Fork mods

[Yow Ling]

My bucket lives in a TZR250 1KT frame. The forks stick 80mm through the triple clamps so as well as looking a bit dorky , they may be a bit hazardous during a crash. So Im getting 70mm chopped off the top.
There are a few things that I would like to know, in stock form the forks were a bit soft, so by shortening the fork tube and shortening the spring by a similar amount, will this firm up the front as there will be less live coils in the spring?
What kind of place shortens springs properly , puts the last turn a bit tighter and grinds it flat

Thanks

[TZ350]

Hi Yow Ling we are also starting to work on our suspension, what a fag, not nearly so interesting as motors.

We are just starting to learn about suspension and know that if you chop 70mm of the top of the forks you will be reducing the air space above the damper oil. Reducing the air space will harden the forks up, the same as adding damper oil. Reducing the number of coils in the spring stiffens the spring up also.

We just cut our springs with an angle grinder (thin blade) and then grind the spring end square, works for us.

[marsheng]

You will need to re-thread the top of the forks. The spring flats are made when manufactured. Can't add later. What you can do is make a bush/sleeve to go inside the spring about 1 coil long to keep the cut end of the spring working straight.

New idea on oil level, fill to covering the working parts and add 10 mm, just in case some leaks out.

Springs will be bit harder and the air gap will be reduced.

Note, the tubes are chromed and it may be a problem cutting the new top thread.

[jasonu]

Instead of rethreading you could cut an internal circlip groove and turn up some simple ali bungs with an oring to make the seal. Some of the forks I have used have this set up.

[diesel pig]

You can get new springs made up in Christchurch, This is what we did for the forks on Black Betty when I wasn't happy with the forks action under braking. Team engineer Garth measured up the old spring rate and work out a new twin rate spring then got a local spring maker to design with a computer program the wire gauge, number of coils and so on to suit the dimensions and spring rate supplied by Garth. The resulting springs work well and I no longer need to pump up the forks with air to get the ride height I need. If you want to know more Garth has "technical debriefing"* most Friday nights in his workshop so let know if you come along Garth would be happy to explain how he did it.

*Some people think this means spinning yarns and drinking beer but since these persons tend to be female I don't think they room talk Aye men

[F5 Dave]

Well for a start there will be spacers in there that will likely be that long (Yam of that era) so you could leave the springs as is if you wanted.

Yes oil level will be raised in relation to room left. I don't think it will be untenable. I totally dissagree with teh lower the fork oil level idea. Oil gets squirted up & takes a while to get back down meaning damping will be air based a lot of the time if it is that low (rather than just some of the time in most damper rod forks).

But to make them stffer yes you can cut coils off. But you must heat & bend the last coil, then grind it. It must then be flatter than flat as sideways pressure will rub heavily against tube sidewalls. OK it will in these types of forks anyway but no point making it worse. Obviously being careful to avoid coil bind. Making a safe test jig with a sliding rod & pressing the spring will confirm this travel, leave some safety room. Have to measure the inner fork to see how much room the springs have when installed minus fork travel.

Jason's circlip idea has merit, my old RZ350 forks were held together like this (maybe the TZR ones are too), but I'd want to use the same tight fitting plug & properly machined groove.


Here was a post I wrote many moons ago on another site cut & pasted. Example was dirtbike soft springs. You can measure spring rate with a press, a tape measure, & bathroom scales. just be careful, use a rod to keep the spring straight & a piece of pipe over the press ram to stop the spring shooting sideways.


Basically the less active coils (all the ones that aren’t touching at either end) a spring has the higher (stiffer) the spring rate. (Bend your ruler at far ends then move your hands closer & it gets harder right?).

So how much to cut off?

(Old spring rate x active coils)/desired spring rate.

That gives me the number of coils I want left. In my case I had .3 springs & 33 active coils & I wanted .33 springs. (works with pounds as well). This = 30 coils so I cut off 3 from the original 33. (Usually the numbers aren’t this round, with rear springs be more accurate).

I just used a disc grinder to lop the coils off & then GENTLY using a bit of heat from a handheld propane torch or whatever bend the cut part down to touch the lower coil & dress it up to look like it did before you cut it.

The springs measured 495mm before & 440 after, so we have to make up the difference to keep the preload the same. The std spacer was 85mm so a new one 140mm was cut from a piece of 34mmID (37OD) plumbing pipe. (OK so I spent $2 so sue me!)

Job done.

You can stiffen Front springs, Rear springs, even clutch springs, as long as you make up a spacer to retain the preload. Just beware if you are going to lop off quite a bit -that there is still room so the spring doesn’t coil bind.

[jasonu]

Shit Dave, aren't you supposed to be working???

[F5 Dave]

I am. I can operate this fission buzz saw and WAP on my Blackberry as well as eat can't I?

[marsheng]

Jason's circlip idea has merit, my old RZ350 forks were held together like this (maybe the TZR ones are too), but I'd want to use the same tight fitting plug & properly machined groove.



The circlip has to take the full impact of the front wheel. The wheel compresses the spring and the spring is stopped by the top plug, so make sure it is good.

[Kickaha]

Jason's circlip idea has merit, my old RZ350 forks were held together like this (maybe the TZR ones are too), but I'd want to use the same tight fitting plug & properly machined groove.


The XJ550 was the same, it really sucks trying to put them back together by yourself though

[Yow Ling]

Thanks guys, I was hoping that I could shorten the spring. I like taking the easy route.
I think the forks are only chrome on the outside, Ill see when I pick them up on Friday if the man is grumpy or going on about broken inserts. Ill post up some sesults when I have them.
I was thinkig for a test jig I could just assemble the fork without oil or a bottom bung and compress them to say 60mm on the scales, little less messy than tubes bars and needing 3 arms to work everything at the same time

[F5 Dave]

The XJ550 was the same, it really sucks trying to put them back together by yourself though
Yeah I found the best way was to put them back into the clamps with the frame propped up so not compressing the forks. I then had an old GSthou piston that I pushed down with my chest on an extension bar, flick the circlip in or out, wasn't too hard, left both hands free.

How is shortening the spring the easy route? Oh you mean reducing the travel? Well you can do that (have to do that) by putting a spacer under the damper rod and shortening the preload spacer, but you wouldn't want to have less than 90mm stroke, remembering that using the hydralic stops in normal use brings issues as it freezes the wheel for too long.

[jasonu]

The circlip has to take the full impact of the front wheel. The wheel compresses the spring and the spring is stopped by the top plug, so make sure it is good.

It does work. Many manufacturers do it as it is cheaper than threading and it works. The bung has a shoulder at the top that protrudes above the circlip so it can't let go.

[F5 Dave]

Crucial piece of information I omitted, its been a while but makes total sense & I can remember it now you mention it. That is why you have to push the bung down to expose the clips.

[Yow Ling]

How is shortening the spring the easy route? Oh you mean reducing the travel? Well you can do that (have to do that) by putting a spacer under the damper rod and shortening the preload spacer, but you wouldn't want to have less than 90mm stroke, remembering that using the hydralic stops in normal use brings issues as it freezes the wheel for too long.

Im not trying to reduce the fork travel, just the extra length sticking up above the top triple clamp. The other thing Im hoping to achieve is firm up the forks as they are hopelessly soft. Under hard brakes into the hairpin the forks are fully compressed and mechanicly solid (really compressed) so the handling is a serious problem. by chopping 70 mm off the top of the fork leg and a similar but to be finally worked out amount from the spring, I hope to shorten the fork and increase the spring rate all in one go

[Bren_chch]

Im not trying to reduce the fork travel, just the extra length sticking up above the top triple clamp. The other thing Im hoping to achieve is firm up the forks as they are hopelessly soft. Under hard brakes into the hairpin the forks are fully compressed and mechanicly solid (really compressed) so the handling is a serious problem. by chopping 70 mm off the top of the fork leg and a similar but to be finally worked out amount from the spring, I hope to shorten the fork and increase the spring rate all in one go

And then you can do my set! :D

[F5 Dave]

Well treat them separately. So you cut off 70mm & then have to remachine the fork tops. That's ok. If you cut 70mm off the springs they would doubtlessly coilbind + be way too stiff.

As I said there will be a spacer in there probably about 100mm long. Then cut off say 10% or maybe 15% of the active coils from the springs (after carefully checking that you won't coil bind whilst preloaded & compressed). I cut off starting with any close wound coils first.

Then measure how much shorter the spring is, lets say 30mm.
Then that 100mm spacer - 70(from the fork tube)
+ 30 (removed from spring)
should be cut to 60mm as a good starting point.

Then try it. I'd stick to small changes as you will also get the increase in air spring from the relatively higher oil level. To that end one will need to do a sensibility check to see that the oil level will not be way too small. You may get away with lowering the level slightly, or you may decide to only cut 50mm off.

[Yow Ling]

Ok made some progress, my engineer guy has cut to top 70mm off and recut the threads inside the forktubes. Now I need to collect the data here and reassemble forks and figure out the perfect springs. Not much eh?

Here is the BSB guy, hes sorting the MotoFXRs for this season, beats endless BSB testing I guess !!

[Yow Ling]

Time for some action on this job, we got racing in 2 weeks !
After shortening the fork tubes I wanted to start with about a 10% increase in stiffness, before I cut anything I put a full spring (unmolested ) into a fork assembly and measured it on the test rig , pic 1. I made about 6 measurments and they averaged out at .50kg/mm. I just applied some weight measured the peak then measured the forktravel with a zip tie.

Ok now calculate the number of turns to remove about 2 1/2, just used an angle grinder with a skinny cut off blade. Heat the ends to reshape the flat end , linish it kinda flat , then back to the test rig.

I made 3 measurments and the average was .54kg/mm so I got around 8% increase, close enough to try out, now i need to mess with the oil levels and fit to my bike

[F5 Dave]

So presumably you ground the ends flat so the spring is as flat as it was before. Then you will check preload is ~ 25-30% rider on board. Sounds promising.