Help! RD400 fork questions......

Only issue I've had with the suspension is hard top out, very hard!! Got them apart and as I thought, no top out spring or bumper or anything.............Progressive springs, that I think should have been shortened, are what's in it now.

Question is, should there be a top out spring (I don't see them on parts schematics or in my manual pictures?!?). So I was thinking I'd cut down the springs (about 1.5") and put the cut piece as the top out! Bike has no sag with me on it, front or rear, so I'm sure it will be much better if it had.

Opinions/suggestions!?

I've rebuilt dozens of forks over 40 plus years. I'm not the smartest tool in the shed, but have never come across what you are talking about. Top out? If your forks are bottoming out, the only thing I can say is, the oil is wrong (either weight or volume) or the springs are wrong. Progressive springs are reasonably priced and have always worked well for me, and if there needs to be "Top out springs" they'd probably come in the kit....

Thanks!.........I bought the bike and previous owner rebuilt the forks and put in Progressive springs.

This isn't a RD fork pictured but #10 is what I'm missing.......aren't I, rebound spring, top out spring.......whatever makes it so the fork doesn't rebound to an abrupt stop?!!??

I see what you're saying. I guess none of my bikes used this type of fork. Are you sure the Rd has this type of set up? In the setup I found, it doesn't look to have one. http://www.cmsnl.com/yamaha-rd400-1979-usa_model8704/partslist/D-05.html#results Maybe there is another part missing. Almost every bike I've rebuilt forks on, the p.o. used motor oil instead of a decent fork oil, and usually the volume is off.

Ya, it looks like it doesn't have one from everything I've seen, I think that's weird. I was worried the damping rod would be damaged. Fork oil weight and level are fine.....the rebound wasn't too quick just too abrupt.

I guess I have some MacGyvering to do! At the worst I'll ruin a Progressive spring...........

RD350/400 forks are an older simpler design. They do not use replaceable bottom bushes (9) and do not use a top out spring #10. They will have a fairly harsh top out when you lift the bike onto a stand or wheelie it but it makes little difference on the street.

It is possible to use slightly more oil or thicker oil to slow down damping but that changes everything.

Now back to the real question you had and that was lack of sag. You can shorten the springs but that comes with two provisos. First, putting the cut off part below the damper rod head will shorten the forks so the front end will be lower, but it won't make any difference to sag if you do that.

Before you cut off some spring length, how much are the springs pre-loaded. In other words, how much do you have to squash them to get the top caps on? If it's "a lot" you can shorten the springs. If there's not much pre-load then shortening the springs will not help much.

So the questions is or are: Are there emulators in the forks? And adapters? and spacers above the springs? What springs are they and what length and rate are they?

I remember having an RD when I was a kid, and I LOVED that bike, but I weighed a whole buck and a quarter back then. Almost twice that now and wonder if I'd still be as comfortable... I just bought a BSA with shoe brakes. Back in the day, thought nothing of it, but now, I gotta say I plan way ahead for stops.

I love old bikes, but have owned some new Aprilia's and other sport bikes, and find the old stuff is still bitchen, but newer stuff is miles ahead of my vintage stuff. Sometimes my expectations for the older stuff might be a little too nostalgic.

I'm not familiar with them, but you might check into (I think they're called) immulators? These (from what I hear) are added to your existing forks and they help bring them up to modern performance.

teazer said:

RD350/400 forks are an older simpler design. They do not use replaceable bottom bushes (9) and do not use a top out spring #10. They will have a fairly harsh top out when you lift the bike onto a stand or wheelie it but it makes little difference on the street.

It is possible to use slightly more oil or thicker oil to slow down damping but that changes everything.

Now back to the real question you had and that was lack of sag. You can shorten the springs but that comes with two provisos. First, putting the cut off part below the damper rod head will shorten the forks so the front end will be lower, but it won't make any difference to sag if you do that.

Before you cut off some spring length, how much are the springs pre-loaded. In other words, how much do you have to squash them to get the top caps on? If it's "a lot" you can shorten the springs. If there's not much pre-load then shortening the springs will not help much.

So the questions is or are: Are there emulators in the forks? And adapters? and spacers above the springs? What springs are they and what length and rate are they?

Click to expand...

Only change to forks seems to be the Progressive springs and an unknown air valved top cap. There is WAY too much pre-load. And on the street over even small bumps it top out harshly, almost dangerously so!

I see on HVC Cycle **(Yamaha RD250, 350, R5, DS7 Must be cut Down 1.5 inches or they will not fit. We offer the cutting service for $30.00 for the set of springs. Select the cutting service on check out.)** but not specific to RD400 for length issues.

Do you think the springs could just be too long, previous owner bought wrong ones for the bike (probably an Ebay buy)? What is the length of stock springs?

The aftermarket cap is also a factor. See if you can determine the distance from the top plane of the fork tube to the spring surface, and adjust the spacer or length of the spring accordingly.

Regardless of whose spring and spacer and cap you have, about 20mm of preload is a decent starting point, but remember to factor how much preload that cap is adding due to its length.

You need to just reset on this fork, don't assume any of those parts are correct, or installed correctly.

Thanks!

There is way more than 20mm of pre-load now. Are Progressive springs overly long compared to stock?

In any of the Progressive's I've bought, and I've bought a lot, the springs were proper length, to say they were the same length as the stock. It's the way they are wound, not the length. They are wound tighter at one end. I'm kinda of a hack, but here how I'd start this out. I'd look real close at that schematic I sent and make sure you have all the parts and in the correct order. If this checks out, I'd call Progressive (I've done this, and they were helpful) and order proper springs. You might also ask around, I know not everybody is as highly satisfied with Progressive as I am. I'm no Rossi, but do ride briskly. If you have the correct parts, get some new fluid, there shouldn't have any issue when finished. I don't remember having air caps on my RD, might want to toss those unless you want to work those into the system. The air has to go some where, which means the p.o. might have adjusted the fluid level to compensate and if the air seals leaked or went bad, leaving too much area causing the forks to bottom. By the way, forks bottoming is always dangerous. you have to figure this out before you go back to the street.

I had a XS once with your issue, it seems somebody put longer spacers in the bike (to lower it) and every time I hit the front brake, it bottomed. I found I was about 80lbs heavier that p.o. I drained fluid, cut spacers to correct length and put proper amount of new fluid. Fluid is very important, being 10 c.c. off can make or brake a ride.

Now, this said, there are much smarter guys on here and they probably have better ideas. In my limited talent, I always go back to ground zero (stock settings) and work from there...

Thanks for the chit chat guys!

The forks never bottomed, far from it! Bottoming or too fast a rebound would be an easy fix, this, not so much. I know a couple of you have said that it's been bottoming but it's top out or full rebound that has been the problem. I would say the fork is too extended with little to no sag and when riding coming off the brakes quickly or while accelerating they smack on the damping rod. Seems to me like the springs must be way too high a spring rate or, more likely, too long so pre-load is far from correct.

It sounds like the wrong springs are fitted

With a fork cap removed, how high above the fork tube is the spring?

How long are the fork springs?

How tall are the new aftermarket caps? measure from bottom of the threads to the lower face of the hex. I'll compare that to stock for you.

Do you have any static sag and if so how much?

It's obvious the front springs are either too long, too stiff, or both.

Until you take those caps off and have a look there's really no point in posting.

True!


So springs are 20mm above the top of the stanchion, the caps are Goki.....just saw a name on them!?!? and they are 30mm so are pretty much at the end of the threads of the tube.........that doesn't seem to be a problem from the info some of you have given me.

Bike has no static sag!

I found this in USA2Strokers that shows an RD400 fork with a rebound spring.....http://www.usa2strokers.com/forum/forum_posts.asp?TID=41434&OB=DESC
this is the only schematic for an RD that shows it........weird but hopeful.

Those springs aren't your problem. The fork sprigs are.

Okay good to know.......nothing to do but a little customization then.

A video of a '78 XS400 fork rebuild show the top out spring and the damping rod looks identical so I'm giving that a try. Putting in a top out spring certainly couldn't do more harm than letting it smack and unsettle the bike.

I agree, it won't do any harm. However, if you are to fix this you will need to correct the problem.

OEM forks have a steel spacer tube on top of the springs. Your springs must be much longer than stock and probably too still too. That picture seems to be from an RD400G Daytona which in the rest of the world at least were different forks. They increased diameter from 34 to 35mm and had different damper rods. I don't remember the top out springs, but it's been too long since I had late model RDs on a different continent to remember much of anything.

We usually make up different spacers to get the correct overall length and if yours don't have spacer tubes, the springs are clearly much longer than stock.

Goki were aftermarket fork caps - usually with air valves to add air springing. Are your's blue by any chance?

BTW, What diameter are your forks and what size are the XS? I think they are only 33mm. The RD has very skinny damper rods, so the XS top out springs may fit. Looks like 82-83 Maxim may have had 35mm forks which might be even better.

teazer said:

OEM forks have a steel spacer tube on top of the springs. Your springs must be much longer than stock and probably too still too. That picture seems to be from an RD400G Daytona which in the rest of the world at least were different forks. They increased diameter from 34 to 35mm and had different damper rods. I don't remember the top out springs, but it's been too long since I had late model RDs on a different continent to remember much of anything.

We usually make up different spacers to get the correct overall length and if yours don't have spacer tubes, the springs are clearly much longer than stock.

Goki were aftermarket fork caps - usually with air valves to add air springing. Are your's blue by any chance?

BTW, What diameter are your forks and what size are the XS? I think they are only 33mm. The RD has very skinny damper rods, so the XS top out springs may fit. Looks like 82-83 Maxim may have had 35mm forks which might be even better.

Click to expand...

Good info, thanks!

Yep the Goki caps are blue, I aired them up once and with little pressure (~5psi) they were locked out! My '89 RZ had air caps too but I put little to no air in those too......of course I was 135lbs back then. LOL

**Update** just measured damping rods are 17mm shaft, 26mm piston and stanchions are 34mm.

I'll have to check in with the guy I got the bike from but I don't recall him saying he changed anything but springs and caps.