Speed Wobble

[binge]

Since fitting my new forks on my pit bike, I am experiencing speed wobble at around 65 - 70mph.

I'm not sure what is causing it. It wasn't doing it with my old forks, BUT the old forks had very tight headstock bearings. So much that when the front wheel was suspended off the floor, it was quite "clunky" to turn the bars from left to right.

I think this may have acted as kind of a steering damper with the old forks.

Now my new forks are nice and smooth left to right. It is wobbling. The forks are also longer than my old ones, However, I have dropped the yokes by 2" to compensate for this.


It sounds stupid for a pit bike, But would a steering damper be my only option of cure? Obviously these bikes weren't designed to do 70mph.



Ben

[TUG]

Damper would work in theory, can you not get the headstock any tighter? Spose some bar end weights would be out of the question to?
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Haz ER-5, innit!

[element]

wider bars might help too..
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04' DT125RE (run-a-bout) // 89' DT125R (rebuilt + supermoto conversion)
SPARES FOR SALE. 2RK TZR // 82 DR125 // DT125R/E .... PM ME...

[binge]

I am running 32" Renthal Play mini bars. They are Gigantic.
The headstock is done up as tight as It will go before it starts chewing the bearings up really. Any tighter and the bearings will need changing in a month or so.

Bar end weights, Would they work?



Ben

[TUG]

[binge]

I never had any on my NC29. I had flush fit Oxford bar ends which were weightless.

Might give them a shot. Just think they might look a bit gay.



Ben

[TUG]

[MarJay]

[G]

Bar end weights are designed to dampen high frequency vibrations as far as I know, not speed-wobble.

A steering damper may well help, but it may just help rather than solve.

If it's not just the loss of damping affect from the bearings, then we can presume it's to do with different characteristics of the forks - maybe the rebound is working a bit differently.

[TUG]

Dont know then, sorry if i'm incorrect but the bike was wobbling without bar ends, but with them was very stable.
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Haz ER-5, innit!

[lllN30lll]

if it's high speed wobble, wouldn't dropping your forks reduce the wheelbase and make it even worse?
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Turbo R1
CRF450R

[iooi]

You got the forks straight ?
Are the new one's bent in anyway ?

Perhaps trying altering the amount you have dropped them and see if it changes anything.
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Just because my bike was A DIVVY, does not mean i am......

[Kickstart]

Hi

Tight steering head bearings normally make stability worse. However one main reason for this is that when the bearings are stiff you land up over correcting all the time.

A full on tank slapper is largely down to the forks flapping from side to side at a natural frequency. Weights on the bars can change this (for better or worse), but not their primary purpose.

I would go with Marjay and say to change how much the forks are dropped through the yokes. 2" is a long way to have dropped the yokes down the forks. Also check the mounting of the wheel (ie, was the axle in front of the forks on the old set).

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[MaybeGuy]

unbalanced wheel?
it is a tiddler after all... lost any of the weights on it?
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Blue_SV650S wrote: it was a sh1te wheelie, but it proves that he can get it up in 3rd and can do angles. In summery, mattsprattuk is a gobby little sh1tebag, dopehead tw4t, but sadly for all of us, he probably isn't THAT full of sh1te!!
Kickstart wrote: Hi I tend to agree with Matt. All the best Keith

[G]

[MarJay]

[Kickstart]

Hi

Looking at the side on views of the bike in the alloy bracket thread there seem to be a fair few differences between the old forks and the new ones.

Also remember that the forks springing / damping is likely to be very different. Entirely possible that when you sit on the bike the new forks are compressing rather more than the old ones, reducing stability even more.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[G]

Sorry, I should have specified dropped the yokes, which is what I was thinking at least .

Also what N30 meant I presume, as that would slightly reduce wheelbase.

[lllN30lll]

I was meant to say yokes
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Turbo R1
CRF450R

[binge]

Yes. The old forks were 660mm long, And the axle bolt was smack bang across the bottom of the fork legs.

The new forks are more like "Big bike" forks. The axle is set forward by about 10mm, It has pinch bolts like my NC29 did.

These new forks also have and adjustment on the top of each leg. One leg has a "Fast - Slow" knob on it, And the other has a "Soft - Hard" knob it on.

The reason the new legs are longer is because they are designed to be run on a KLX110, which has a 6.25" headstock. Now my bike only has a 5.5" headstock, so I got the shorter headstock bolt when buying the forks. And obviously I needed to move the yokes down the legs a bit otherwise it would be like riding a Harley. And lets face it, That would be ghay.


I will have a tinker tomorrow, Will raise the yokes back up by 10mm, Balance the front wheel and go from there.


My old forks had 20 weight oil in them, And a set of 1/2 spacers above the springs. I did this because they were too soft when I bought them.


My new forks, I have no idea if there is even oil in them! I've heard some of the high end pit bike forks (Fast ace, Marzocchi, Fox, Elka, etc) are Nitrogen filled.

The new forks seem to be very adjustable though.


Maybe the suspension and Geo needs setting up properly. I've not really set anything up yet. Just set it so it bounces down evenly when I bounce up and down on the foot pegs.




Ben

[Kickstart]

Hi

If the steering stem is meant to be 0.75" longer then dropping the forks 2" is a hell of a lot.

Suspect the Fast / Slow knob is to adjust the springing while Soft / Hard is the damping. Guess the forks are set up with one doing springing and the other damping.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[binge]

Yea one leg adjusts one, and the other leg adjusts the other.

No idea what which is though. Rebound, Preload?




Ben

[Kickstart]

Hi

Suspect it has no seperate compression and rebound damping.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[binge]

All I know is each fork leg has a valve so I can pump them up with air. Got a Fast Ace suspension pump.

The one leg has a fast and slow adjustment on it.

The other leg has a hard and soft adjustment on it.

No idea what each adjustment is called though.


Im also not sure on tyre pressures either. I always stick 30 front and rear. I cant see it being that important in the speed wobble factor mind.





Ben

[G]

I'm not quite sure how it works, but set-forward front wheels tend to be used on trail bikes, while supermotos tend to get standard ones in line (and also tend to be more stable).
Negative caster, or whatever, but a while since I looked into it all.

Suspect this could be part of the problem.