XR400 preload spacers

[crazybrit]

In thread http://dirtrider.net/forums3/showthread.php?t=121664 John Harris says:

"The better? way is to put the springs in the forks and screw the cap onto the center rod, then measure the uncompressed distance between the top of the springs and the cap for the spacer. Add to this dimension the amount of preload you want on your XR springs, usually 3 to 10 mm and then cut your spacer. Be sure you have the rings/caps on both ends of the springs (from the stock springs) before you do your measuring and then of course use them in the installation. To help you out--if I remember correctly with 10mm of preload my spacers were about 1.5" long."

'between the top of the springs and the cap for the spacer' ... I'm assuming John means between the top of the spring and the bottom of the fork cap.

Click here for a picture of my fork (hanging uncompressed) with the XR400 spring installed, fork cap screwed onto the rod and the two spacer washers (for the spacer) installed above the spring (top one upside down so flange is inside fork cap - for purposes of getting a correct measurement).

I have maybe 0.5 mm of gap so I'm confused as to how John got to a 1.5" spacer length. He seems to know what he's talking about so that leaves only one thing, spring length. My stock springs are 18.5" long and the XR400 ones (supposedly from a 1998) are 21" long (2.5" longer). In the old thread I referenced above the difference in length was 1.5".

Clearly I'm making a smaller spacer :-) Just wanted to confirm I'm doing the measurements correctly and I'm somewhat curious on the longer length of the springs I got.

Tony

 

[canyncarvr]

Can I presume the 18.5, 21 and such to be exact measurements? Not...'about'?

So...that's 470mm and 533mm respectively?

My XR springs were 520mm as I recall..and I had to leave out one of the flanged washers to get a piece of PVC long enough to deal with. I'm saying that cutting a spacer out of PVC that is only (example) 1/16" is difficult.

Some washers made for the purpose (spacing) such as are more commonly used in the USD setup would work better. I didn't have any such washers on hand at the time.

So...that extra 13mm (over what my 'I think' length was) is going to make the required spacer tougher to make.

Are these Honda OEM springs, or springs made by someone else FOR a Honda? That mostly just a curiousity on my part.

To make this easier...what size spacer do you have in mind?

 

[crazybrit]

Can I presume the 18.5, 21 and such to be exact measurements? Not...'about'?

So...that's 470mm and 533mm respectively?

I don't have a mm tape handy. The 21" is pretty exact (supposed XR). The 18.5 for the OEM was about, it's somewhere between 18 3/8 and 18 1/2.

So...that extra 13mm (over what my 'I think' length was) is going to make the required spacer tougher to make.

Yep

Are these Honda OEM springs, or springs made by someone else FOR a Honda? That mostly just a curiousity on my part.

I bought they from a guy on ThumperTalk. He seemed cool and all that. I asked several times. He said they were from a 98 XR400, he removed them to add aftermarket springs. I deduced from this that they were OEM. Depending on what feedback I get here, I can I guess go back and ask him.

To make this easier...what size spacer do you have in mind?

Well. It all depends on whether or not hanging the fork (uncompressed) in free air is a valid measurement technique. If so, I have about 1/2mm of gap. I was thinking of using 4mm of preload, so thats a 4.5mm spacer. If this isn't a valid way to measure the gap, I'm possibly already preloading the spring just to get the fork cap on (with the two spacer caps installed -- clearly they can be removed).

More importantly I need to get out my dial gauge and measure the thickness of the two springs and the distance between coils as clearly, given the length mismatch, I need to confirm the spring really is a .38. No point even bothering if it isn't. The whole point of this exercise was to go from stock .35 to .38.

Tony

 

[glad2ride]

Is the fork cap and rod nut in final position?? To me, it looks like the cap is not touching the nut.

Hanging the fork in free air sounds valid to me. If you want to get real technical, though, the forks mounted in the clamps, with the qeight of the front wheel pulling down on the top out springs would be most accurate.

I would suggest a washer on each end of thespacer, like OEM, so if your springs are so long that it is not possible, then start looking for washers.

 

[canyncarvr]

re: 'Is the fork cap and rod nut in final position?? '

Matters not where the jam nut is if what you're doing is measuring up your preload spacer.

re: 'measure the thickness of the two springs and the distance between coils as clearly, given the length mismatch, I need to confirm the spring really is a .38.'

If you have some info on what the spring is made out of, how it was drawn, then I suppose the thickness and distance would tell you something.

I don't know how much spring stock varies, but it sure ain't all the same.

Length mismatch? One from the other? (I don't think that's what you mean).

Different from some other measured XR spring? That's to be expected.........I'd expect.

To measure the spring rate, makeup some cool tool consisting of PVC and/or dowel for guides, mark some given mm stopping point, squeeze the spring on a good scale and measure it up.

That not meant to be unclear..but I'm sure you get the idea.

The '98 XR400 is supposed to have a .38 spring. Curiously, RaceTech shows it to be a 3647 series spring, same as the KDX series..and 470mm long.

Figures lie and liars figure!

**edit**

Ha! I KNEW it was 'out there', didn't think I'd find it!!

...you can thank me later:

To figure spring rate:

RATE 11.5 x Wire Diameter to the fourth power (D4) ÷ by 8 x Number of Active Coils x Mean Diameter cubed.

From: here!

Very good info there!

 

[glad2ride]

re: 'Is the fork cap and rod nut in final position?? '

"Matters not where the jam nut is if what you're doing is measuring up your preload spacer."

But I'm not. :-) I am trying to help a guy who is though, and in trying to better figure out how to help him, I (meaning I, me, myself) can benefit from knowing exactly how far down the fork cap is. It doesn't look right to me.

Now, go get DirecTV. :-)

 

[steve.emma]

have recently installed a set of (oem) honda xr400 springs into my 220 forks. the springs were a lot longer than my old kdx springs ,537mm from memory? with the spring in the fork and the washers on and the cap screwed down until it bottomed out on the damper rod there was approx 2mm gap
between spring (including 2 washers) and bottom of fork cap. i went for 8mm of preload so the spacers worked out at just under 10mm long. results were well worth the effort!
good luck.

 

[grump99]

I think you are on the right track. I installed the later model xr 400 springs in my kdx. With the rod fully extended I had about a 1 mm gap, maybe 2.

I could be wrong here, but I think the correct way to set the spacer length is by front race sag (assuming you have the correct spring rate for your weight). I read somewhere that about 70-75 mm of sag is ideal, but dont quote me on that.

As an example, I weigh 180# without gear. Initially I used about an 8 mm spacer (about 5 mm of preload). My race sag in the front ended up about 90-95 mm. I felt that this was too much sag, because it felt like the bike was riding too low in the fork stroke and would nose dive upon hard braking.

I then went to an 18 mm spacer (about 16 mm of preload). That gave me about 60-65 mm of race sag. I liked the change much better. Much less nose dive and the front felt much more balanced with the rear (although a little harsher over rocks and such). My next step will be to try about 10-12 mm of preload.

The point I'm trying to make is that pvc is easy to cut to length and is pretty cheap. You should try different lengths and find what works best for you. As a start maybe try 1/2"

 

[canyncarvr]

Glad2..Direct TV? Ha! I missed that at first........ ;)

Siriusly, if someone is going to be trying to determine a preload situation and NOT threading the cap down all the way then we need to go back to WAY before square 1.

Re: Sag is the way.

Yep. But you gotta start somewhere with forks. They are not as easy to set/change as the shock is.

BTW...
Bike Type Front % Front mm Rear % Rear mm Rear Free Sag mm
Off-Road Bikes 22-25% 65-75mm 30-33% 95-100mm 15-25mm

That from RaceTech, here.

Please DO note the same page says there is 'no magic number' when it comes to sag. I determined my preload requirements by riding and guaging front-end suspension reaction then making changes to suit me. I have reason other than sag #s to preload a fork spring very little.

So...Tony....done yet?

 

[grump99]

So...Tony....done yet?

Hehe, well... not quite. I've got a '98 wheel but no forks or triples yet. We are in the process of moving, so things have been a little hectic. I've been outbid a couple of times, but I'm trying to find a good deal, so that's ok. With a little luck the right set of forks will show up soon :)

 

[crazybrit]

Hehe. No, sorry, been busy on non-bike projects.

Agree w/ CC, need to start somewhere. Sure, sag is correct way but changing preload isn't as easy as on other forks.

As for the comment regarding the nut, I don't believe the fork cap will screw down all the way to the nut. If you hold the nut and screw down the cap, all you are doing at the end is backing the nut up towards the cap (since the rod moves). I tried holding the rod, the cap won't screw down all the way to the nut.

Anyways, here is another pic. With the 2 preload spacer caps removed this time. Fork is standing on floor (rather than free hanging), rod pulled all the way up, cap screwed all the way down. So I'm definately not preloading the spring with just the cap, have about 1.5mm of gap which agrees with what grump99 said (thanks). Odd thing is that the guy assured me these were from a 1998 XR400 which doesn't seem "late model" to me.

Here is another pic. showing what I said above.

Anyways, this is all not that interesting other than to indicate that some XR400 springs are clearly longer than others and the 1 1/2" total spacer length mentioned in other XR threads may not be correct for you. :rotfl:

I still need to double check that the spring rate on these springs, is OK, will do that next.

Thanks for everyones help, especially you Brad :nener: :nener:

Tony

 

[canyncarvr]

Hehe. No, sorry, been busy on non-bike projects.

Agree w/ CC, need to start somewhere. Sure, sag is correct way but changing preload isn't as easy as on other forks.

As for the comment regarding the nut, I don't believe the fork cap will screw down all the way to the nut. If the nut is at the bottom of the rod threads. If you hold the nut and screw down the cap, all you are doing at the end is backing the nut up towards the cap (since the rod moves). I tried holding the rod, the cap won't screw down all the way to the nut.

There are more threads on the rod than there are threads in the cap. The cap will not thread 'all the way down'.

Discussed before, but this is what happens: Hold the jam nut, loosen the cap. That loosens the cap/nut, but the cap itself is still 'stuck' on the rod. As you continue to 'loosen' the cap what you are doing is moving the jam nut DOWN the rod threads. When the nut jams at the end of the threads you feel another 'stick' when the cap loosens from the rod.

The same (opposite) happens when you thread the cap back on.

You don't ever need to hold the rod, just the jam nut and the cap.

Anyways,here is another pic. With the 2 preload spacer caps removed this time. Fork is standing on floor (rather than free hanging), rod pulled all the way up, cap screwed all the way down. So I'm definately not preloading the spring with just the cap, have about 1.5mm of gap which agrees with what grump99 said (thanks). Odd thing is that the guy assured me these were from a 1998 XR400 which doesn't seem "late model" to me.

The length of your springs is fine. The gap you got is fine.

It's all good.

Here is another pic. showing what I said above.

Anyways, this is all not that interesting other than to indicate that some XR400 springs are clearly longer than others and the 1 1/2" total spacer length mentioned in other XR threads may not be correct for you. :rotfl:

I still need to double check that the spring rate on these springs, is OK, will do that next.

Thanks for everyones help, especially you Brad :nener: :nener:

Tony

You're welcome.

Check your tag code...I'm assuming 'Here's another pic' means there is another pic?



Something missing, 'eh? :nener: yourself.

 

[crazybrit]

Check your tag code...I'm assuming 'Here's another pic' means there is another pic?

Yes, use the second occurance of the phrase aka "Here is another pic. showing what I said above.". You're correct the first one didn't have a link (cut'n'paste error) anyways, you're a smart guy, I'm sure you figured it out :-)

Something missing, 'eh? :nener: yourself.

Or maybe not :)

Tony