OzVMX Forum
Clubroom => Tech Talk => Topic started by: Nathan S on June 05, 2012, 09:03:26 pm
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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.
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Maybe it's just the amount of overall weight pushing the front wheel.
Or maybe the nut on the handlebars is loose. ;)
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sit back a bit more, don't weight the front so much,
and if you come off,
then try something else... ;D
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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
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sit back a bit more, don't weight the front so much,
and if you come off,
then try something else... ;D
LOL ;D
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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.
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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.
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A lot of problems at the front are caused by the back [rear suspension.p
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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. :)
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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 ;D
Sorry Nitram :)
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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 ;D 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
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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]. :D It couldn't be any worse than rallying a Volvo. ;)
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may be forks up the clamps 5mm and air caps cheap fix on soft front end
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It couldn't be any worse than rallying a Volvo.
;D ;D ;D ;D ;D ;D ;D
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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.
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Do these have cartridge forks?
Yes. Ballard and Jess Gardiner sometimes run TTR forks on their WRFs - no doubt the revalve and resping them like crazy, but they're pretty good.
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Old racer explained it in terms adjusted for me ::)...."Trouble with the front, more throttle. Trouble with the back, less throttle"
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Old racer explained it in terms adjusted for me ::)...."Trouble with the front, more throttle. Trouble with the back, less throttle"
Back in the Moorebank days in the heavy sand section the criteria generally accepted was 'power on , get the front wheel up out of it'
pancho.
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Nathan, I'm not 100% sure whether yr describing the front end tucking under (falling in) or washing out, but I think you mean tucking under.
My maths/physics teacher always used to say, "work it out from 1st principles", so here's a go (as I understand it):
As you turn the bars, you lose trail till it becomes zero then goes negative (ie lead not trail).
When you lean, it also brings into play the effect of the bike's C of G & the mass of the forks etc in front of steering centre-line (pendulum effect). Both those gravity forces work to overcome the self-centring effect of trail when you turn & lean.
That's why the wheel flops to the side when you put the bike on the side stand. Its the same effect as tucking under in turns.
The usual fix is more trail, usually via more rake & as you know there's several ways to do that - push the forks thro triples, softer rear, sit further back (a trade off w losing front grip tho of course), gas it more etc, mentioned above. Even tyre pressures can make a diff.
However one of the 1st things I'd be checking is if you can mount the big lights closer to the steering axis. As is, that extra pendulum effect will be overcoming the self-centring effect sooner as you turn & lean, making the tucking-under worse (happen sooner). Be mindful of the centre of gravity too.
Why does it happen in soft ground/sand & not on hard track? 2 things: as the tire sinks more under weight transfer in cornering, the effective dia becomes smaller so trail is less, but also, the contact area is considerably longer having another (perhaps larger) effect on shortening trail & reducing the self-centring effect. (Too-low front tire pressure has similar effect.)
Thats why its usually a good policy to sit back further & keep the power on in sand
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I'm think of another angle .... front tyre, what size is it? 3.00, 2.75, 80/90, 90/100, 90/90? Is it trail or knobby? Maybe a wider tyre might sit higher in the sand and not tuck?
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Yeh, good point - effectively increases trail in the sand
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I reckon JC and JO are on the money. I would be going for floatation and height with wide and high 90/100 on the front. Basically experiment. Drop the rear spring tension off a mic hair if there is any leeway available and up a click or two on the rear rebound too. Make sure your forks are as low as you can get them in your top triple. Definitely different riding technique for steering 2S's and 4S's in the situation you describe.
That’s how we used to set up our big bores for Stockton Dunes. We used to live on a 100 acres on the dune fringe. I could really notice the difference in my 465G on the sandy/loamy access tracks if I didn’t take the time to put my forks back down in the triples after trailing on clay based Terra fir-ma.
If you decide to try Stockton Dunes while you still can? You will need two really good rim locks on the rear. Fair dinkum, it is a blast that you will not forget :o
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Also rode a stripped down DR500s on the dunes for a while. Bloody hard work I remember, compared to the pleasure of blasting the 465G.
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Thanks for all of the input gents.
Will take it out and experiment with fork heights next weekend. Another interesting discovery was the compression adjustment on the shock was actually two clicks out from full hard >:(, which is probably a significant part of the problem.
During the week, I made new headlight mounts to bring the lights back as far as I can - got them back about 30mm.
(http://i46.photobucket.com/albums/f128/annaandnathan/Bikestuff/TTR250/160620122722.jpg)
I also removed the speedo and idiot lights.
The new footpegs:
(http://i46.photobucket.com/albums/f128/annaandnathan/Bikestuff/TTR250/160620122723.jpg)
(http://i46.photobucket.com/albums/f128/annaandnathan/Bikestuff/TTR250/160620122727.jpg)
(This peg is a bit clunkier/uglier than intended, because I built it from memory and forgot that it sat inboard of the mount and then had to modify it).
The high seat - reusing original cover is ugly but also very cheap:
(http://i46.photobucket.com/albums/f128/annaandnathan/Bikestuff/TTR250/160620122729.jpg)
The new top triple. Its from an '89 CR125. Compared to the stock TTR triple with the bars in the foward-most position, it puts the bars foward another 5mm and up by 5mm. I'd prefer another 5mm further foward, but this is still a useful help. It went straight on, except for the centre hole needing to be 0.5mm larger.
(http://i46.photobucket.com/albums/f128/annaandnathan/Bikestuff/TTR250/160620122725.jpg)
The lights:
(http://i46.photobucket.com/albums/f128/annaandnathan/Bikestuff/TTR250/160620122732.jpg)
Obviously, they're pretty bright, but the back ground of this photo (which was taken seconds earlier) shows how much the camera has compensated for the lights :) :
(http://i46.photobucket.com/albums/f128/annaandnathan/Bikestuff/TTR250/160620122730.jpg)
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Re #23 The 1st pic titled 'the new footpegs'-- That is not a crack in the weld on the frame I hope?
pancho
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No. Just the way the camera flash caught the slighly lumpy weld.
The bike has had a very easy life (until now...).
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Dakar?
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Dakar?
Maybe oil the chain first.... :'(
(http://i46.photobucket.com/albums/f128/annaandnathan/Bikestuff/TTR250/160620122723.jpg)
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I have found on my current model YZ 2 stroker that the forks come with far too much compression damping and way to much pre load.
I turn them to the softest setting used the lightest oil and cut the springs down. I can now feel the front and turn at will the bike is so much fun to ride.
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Update, if anyone cares: MASSIVE improvment in the way the bike rides.
Previously, you could go respectably fast on it, but it took a LOT of effort (both brain and physical). Now you can go faster with less effort - its so easy to ride at a decent (not amazing) pace that I'm worried I'm going to have a big crash on it... I mean, up to the point where the average suspension is an issue, you can go pretty quick with no real effort - but it gets ugly very quickly and you mind starts to wander before it gets ugly
Had fun hunting down a KTM350 (among others) this morning - but then failed to catch an '83 KDX250 too...
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What did you end up doing to get the result, was it just the head light remount?
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To be honest, I don't really know - I did the whole "change lots of things at once" trick... ::)
My gut feeling is that the headlight mounts, shock setting and triple clamp made the difference to the steering problem. The gearing, seat and footpegs are more about riding it faster, more comfortably.
Today, the new tyres and mousse tubes were fitted, along with that service I threatened (yes Ross, including lubing the chain). The top shock bearing is squeaking, so I've gotta lube that up, and I need to work out what they want me to do for mounting the number and make that happen.
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Good luck on the 24hr in a week and a bit. Lighting shouldnt be a problem as I noticed a HID ballast in your photo. Hope the fog and rain stays away for everyone n keep those hands warm. Have you done a few 24s.
Regards Troy
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Make sure you take your thermals... and a torch [in your pocket] to help you find the bike if you happen to bail in the dark...
Keep us up to date with your progress. Good luck!!!
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talking to a bloke at Morgan last weekend thats done the 24 a few times and was saying, make sure you always put in new wheel bearings, a $2 part that has undone alot of enterants