Steering issues.

[Nathan S]

Sorry, not VMX, but hopefully the brains trust here will be able to help.

I've got an '03 TTR250 that I'm riding in the SA 24 hour trial next month. Its stock except for some stiffer fork springs and the big lights.

Overall, I'm quite pleasantly surprised with it, except for the way the front wheel knifes into loose dirt/sand. You'd tip it into a corner and it would just want to keep turning in - then you'd have to use lots of body english and back off the throttle to hold it upright.
Its fine on harder ground, but bloody difficult and tiring to ride fast(ish) in the loose stuff.
I rode three other bikes on the same track and didn't have a drama with any of them, so I figure its not a technique problem.

Any suggestions on the source of the problem, or how to fix it?

FWIW, the handlebars are mounted a fairly long way back (centre of the bars is roughly in line with the steering axis) - this is the more foward of the two options. I'd like to move them further foward to give myself a bit more room, but won't if it makes the steering worse.

[Curly3]

Maybe it's just the amount of overall weight pushing the front wheel.
Or maybe the nut on the handlebars is loose.

[vandy010]

sit back a bit more, don't weight the front so much,
and if you come off,
then try something else...

[Nitram]

Hi Nathan,
Careful with those heavier fork springs.  Myself and a mate did about a million dirt miles on TTRs a couple of years ago.  We had a big (20litre ?) Acerbis tanks and we carried a fair bit of gear.  The forks are pretty ordinary but I never felt the need to get heavier springs.  The stock back shock, on the other hand is pure junk and is desperate for a re-valving job. 

I then got a DR650 for dirt touring, and the forks seemed way too soft-bottoming out on the hard landings with a full load on etc, so I got some heavier ones of the recommended rate.  With the heavier springs the front didn't bottom out, but the steering turned to shite.  It seemed to understeer and then fall into the corner on loose surfaces, a bit like you describe.  I'm pretty confident that because of the heavier springs, the back is squatting more than the front in the bends, causing the steering geometry to go strange (i.e. front raked out).  My mate's DR with stock springs steers fine..............except that it bottoms out occasionally.

My philosophy with front ends is that they should be as soft as possible, without bottoming out in whatever conditions you are riding in. i.e. if you're not going over jumps, then it doesn't matter if it bottoms out over jumps.  Know what I mean ?  Unless your original TTR springs are fully sagged, I'd whack them back in and see how it steers then.

Oh and also, keep an eye on the steering head bearings in the TTR.  Mine was feeling a bit weird at one stage. I didn't notice that there was a notch in the head bearings until one day I had the front wheel off-then it was pretty obvious, and replacing them worked wonders for the steering in loose stuff.

Good luck in the 24hr trial by the way.  Sounds like fun.  Make sure to do a write up and let us know how you go.

Cheers,

Martin

[Simo63]

sit back a bit more, don't weight the front so much,
and if you come off,
then try something else...

LOL 

[RED ALERT]

Have you tried playing with different fork tube heights in the clamps? If it oversteers (knifes in) slide them down a little.
 If it understeers (washes out) slide em up.

[Montynut]

Nathan you mentioned the big lights. This extra weight ahead of the steering head axis would seriously contribute to the effect you describe. Maybe look at the rear suspension sag setting (introduce more rider static sag) this should compensate for the over centre effect that the extra lighting is causing on the frontend.

[paco]

A lot of problems at the front are caused by the back [rear suspension.p

[Nathan S]

OK, thanks for the input so far...

Remembering that I'm ~20kgs heavier than Nitram:

Front.
In standard form, the front sagged a lot - made the bike very twitchy and impossible to ride at more than sedate trail pace. Stock springs are 0.42s, and I've got 0.46s. The sags are now good.
Forks feel quite good on compression - soaks up the little stuff well, but didn't bottom noticably in the bush (and only very mildly on the MX track).
It rebounds a bit too quickly, but I'm quite happy with the forks overall.

Rear.
The rear sags are fine with the stock spring - its a lot stiffer than the front, presumably so that the bike can cope with a pillion. The rear doesn't squat much under power, but I had been assuming that was due to the lack of power?
Had to wind a lot more rebound dampening into the rear shock after it tried to kill me on some moderate whoops. I think its 6 turns out on the rebound (of 22 possibilities). Compression adjuster isn't very sensitive - its set just to the hard side of the middle, after both extremes were obviously wrong, and no mid-range setting was obviously better than other mid-range settings.

Lights.
The lights are only marginally heavier than stock, but they do sit a lot further away from the triple clamps than stock. Obviously this increases the pendulum effect - their weight is noticable once the bike is knifing in, but I don't think they're the cause of the problem.

Back to the brains trust.

[TM BILL]

OK, thanks for the input so far...

Remembering that I'm ~20kgs heavier than Nitram:

Geez Nitram must be a big boy if your only 20 kgs heavier than him 

Sorry Nitram 

[Lozza]

I've got an '03 TTR250 that I'm riding in the SA 24 hour trial next month.

Any suggestions on the source of the problem, or how to fix it?

Have you no shame?  yes get a proper bike I'd go with Nitram's diagnosis when you chop the throttle weight goes to the front giving you steering, like a front wheel drive car.
A quick check of the laden weight bias would show neutral or rearwards bias. Short term fix would be to push the forks through 5mm at a time 

[vmx42]

Typical 4-stroke traillie...  they usually have short wheelbases and a high centre of gravity.

In the soft stuff, try to brake when you are travelling straight - your weight centred [or back slightly] and knees gripping the tank.

As you initiate the turn, make sure you have the throttle open - the compression braking [in combination with soft trail bike forks] will always make the front end knife under with the throttle closed. Slow in - fast out is the best way in the loose stuff.

Just keep the throttle open, rear wheel spinning and you should be sweet... you have to try to flow, not stop and start

You can try raising the idle speed as well to minimise the the compression braking.

I reckon by the end of the 24 hours it will be second nature, and you will wonder what you were worried about [or it could just drive you nuts].    It couldn't be any worse than rallying a Volvo. 

[bazza]

may be forks up the clamps 5mm and air caps cheap fix on soft front end

[firko]

It couldn't be any worse than rallying a Volvo.

[brent j]

Do these have cartridge forks?
If not PD valves or Emulators will help as they will provide more compression damping. You can up the preload on the emulator spring and that will increase the low speed comp damping.
Worked on my XT500's.