OzVMX Forum
Marque Remarks => Yamaha => Topic started by: Simo63 on October 23, 2013, 08:47:15 pm
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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
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yes they do
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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.
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the short one is the top out spring , the long one is a negative spring. You need to retain the short one.
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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.
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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.
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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.
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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".
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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 :)
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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?
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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. :)
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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)
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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
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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
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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.(http://i1139.photobucket.com/albums/n542/fastesthead/PB240007.jpg) (http://s1139.photobucket.com/user/fastesthead/media/PB240007.jpg.html)
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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.(http://i1139.photobucket.com/albums/n542/fastesthead/PB240007.jpg) (http://s1139.photobucket.com/user/fastesthead/media/PB240007.jpg.html)
That's the best way to do it and the way I've done my RM250 & 370 forks last year. You may have to alter the length of the preload spacer for the main spring to suit.
You also need to check that the new spacer doesn't block the rebound holes in the top of the damper rod.
We used to remove the long preload spring from suzuki forks back in 77 to obtain more travel..
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Hmm this works for me. 50mm top out springs that I made myself from cutting up springs plus spacer. Some cut up old YZ 465 springs and double valve springs on top under the caps very little pre load. Gives just under 9 inch. ATF 140mm from the top springs out.
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You guys are awesome .. thanks for the info
When I have more time I'm definitely doing a similar thing to the shorter dampner rod pictured
Just one thing .. where was all this wonderful advice when I had the bloody forks apart BEFORE I reassembled it back into the bike ;D ;D ;D
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Hmm this works for me. 50mm top out springs that I made myself from cutting up springs plus spacer. Some cut up old YZ 465 springs and double valve springs on top under the caps very little pre load. Gives just under 9 inch. ATF 140mm from the top springs out.
I have done it similar to that in the past also using a car valve spring cut to length instead of a solid spacer with the short top out spring. I use PVC pipe cut to length for the fork preload spacer for firmer fork action.
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I'll keep mine standard and play at club level only . It was made with the best at the time why go backwards . Iain
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I'll keep mine standard and play at club level only . It was made with the best at the time why go backwards . Iain
My sentiments as well. Yamaha always seemed to be at the forefront when it came to front suspension and brakes. That is why I am going to use '77 YZ front ends on my '77 Huskys and VB Montesas and 43mm '81 YZ front ends on some of my Evo bike projects. But I will probably only ride the pre '78 bikes at club days, because I don't feel like complying to stupid rules.
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Dont start me on the pre 78 suspension rules I have just been shortening up my Husky rear end all week trying to get the balance correct. The bike looks funny now but does fit on a milk create. >:(
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I'll keep mine standard and play at club level only . It was made with the best at the time why go backwards . Iain
Iain I completely understand what you are saying and I also have an issue with having to lower it to meet the mom's. Remember the thread from 12 months ago where I won no friends with the 2012 Nats organisers and scrutinisers? But at the end of the day, if you don't do it, you are technically not complying with the rules.
And the fact is, we all need to comply otherwise it's not fair.
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None of us agree with the pre 78 rule but you guys are the ones short changing yourselves by staying home while we ride the big meetings...
I personally think the rule should be set at 10" so all std bikes and aftermarket fork kits and shocks comply without modification.
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None of us agree with the pre 78 rule but you guys are the ones short changing yourselves by staying home while we ride the big meetings...
Agree entirely JO
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I do go by the rules my pre 70 and 75 bikes are within the rules and I ride them at the nats . Simo I felt for you at 2012 I would have had no probs with your bike as Im of the idea that the better rider can win on any bike within reason . I just don't want to change my bike backwards and as Im usually fighting for last place whats the point .
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All good Iain :)
I might ask to lock this now as I have the info I needed and don't want to risk it turning into a rule based thread ;)
Thanks for everyone's input, it's greatly appreciated.
Cheers
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Slakewell or Husky500Evo can you tell me what the standard shock length should be on a 77 390 Husky?
Mine has 14.25 YSS shocks with only 4" shaft travel giving 7.25" rear wheel travel. Obviously not the correct shocks for the job..
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Standard shock length for a '77 CR390 is 380mm, with around 140mm of shaft travel. They came out with Gas Girlings with dual rate springs, with the smaller springs being a bit strange because they were wound with square section material , rather than the usual round section material. I run remote reservoir Ohlins on my '77 CR 390, which are 383mm long and have around 140mm shaft travel. I think that these Ohlins came out standard on '79 WR Huskys.
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I have 340mm shocks on the Husky now and slide the forks pass the bars. It does steer a little bit better being lower. It does comply with the rules. I will submit to MA that the suspension limits in Pre 78 are not necessary and no limits should apply. Please start a new discussion if you want to bash the rules thou. Hope you got your forks fixed.
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So Simo what did you end up doing to them and what did you do with the rear end .
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So Simo what did you end up doing to them and what did you do with the rear end .
The rear shocks on all my three 77 yz's have been lowered internally to limit travel to under 229mm. The forks on two of them have been done professionally as we'll but this one I was trying to do myself hence asking for help. :)
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Can anyone here provide exact free length dimensions +/- 1mm of the D or E negative and rebound springs and for those that have removed the stock negative/rebound springs, would you be willing to sell them?
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Most of the YZ fork travel is at the brochure. If you use the tired and true zip tie test the travel is barely over 9 inches standard. Given that MA gives you 10% leeway they could pass in standard trim. If you are not going to win I would leave them standard. Saying that if you follow my set up in this thread they end up better then standard. Air assists forks never took off for a reason.
The top out springs need to be 50mm + spacer. If you want to use air assists method. Find some 30mm fork springs and cut them 150mm. All the old YZ stuff I have pulled apart are all stuffed and unusable.
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So the short is 50mm and long 150mm?