Help! RD400 fork questions......

Anyone ever replace the inner bushing?? HVC said they aren't shown as replaceable, which I believe but if it's possible to get them out and put in new, that would be awesome! Ideas?
 
RD400 forks do not have replaceable bushes. The fork leg rides inside the sliders. No bottom bush and no top bush.
 
Guys are kinda one dimensional!!?? Just cause they say it isn't available doesn't mean it can't be done. I'll check around at some machine shops.
 
No, not so much. I think you need to keep in perspective what it is that you are trying to gain. Why not buy a set of springs? Then maybe emulators? And once you get all that more or less dialed in, see if the bushings need done.

For my RD350 I mounted FZR600 forks. My oem forks were shot. The FZR units cost $100 with brakes. I got more rigid forks, better brakes, and they are lighter. It took a bit of work though. Since then I have tested Racetech Emulators, YSS PD Valves, and the Taiwanese knockoffs of the emulators. The suspension isn't just good, it's great, especially for a 40 year old bike. So again, keep perspective on where you are going to find gains and pick the lowest hanging fruit first.
 
RD425gehr said:
Guys are kinda one dimensional!!?? Just cause they say it isn't available doesn't mean it can't be done. I'll check around at some machine shops.

RD forks are primitive at best. Of course you could get teh legs and sliders machined, but do you have a set of suitable teflon coated bushed to use? I don't know of any because it's not a modification I would spend time on.

I have a set of drag race forks here with 4" long lowering spacers around the damper rods and the springs stick out the top of the forks by almost 30mm and they are almost rock solid. I would cut 20mm off those springs and try them and if there is still not enough sag, try another 5mm. Or cut 100mm off and add spacer tubes on top made form PVC tube and adjust them until the suspension is better.

In an earlier post you said that the oil quantity and viscosity were fine? What was the oil level without springs and forks compressed and what viscosity oil is in there?
 
teazer said:
Seriously!

RD forks are primitive at best. Of course you could get teh legs and sliders machined, but do you have a set of suitable teflon coated bushed to use? I don't know of any because it's not a modification I would spend time on.

I have a set of drag race forks here with 4" long lowering spacers around the damper rods and the springs stick out the top of the forks by almost 30mm and they are almost rock solid. I would cut 20mm off those springs and try them and if there is still not enough sag, try another 5mm. Or cut 100mm off and add spacer tubes on top made form PVC tube and adjust them until the suspension is better.

In an earlier post you said that the oil quantity and viscosity were fine? What was the oil level without springs and forks compressed and what viscosity oil is in there?

Good info! Measured volume of oil was about 120mL per leg and whatever was left in it would have been correct, previous owner said 15W and what came out was red, almost like Rock Shox red rum! LOL
I hear what you guys are saying, I was just seeing if anyone had ever tried to update/modify or work around old RD forks and there oldness.

I'll experiment with what I have and see if i can improve on it and if not swap out to something else in awhile.
 
I found that heavier oil tended to work better but old forks are always a compromise. Best solution for those forks is Gold Valve cartridge emulators with modified damper rods and softer/shorter springs.

I just checked the Progressive Suspension site and it says do not dispose of OEM fork spacer. That suggests that the springs in it now are not the ones that P-S specifies for that bike. http://www.progressivesuspension.com/product/1465/fork-spring-kit

They list specs here:
http://www.progressivesuspension.com/pdfs/7100-105.pdf

Says they are 19.00" or 483mm long. How long are yours?
 
Thanks for your posts, they're are always insightful. :)

They are 483mm and no fork spacer.........there is no way the fork spacer would fit.

I'm going to cut these down about 20mm and up the oil weight. If that doesn't make it even a little better I'll try the emulators.

I've seen a few aftermarket stanchions available but still worried about the fork lowers inner bushing. I'll use what I got until it's fried, hopefully a few years and then look into other options.

** anyone use Forking by Frank I've heard good things, if not pricey.**
 
Hmm. That's odd. I just checked a set of RD350 springs and they were much shorter and of course P-S don't list a spring for that model. They are around 390mm by comparison.

What year did you say your forks are off? I wonder who else has a 400 that could measure their fork springs for you.
 
They seems to measure up correctly for a '76 to '78 RD400, no idea exactly what year they are from.

My buddy bought lots of parts bikes back then. I got two extra frames, 2 oil tanks, 1 gas tank, 1 triple tree and boxes and boxes of carbs and parts of oil pumps and it goes on.
 
And yet stock RD forks use a spacer. Something odd going on. I wonder if your buddy used different damper rods or shortened the fork tubes. Someone here must have a set of RD forks they could strip to compare parts and work out what is going on. You could also post on one of the RD forums like 2strokeworld.com
 
My limited and sketchy knowledge is that the spacer is removed when the progressive springs are used.

I cut the springs down to 444mm and I got 20W oil.........just waiting on my order with the seals to put it all back together.
 
RD425gehr said:
My limited and sketchy knowledge is that the spacer is removed when the progressive springs are used.

I cut the springs down to 444mm and I got 20W oil.........just waiting on my order with the seals to put it all back together.

???... My memory (although it's been quite awhile) the spacers stay, and I think I remember a little paragraph about using different length spacers (made from PVC) could shorten the forks if needed. I do remember forgetting to put the spacers in (on a GS1000) and the bike pogo'd horrendously over speed bumps, went home immediately and dropped those spacers in, and all was fine....
 
At 483mm the Progressive springs were 20mm above the top of the stanchions, there is no way the stock spacer could be used as it was difficult to even thread in the top caps without them.

Forks must be stock length as you can see from the picture, they almost look a little too long. I'm hoping with my changes there will be some sag and less abrupt top out.
 

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While you are waiting for seal, it would be a good idea to strip the slider off and inspect the damper rods and clean out the slider of old oil and debris.
 
It's all apart right now, including the engine, which I just got the cylinders back from USA PowerSeal with two new Wiseco pistons. ;)
 
Since the forks are rebounding too quickly, to the stop, and since there is no sag, doesn't this just mean the springs are too strong and you need weaker springs? The rebound damping, whether it is too much or not enough should not affect the sag. Am I missing something here?
 
Nope you are right. Springs have too much pre load and too stiff for my weight and possibly oil weight was too light.
 
Yes, the oil weight can affect the rebound speed but not sag but if you increase the amount of oil in the fork then you might also affect the rebound speed from a compressed fork , I am thinking, because there would be more force pushing to extend at the beginning of the fork extension movement from the compressed state and therefore increasing the inertia.
 
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