much of your front tire is being utilized. Despite removing the 
strips from the rear tire the front was hardly scuffed. Lowering the front moved things into the perfect spot on the sticky part of the Pirellis, with just scuffs to the edge. The bike turns much better but stability is at the edge with the tc moved down 10 mm and stock suspension/loose steering head. Just a hint of tucking once or twice but nothing heart pounding. Use the bars and throttle while leaned over and can see where the damper would be really nice right now! Will see with the more robust setup from Jaime & Race Tech when the weather turns colder, but i might miss the squirming and shaking!
strips from the rear tire the front was hardly scuffed. Lowering the front moved things into the perfect spot on the sticky part of the Pirellis, with just scuffs to the edge. The bike turns much better but stability is at the edge with the tc moved down 10 mm and stock suspension/loose steering head. Just a hint of tucking once or twice but nothing heart pounding. Use the bars and throttle while leaned over and can see where the damper would be really nice right now! Will see with the more robust setup from Jaime & Race Tech when the weather turns colder, but i might miss the squirming and shaking!
11-01-2010, 05:29 PM
#22
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If you have dropped the nose of the bike by 40 mm overall, that could very well be the problem.
I'm not sure what would cause a traction problem, but you are exceding your avalable traction. That's not really the problem we're trying to solve. The problem is, you are losing the front without warning.
The problem with steepening your head angle (decreasing your rake angle) that much is that you have lost a lot of your trail. Trail will cause the handlebar to rotate when the tire starts to slip - as Tim was saying. This gives you feedback about what the front end is doing. With too little trail, you don't get the feedback and by the time you know it's sliding, it's too late. If you want to run the head angle that steep, you need to decrease your triple clamp offset.
I don't know for sure that's the problem. But that's my guess.
I'm not sure what would cause a traction problem, but you are exceding your avalable traction. That's not really the problem we're trying to solve. The problem is, you are losing the front without warning.
The problem with steepening your head angle (decreasing your rake angle) that much is that you have lost a lot of your trail. Trail will cause the handlebar to rotate when the tire starts to slip - as Tim was saying. This gives you feedback about what the front end is doing. With too little trail, you don't get the feedback and by the time you know it's sliding, it's too late. If you want to run the head angle that steep, you need to decrease your triple clamp offset.
I don't know for sure that's the problem. But that's my guess.
11-01-2010, 06:25 PM
#24
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RCVTR...That was very good information and I feel like that is the closest to my problem about having to little trail. I feel like i get no feedback from the front until the front wheel does slide out and then grab traction again because my initial reaction is to shut off the gas and stand the bike up. I never measured how big a difference it is, but, like Tweety said, it has to be over 40mm shorter than the stock forks where. So I guess the only way to gain any length in the front would be to get a gullwinged upper.
RIPI...What in the Hell are you talking about? haha
RIPI...What in the Hell are you talking about? haha
11-01-2010, 09:21 PM
#25
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there's also a potential issue with the torsional rigidity of the chassis. The VTR chassis is very soft in torsion, compared to a race chassis. The race VTRs had to be heavily braced.
when the bike is leaned way over, torsional flex in the chassis becomes a major component in your suspension. It acts to keep the tires loaded.
I don't know all of the dynamics, but the VTR chassis may not react well to very steep head angles. This may be part of the underlying traction problem.
I think if you raise the front, you'll make both problems less severe.
when the bike is leaned way over, torsional flex in the chassis becomes a major component in your suspension. It acts to keep the tires loaded.
I don't know all of the dynamics, but the VTR chassis may not react well to very steep head angles. This may be part of the underlying traction problem.
I think if you raise the front, you'll make both problems less severe.
11-01-2010, 09:25 PM
#26
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it's hard to make comments on feedback or make any suggestions because i think that the PP tires themselves give very little to start with.
it would be interesting if you ran a sticky tire with the same setup if you'd
make the same comment about the tire just letting go. i ran a set of PP's and took them off after just a couple hundred miles, of course i've been running pirelli SC2's & SC1's so my requirements or what i'm used to getting is on the other end of the spectrum.
a sticky tire won't cure the understeer though.
tim
it would be interesting if you ran a sticky tire with the same setup if you'd
make the same comment about the tire just letting go. i ran a set of PP's and took them off after just a couple hundred miles, of course i've been running pirelli SC2's & SC1's so my requirements or what i'm used to getting is on the other end of the spectrum.
a sticky tire won't cure the understeer though.
tim
11-01-2010, 09:34 PM
#27
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there's also a potential issue with the torsional rigidity of the chassis. The VTR chassis is very soft in torsion, compared to a race chassis. The race VTRs had to be heavily braced.
when the bike is leaned way over, torsional flex in the chassis becomes a major component in your suspension. It acts to keep the tires loaded.
I don't know all of the dynamics, but the VTR chassis may not react well to very steep head angles. This may be part of the underlying traction problem.
I think if you raise the front, you'll make both problems less severe.
when the bike is leaned way over, torsional flex in the chassis becomes a major component in your suspension. It acts to keep the tires loaded.
I don't know all of the dynamics, but the VTR chassis may not react well to very steep head angles. This may be part of the underlying traction problem.
I think if you raise the front, you'll make both problems less severe.
tim
11-02-2010, 05:43 AM
#28
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No braking used in the corner at all, just engine braking. Using little throttle in the apex and that is where it happened. I never really set the damper very tight as it feels harder to turn in initially. Tires fully warmed up as I had been riding twisties all day. Ride height is lower than stock. 1000rr forks are shorter and took 10mm out of rear to match close to it. Non gullwing type upper tripple. Temperature was about 65 degrees and sunny. The front end tucked three times that day and caused it to get real shakey in the front. Tire pressure was 31 front and 30 rear on Pilot powers. I haven't had it on the track since I did the front end and braced the swingarm, but I didnt exect all these to be a factor in riding on the street in average weather.
When you add even a little throttle leaned at the apex considering these adverse circumstances, this would only exacerbate any suspension problems causing front end slip. If for example, your forks springs have no rebound damping adjusted in, this could serve to unload the front tire to the point of losing contact patch when throttle is added at full lean, and even more so with the heavier sprung cbr1000rr forks you've added.
Not trying to be critical in a negative way but maybe food for thought hopefully taken in a constructive spirit, if even applicable at all. The truth be known, i've been sliding and bouncing down the road on at least 10 occasions and twice on the track, so I'm hardly a master of control. haha
The one superhawk tuck I had(stock suspension)was a an 80-90mph right hander when I added a little throttle at full lean, thus unloading the front fork springs followed by the front tire. But there was no shaking or other instability, just like the front tire hit ice, maybe the result of higher speed than your git-off; hard to say. Now i keep the front brakes applied while adding throttle to keep the front springs from unloading too abruptly when accelerating out of the apex, and this holds the contact patch for better front tire grip.
11-02-2010, 07:36 AM
#29
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there's also a potential issue with the torsional rigidity of the chassis. The VTR chassis is very soft in torsion, compared to a race chassis. The race VTRs had to be heavily braced.
when the bike is leaned way over, torsional flex in the chassis becomes a major component in your suspension. It acts to keep the tires loaded.
I don't know all of the dynamics, but the VTR chassis may not react well to very steep head angles. This may be part of the underlying traction problem.
I think if you raise the front, you'll make both problems less severe.
when the bike is leaned way over, torsional flex in the chassis becomes a major component in your suspension. It acts to keep the tires loaded.
I don't know all of the dynamics, but the VTR chassis may not react well to very steep head angles. This may be part of the underlying traction problem.
I think if you raise the front, you'll make both problems less severe.
11-02-2010, 08:23 AM
#30
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i hope you do experiment with the damper. feedback from the front tire is so important when you start increasing speed & lean angle. as you clamp down the damper you'll increase that feedback.
tim
tim
