TY250 Fork Improvement

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TriCub
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Re: TY250 Fork Improvement

Postby TriCub » Sun Jun 29, 2014 4:16 pm

John.
I have been helping a few guy's lately with classic MX forks fitted with emulators. One in particular where at his test track armed with different oils and all the required tools spent half a day then tossed the emulators in the bin. Then moded the stock dampers and came up with a good working set of forks.

Main problem with the emulator was the fact that it sat quite high in the fork and needed quite a lot more oil in the fork to control the damping at the top of the stroke, this in turn limitted the down travel of the fork due to the lose of air space when compressed. Other problem was if the spring was backed off enough to make it feel good mid corner then there was no damping left for big hits.
Trials forks are a bit different in their needs and if you believe the sales hype the em's will transform your old forks into upside down cartridge forks but I'm not convinced.



JC1
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Re: TY250 Fork Improvement

Postby JC1 » Sun Jun 29, 2014 5:04 pm

Very interesting, George

TriCub wrote: if you believe the sales hype the em's will transform your old forks into upside down cartridge forks but I'm not convinced.

Your'e not alone there either.


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David Lahey
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Re: TY250 Fork Improvement

Postby David Lahey » Sun Jun 29, 2014 9:14 pm

The KT forks do work well once they get a decent set of springs in them - the standard KT springs are very soft and if you try and increase the preload by much they coil-bind. TY250 springs work very well in the KT forks.
Other things to consider when comparing the actions of KT and Yamaha TY standard forks is that the KT forks have more travel, and the KT front wheel is a fair bit heavier than the Yamaha front wheel.
JC I thought the four KT compression damping holes were smaller diameter than the two TY holes. I'll check them out tomorrow. Another difference with the KT forks is that they don't have anti-topping springs.
Tricub yes the oil level needs to be higher to cover the gold valves, but it hasn't been an issue. Next time I fiddle with the gold valves I will measure how high the oil is with them fully compressed to compare with the 125mm I usually use.
Yes the beaut supple action in rocks will come at a cost on big hits, but that's why I have been testing them at trials, and so far they have worked great in sections. I did go for a bit of a high speed (70 km/h) run today across rough ground and there is no way I would want those forks on an enduro or MX bike, because it felt like an unexpected high speed hit with the front end would be unpleasant, but I'm not sure a modern trials bike front end would work much better in those conditions.
Next weekend I'm riding a trial that is likely to have big obstacles and high impacts so that should be interesting


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Stanm
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Re: TY250 Fork Improvement

Postby Stanm » Sun Jun 29, 2014 10:32 pm

I was all keen to buy some emulators until a mechanic advised me that his suspension guru told him that he thought there would be little benefit for use with trials bikes.
So I started mucking about with oil levels and weight again on my TY250 forks. I calculated the air gap left under full compression. Is there any formula for this? Or does it all go on feel?
I am now using 190ml of oil. I am using ATF and 10/30 combination because its cheap. I have used 100% ATF for quite a while ( 10-12W) which generally felt Ok but was not so good with rocky multi hits and I was loosing control. This is probably also a skill issue. I tried the 10/30 motor oil that gave me some unacceptable kickback on big obstacles so it only lasted two rides.( Maybe I had to much oil?) Today I tried 50% ATF and 50% 10/30 ( Approx 20W??) This seemed good but yet to be proven in the rock garden. So Many combinations to try!
I have not altered any porting in my damper rods but I did hear the Magicals had some special damper rods. I have plenty of damper rods from rusty forks to experiment with but I don't now where to start with the porting. I if I could get advice on this I would try and give feedback.
Cheers



Guy53
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Re: TY250 Fork Improvement

Postby Guy53 » Sun Jun 29, 2014 11:20 pm

My two bits: First there is always a guru that can tweek any part of a bike but, unfortunately for me I don't know any
Second you must have a good supply or spare parts to be able to realy test an go overboard and come back to the uptimum setting and than, each mod have to be tested with different oil, sprint setup and, and, and.
I just don't have the time, patience, and the frankly knowledge.
I agree that dit is more fun to find the right setup but on that case the emulator make it a lot easier fort me
Sorry for realy bad spelling,I write tis reply on my tablet ans the *?!/?! correter from french......
Guy



TriCub
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Re: TY250 Fork Improvement

Postby TriCub » Mon Jun 30, 2014 9:43 am

One of the tricks to try with stock dampers is to make the holes at the bottom smaller by about half the area. If there are 4 holes, block 2 by welding and then drill 2 new holes about half way to 2/3rds down the damper rod. This gives the forks a progresive damping as they travel down further. Some even have a series of hole down the rod. The idea is to have a nice supple action in the top half of the travel to keep the wheel in contact with whatever it is rolling over and then on a drop off to stiffen up to control bottoming. You don't ever want the forks to bottom as this can causes a lose of traction at the worst possible time.
There are a few other tricks but they are top secret.

Stanm.
The oil level and air pressure was used in some of the best early 80's forks to give an adjustment of spring rate. The air in a sealed fork is pressurized as the fork compress this pressure adds to the spring force. If you have more oil the pressure increases more. Don't know of any formular but you could work out the volume of the spring from it's weight. Steel is 7.85g/cm3, that means if your spring weighs 78.5 grams it would take up 10cc of oil space. I will leave the rest of the math up to you.



Stanm
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Re: TY250 Fork Improvement

Postby Stanm » Mon Jun 30, 2014 9:37 pm

Hi Tricub
To work out the airgap I got a PVC tub about the same size diameter as the spring and blocked it at one end. Pored in some water about the same height as the spring. Marked the height and them drop in the spring and spacer to work out the oil / water displacement of the spring. I could then work out the air gap left under full compression. But there is no real point if I don't have any guidelines to work with.

So the TY250 has two 2mm holes at the top and two 6mm holes at the bottom. So you are suggesting to block off one of the 6mm holes and drill another hole in about the middle. So what size would you suggest for the middle hole?

Thanks for your feedback



TriCub
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Re: TY250 Fork Improvement

Postby TriCub » Mon Jun 30, 2014 10:19 pm

Stanm wrote:Hi Tricub
To work out the airgap I got a PVC tub about the same size diameter as the spring and blocked it at one end. Pored in some water about the same height as the spring. Marked the height and them drop in the spring and spacer to work out the oil / water displacement of the spring. I could then work out the air gap left under full compression. But there is no real point if I don't have any guidelines to work with.

So the TY250 has two 2mm holes at the top and two 6mm holes at the bottom. So you are suggesting to block off one of the 6mm holes and drill another hole in about the middle. So what size would you suggest for the middle hole?

Thanks for your feedback


If there is just no air gap then you will be at the limit of travel so that is your max oil quantity. With no air gap you will notice a marked increase in the air pressure inside the fork near bottoming. You can play with the levels back from that to get the desired effect.
The 2- 6mm holes at the bottom control the compression damping, if you are happy with the damping at the top of the stroke but want more at the bottom match the top hole with the one you filled at the bottom. That is a 6mm hole about half way down, when the valve passes that hole the damping will increase as you now only have 1 hole for the oil to go through. If the stock damping is to hard at the top you could put in 2 more hole one half way and another 3/4 down but still fill one of the bottom 2.
You have to think about the rebound damping as well, if it is to slow you could drop the oil viscosity to correct it and then make the compression holes smaller to get the compression where it needs to be or the reverse if the rebound is to fast.
Sorry I can't be more specific on the TY forks but it's been a while since I've been on one.



Stanm
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Re: TY250 Fork Improvement

Postby Stanm » Tue Jul 01, 2014 7:46 pm

Hi
Thanks for the feed back on the damper porting. I will ride with the oil set up I have which seemed OK and try work out what I need to do with the porting.

I hope i can supply some feedback in a few weeks

Cheers



David Lahey
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Re: TY250 Fork Improvement

Postby David Lahey » Tue Jul 01, 2014 8:30 pm

JC1 wrote:When I got my KT going recently, I intended to try this very thing, but first I compared its damper rods to those of a spare set I have from a TY250B. The TY had only two holes (6mm dia ea) whereas the KT had four (ea slightly larger than the TY's if I recall correctly but like a dill I didn't record what dia!).

I measured the holes in the damper rods the other day but am having problems getting photos off my phone so a description will have to do for now. Both TY250 and KT250 have the same holes in the top end of the damper rod where the oil comes out while the forks are being compressed (in the aluminium part above the piston that the spring seats on). They both have four 5.7mm holes in a cross pattern in that aluminium part. That part looks identical on both bikes
The holes in the bottom end of the damper rod where the oil goes in during fork compression are different though. The TY has two 6.3mm holes while the KT has four 5.7mm holes
The TY250 damper rod is slightly longer than the KT damper rod, but the TY rod has an anti-topping spring that reduces the effective travel to about 6mm less than the KT, before the anti-topping spring starts compressing
It sounds like the Yamaha rods I have measured are a bit different in the bottom hole diameter to those that JC1 has measured. I think mine are from a TY250B or TY250A

Just worked out a way to get the photo up here
Attachments
damper rods.jpg
damper rods.jpg (141.17 KiB) Viewed 6146 times


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