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trialguy

Ty 175 Fork Swap And Now Terrible Steering

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Just took my heavily modified 78 175 out for its maiden run yesterday. Thrilled with how it runs and performs now but the chassis performance has gone all wrong.

Hoping to get a new gameplan to make it work like it should. Steering has become incredibly heavy and floppy from side to side. I've lengthened the swingarm as per standard 1.5 inches +/-, lowered and moved rearward the pegs and moved top shock mounts forward about 2", running 13.4" betors. These mods are proven successful by many others already.  Where it goes wrong is in the forks and if I hadn't already given away the wimpy (but working geometry wise) standard forks, I'd just put it back to stock...

 

The forks on it now are 1980 xr200 35mm forks with leading axle and damper rods shortened to give 7" travel and a decent spring rate.  These forks use clamps that have the tubes running parallel with the ty headstock (28 degree's) and the trail is achieved assumably by the leading axle design. I thought this was going to work great because the trail is about the same as stock (3.5") and the effective fork angle is now steeper than stock by about 3 degrees.   should have worked I thought but in reality the front end feels incredibly heavy and all wrong. Not to mention but the lower clamp hits the frame and hinders greatly the lock to lock steering which I knew going in but could correct by relieving the clamp and or frame for clearance. That of course does nothing for my obvious geometry issue. I've powdercoated the frame and don't want to get into radical change like head angle altering etc. Anybody willing to suggest a doable solution would be greatly appreciated. Idea's I have are pre mono ty250 complete fork and clamp set, bigger tubes/ stiffer/sturdier etc. but with no improvement in steering etc. over stock or perhaps non leading axle 35mm fork legs in my current honda triple clamps (parallel with ty head angle) Or...?

 

Please Help!!

 

 

PS  Thought I should make comment on the positive things that I've done on this bike which might be of help to others out there so here they are.

Motor seems to work brilliantly, great feedback from a couple riders (experts with vast history in trials both vintage and modern)

68mm overbore dt/mx piston,(contrary to some forum opinion, kdx 220 69mm piston is NOT a workable option for a bigger displacement ty)  mild porting as per post on this site, 1/4 reed block spacer plate and boyesen reeds, flywheel lightened by 18 ounces (from 6 pounds 2.5 ounces stock) and a self made carbon fibre and alloy rear muffler mated to stock ty stuff otherwise. Tried boring my stock ty 22mm carb to just under 24mm as suggested as possible by others... carb now junk as you can see light up the sides of the slide when looking through new larger bore. (the lesson here is that the dimension of the throat/bore of a round slide carb must be smaller (just) than the vertical bore (where slide goes) or else you have major air leak and a ruined carb. So, rather than buy a new carb, I thought I'd see if the flatside mikuni vm 28 carb that worked very well on a 89 beta tr34 that I owned 20 years ago would work. I thought I'd be in for a whole mess of fiddling and jetting etc. if it even happened to work in the first place. Common wisdom would say that a 28mm carb on a 181cc ty, is way to big. That said, I restricted the carb with an aluminum sleeve, installed as a friction fit into the engine side throat of the carb, thereby reducing the carb's venturi to a more appropriate 24mm. I'm happy to say that the motor works tremendously and I have no complaints with it. Very responsive, no bog or hesitation, lots of power and sounds great! (carbon muffler)  (Carb jetting in case useful for anyone is 180rd main,15 pilot and 2.5 slide with middle clip position on needle)  Gearing of 11/50 with 428 setup seems great.Other things of note are a homemade carbon fibre front fender and fork brace, using my gas gas stuff as the mold/pattern and homemade footpeg brackets featuring raga alloy footpegs and a slick little detent pin to hold right peg out of kickstarters way when starting.  I'll attempt to post some pics of some of these items at some point soon.

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Sound like the trail is wrong. Go with a set of ty250 triple trees and ty250 forks. Fixed mine. I to tried a set set of triple trees that were parallel to the headstock the bike felt like I was falling off the front end and sloppy side to side just terrible put the ty250 set up on it much better. Also use the upper triple tree that moved the handlebars in front of the pivot point..

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A couple more details important to the case of my ty project and heavy steering. 1) remeasured my trail and it seems to be 4.75" +/- and 2) The triple clamps although being in line top and bottom and mirroring stock ty head angle, have very little offset. About 12-13 mm (half inch) is how far forward of the steering axis the fork tubes are. centre to centre.

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Too much steering trail and you have made the steering rake too great by making the front of the bike too high.

The trail might come down enough to make it feel OK with that front end if you can get the rake back to being a bit steeper than standard TY175 (slide the fork tubes up higher through the clamps)

 

Rake is measured at the steering head not the fork tubes

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Unfortunately I've tried sliding the forks up in the clamps a further inch and no improvement. It feels so ponderous that I thought that under load the headstock bearings were binding possibly but they turn out to be fine.

I realize that rake is measured at the head but with clamps being parallel with steering head (unlike stock ty clamps) the angle of the tubes is naturally the same as the steering head in this case.  Thanks for your inputs, I'll get it right eventually

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It needs a lot more than an inch extra fork tube protruding through the top clamp to get the rake right for those triple clamps and axle offset

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have a look at the bike that the front end came off and you need to have less rake than it has

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Can't keep sliding tubes up in clamps as much as might be helpful because of the fender/tire coming into hard contact with the lower yoke as the fork approaches it's bottomed state.(running out of room)

With regard to kdx220 piston, yes-bore and pin diameter are right but crown is about 1.5mm higher than ty and would likely hit head at tdc, not such a big deal to machine it lower as there's a fair bit of "meat" on that crown but the bigger hitch in using this piston is the major difference in the rear skirt. KDX has one smallish, round "window" dead centre while yam piston uses two rectangular "windows" for fuel charge transfer.  It may have worked but I wasn't willing to take that gamble, thereby ruining a good cylinder by boring it to a size that no further piston options are available.

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I'm having similar issues with a Honda TLR, lts got 35mm marzzochi forks in at present but was getting strange steering issues when using Fantic yokes

I have hopefully resolved it with hybrid parallel yokes honda offset from steering stem centre to fork tube centre but Fantic fork tube spacing.

How are you measuring trail? It's a strange measurement just about everything affects it except the colour of your underpants.

If I'm about to teach you to suck eggs ignore the next bit!

It is measured by projecting the centreline of the steering head to the floor. ( flat and level ). Then project a vertical line through your front spindle again to the ground.

The difference between the two lines on the floor is your trail measurement.

A common mistake to make is to use the centre of the fork leg. The fork leg is almost always in front of the steering head and on production yokes not even parallel to it.

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The guys are correct saying too much trail. Honda trail/enduro bikes of the era using 21" front wheel typically had 51mm total offset between axle & steering axis. A TY twinshock has 70-72mm. Its this offset, causing trail, that gives the self-steering effect, ie when you lean, it steers into the corner.

 

However the amount of offset you need for that effect is very different at slow speed to that for high speed. What works for hi speed feels very floppy at lowspeed, ie it wants to flop into the turn too far with just a little lean. And conversely what is good for low speed feels very twitchy/unstable at hi speed. Its always a compromise

 

Using other 35mm fork sliders without offset axle in those same Honda triples will make the floppy steering infinitely worse.There's really nothing you can do with those Hon forks/triples that will make it work, short of reducing rake to about 22deg! (which of course will stuff others things - wheelbase, weight distribution etc). Or getting the triples machined to 'kick out' the forks a couple of degrees, thereby increasing axle offset & reducing trail.

 

 

I'd be taking up your option of fitting the TY250 triples & forks. That'll cure it. Or if you still have your Ty175 triples they can be reamed out to take the 34mm TY250 forks. (Same triples) Another option is TL250 forks & triples but they are longer than TY forks so you'll have to compensate. They're 33mm tubes. Probably simpler to stick with TY ones.

 

There is a good thread on understanding steering for trials here. http://www.trials.com.au/forum/viewtopic.php?f=16&t=9348

It basically summarizes what the gurus (eg Tony Foale) said in print, after much experimentation, & applies it to twinshock trials.

(Just to complicate matters, trail also caues the self-centering effect which is dealt with early in the thread. The self-steering effect is dealt with at top of pg2)

Edited by jc2
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I remember talking to Mick Andrews about leading axle ( boring the std yokes to fit 35 mm TY Mono fork )  on my TY TS and the first thing he told me is that I would have to make the steering angle 1 degree steeper.

 

Guy

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Wow, that's quite a read (the Foale piece) thank you to all.  One thing for sure, it seems that above all I need to lose the XR yokes and fit something with a lot more offset, reduce the trail significantly. Anyone have an idea how much one would have to lengthen the top frame tube to affect a 1 to 2 degree change in rake?

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about 25mm on a TY175 frame, if you make the bend at the front engine mount.

If you are not confident, just draw a scale diagram

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Here is a quick test you can try to see if reduced trail will improve your bike's steering.  Take off the front wheel and swap the fork tubes.  Put the axle mounts behind the fork tubes.  Remount the wheel.    I had a 1971 Kawasaki Bighorn that had forks that allowed you to change the axle position.  Moving the axle back made it steer more like a trials bike.   Google it or look here.  You could also raise the tubes in the triple clamps to get back to the same height, because the axle will now be lower. 

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