Author Topic: YZ250D fork top out springs - advice required please  (Read 4912 times)

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Simo63

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YZ250D fork top out springs - advice required please
« on: October 23, 2013, 08:47:15 PM »
Just pulled my new US import YZ250D front forks down so I can service them and make up an internal spacer to lower them to the required 9 inches (stupid rules) and discovered that there are in fact 2 internal top out springs fitted.  One around 50mm long and the other around 150 mm long?

I have lost my manual and need to urgently know if this is correct?  Do they have 2 top out springs standard?

Thanks in advance.
Simo
« Last Edit: October 30, 2013, 12:01:04 AM by Simo63 »

Offline ARN

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Re: YZ250D fork top out springs - urgent advice required please
« Reply #1 on: October 23, 2013, 09:02:49 PM »
yes they do

Offline John Orchard

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Re: YZ250D fork top out springs - urgent advice required please
« Reply #2 on: October 23, 2013, 09:15:12 PM »
The forks must have originally had air-assist (air valves in the caps)?  Just like my KX250A5 forks, with my .46kg fork springs I only run a 50mm top-out spring.  Race Tech suggest going to a conventional top-out spring when running normal (no air) fork springs.
Johnny O - Tahition_Red factory rider.

Offline motomaniac

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Re: YZ250D fork top out springs - urgent advice required please
« Reply #3 on: October 23, 2013, 09:23:11 PM »
the short one is the top out spring , the long one is a negative spring. You need to retain the short one.

Simo63

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Re: YZ250D fork top out springs - urgent advice required please
« Reply #4 on: October 23, 2013, 09:41:25 PM »
the short one is the top out spring , the long one is a negative spring. You need to retain the short one.

Thanks for everyone's advice so far.  So the 2 springs are standard?  Okay now I'm not sure how that works because the 2 springs are sitting side by side and look (to my untrained eye) to be doing exactly the same thing eg acting as top out springs?

What is a negative spring motomaniac?

John are you saying I can remove the longer one and just run the shorter one without any negative impact?

And if I remove the longer one and install a 25mm spacer between the top of the dampener tube and the top out spring, is that going to give me the required 1 inch reduction in suspension travel?

Thanks again in advance.

Offline Nathan S

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Re: YZ250D fork top out springs - urgent advice required please
« Reply #5 on: October 23, 2013, 09:51:11 PM »
Have you measured the actual travel? I don't know if it's true for the D models, but Yamaha sometimes included the travel from the top-out spring being fully compressed, not from the usual fully-extented height.

This adds around 25~30mm to the travel at the brochure, even though you can never 'use' the extra travel.
The good thing about telling the truth is that you don't have to remember what you said.

Simo63

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Re: YZ250D fork top out springs - urgent advice required please
« Reply #6 on: October 23, 2013, 09:58:32 PM »
Have you measured the actual travel? I don't know if it's true for the D models, but Yamaha sometimes included the travel from the top-out spring being fully compressed, not from the usual fully-extented height.

This adds around 25~30mm to the travel at the brochure, even though you can never 'use' the extra travel.

Actually No I didn't Nathan but I've got 2 other YZ D models and both have been lowered by suspension guys who put in a similar spacer to the one I am making.  I will now go out to the shed and check though just to be sure.

Offline motomaniac

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Re: YZ250D fork top out springs - urgent advice required please
« Reply #7 on: October 23, 2013, 10:19:39 PM »
The negative spring was the go for a while back then , Simons kits and Forks as well as Fox Forks had then . They just act against the main fork spring and make the initial travel a bit softer , more progressive . The front end also sits a bit lower . Nathan is right in that Yamaha used to calculate this in the overall fork travel , the front ride height is lower so you don't use all the travel that they claim when you hit a bump but the fork should extend a bit further on rebound after the bump and the theory was the front wheel would therefore follow the ground a bit better.
I would also check the actual travel first.I would take the longer softer neg spring out and measure the travel with the forks fully extended to the top out spring , the take the neg spring and compress it fully in a vice and measure the compressed length then take this measurement off your fork travel already measured without the neg spring . That's your actuall travel then chuck the neg spring and make a spacer long enough to reduce the travel back to 9".

Simo63

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Re: YZ250D fork top out springs - urgent advice required please
« Reply #8 on: October 23, 2013, 11:56:32 PM »
So I just measured with the fork complete but no oil.  It was 220mm travel .. just  under the 9 inches however the fork extends another 20mm to 240mm when you pull against the top out spring/s.

So I will now go out and remove the negative spring and remeasure.

back in a mo :)
« Last Edit: October 24, 2013, 11:00:19 AM by Simo63 »

Simo63

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Re: YZ250D fork top out springs - urgent advice required please
« Reply #9 on: October 24, 2013, 12:13:12 AM »
Okay so I removed the negative spring and the first thing I noticed is that the fork no longer was under tension eg I could screw the air cap on without any resistance and the fork had around a 20mm freeplay.  It was clearly longer at 265mm static and 290mm when I pulled against the remaining top out spring.

So I am thinking that I could do 1 of 3 things:

1. Put it back together as standard where it has 220mm static travel (but 240 when pulled against the 2 springs)
2. Put it back together as standard but add a 20mm spacer to the 2 springs thus ensuring it cannot be extended past 220mm
3. leave the negative spring out and insert a 70mm spacer to limit the travel to 220mm

I want to make sure the bike complies with the regs so that we don't have issues with scrutineering or protests (I'm not riding it but a fairly fast young lady is and I don't want to sour her first taste of VMX).

I think option 2 is the go but am seeking opinions on what we should do?

Offline Nathan S

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Re: YZ250D fork top out springs - urgent advice required please
« Reply #10 on: October 24, 2013, 10:23:43 AM »
Option 3 is crap. Don't do it.

Personally, I'd go for Option 1 and be willing to have the argument if it ever eventuated - but I understand why Option 2 is appealing to you.

The reality is that the extra 20mm of "negative travel" is simply unavailable. Maybe you might get an extra 5mm in certain circumstances, but you'll never ever feel the benefit of it - the fork will be back at its equilibrium point before the wheel hits the ground.

And thanks for confirming that the YZ-D forks have less real travel than brochure travel. :)
The good thing about telling the truth is that you don't have to remember what you said.

Simo63

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Re: YZ250D fork top out springs - urgent advice required please
« Reply #11 on: October 25, 2013, 09:04:52 AM »
I'm going with option 2, retaining the top out and negative springs and machining up a small spacer to limit fork extension to a maximum of 228mm.  That will put maximum fork travel at 1 mm under the maximum 229 mm according to the MOM's and eliminating any issues.

Thanks for everyones help including those anonymous that sent me PM's with technical information that is often closely guarded.

Very muchly appreciated   ;) 8)

Offline crossedup2

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Re: YZ250D fork top out springs - urgent advice required please
« Reply #12 on: October 25, 2013, 10:47:12 AM »
Simo
Can you put a photo up when you have finished this mod. I'm interested in the damping rod.

I'm running heavier main springs with standard 2 springs (as above) and YSS valves. Not totally happy with the how they work and looking at other options or improvement.

Thanks
Peter B
Anything Yamaha. AT1, CT1 (X3), RT1, DT3 (x3), YZ125X,  YZ250E, YZ400D, IT175E, IT250H, IT490K . Always looking for Pre 78 Yamaha stuff....

Simo63

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Re: YZ250D fork top out springs - urgent advice required please
« Reply #13 on: October 27, 2013, 02:07:37 AM »
Simo
Can you put a photo up when you have finished this mod. I'm interested in the damping rod.

I'm running heavier main springs with standard 2 springs (as above) and YSS valves. Not totally happy with the how they work and looking at other options or improvement.

Thanks
Peter B

Sorry Peter I have already reassembled before reading your post.  And I'm not going to disassemble them again as you would understand :)

Basically I just machine up a small spacer and fitted it.  Now the absolute maximum length the fork can travel including pulling against the top out spring is now 228mm .. a mere 1 mm under the 229mm limit set in the MOM's.  At least it will pass any technical scrutineering as it is now and will see how it goes but I have another idea that I might do in the near future  ;)

Cheers
Simo

Offline head

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Re: YZ250D fork top out springs - urgent advice required please
« Reply #14 on: October 27, 2013, 07:21:18 AM »
I went for Nathens "CRAP" option. Removed the long top out spring, shortened and modified dampener rod. Now they operate look like forks that are one everyone's  VMX bike. Crap option for me.