35 mm fork damper rod diameter

[arcticXS]

Hi!
I am trying to find a source for a "top-out" spring, and do not have access to my bikes right now. I do have a set of OEM top out springs from a later bike, but I would like to experiment with top-out spring length and rate. (Basically from an industrial supplier of springs)

So, can someone help me with the OD of the damper rod in a 35 mm XS650 fork
(mine is a -77)

 

[gggGary]

I get 17mm I'm pretty sure this is a 35mm damper...

 

[XSLeo]

Don't the top out spring just keep the fork from knocking when they fully extend? If so changing them from stock won't change anything will it?
Leo

 

[arcticXS]

XSLeo/solo2, don't pat yourself too much on the back....

You are sort of right, provided there is one installed from the factory, and the stock (too soft) fork spring is retained.
As I said in my original post, I have a 1977, which I believe is the first year with the 35 mm fork.
I asked Jerry Heiden about this, and he confirmed that the early 35 mm forks came WITHOUT a top out spring.
This was introduced on later versions of the 35 mm fork.

Also there was no top out spring in my fork when I dismantled it, which confirmed the info I got from Heiden.
And my XS did indeed have a very annoying "clunk" when accelerating hard. Which I like to do fairly often.

So I am going to take my forks apart, and install top out springs. I may even use a longer than stock top out spring, to lower the front slightly, maybe 20-25 mm. In my opinion, using a longer top out spring is a better way to lower the front than solid spacers (like HHB offers)

Also, the top out spring rate needs to be considered when altering fork spring rates and pre-load. Using the Racetech spring rate calculator, I end up with a 0.9kg/mm spring rate and 15 mm pre-load, while the spec for the stock spring is 0.56 kg/mm, pre-load unknown.
Obviously, the force trying to compress the top out spring is very much depending on the pre-load as well as spring rate.
In any case, the top out spring is not doing much good if it is bottomed out completely. It needs to have some "reserve" travel in order to absorb the kinetic energy from the extending fork as well.
The stock top out springs are very soft, I suspect too soft. Especially with a stiffer fork spring and maybe more pre-load lie mentioned above. 0.9 kg/mm and 15 mm results in a "force" of 13,5 kg (actually 132,6 Newton, which is used for measuring force) This will surely compress the OEM top out springs a lot, if not completely (will check and report back)

 

[gggGary]

when doing forks I have found it best to bottom the tube in the lower without a spring, THEN tighten the Allen bolt, this assures the tapered cup is centered with the damper. if it isn't centered you can get friction rubbing of the damper on the side of the cup and even a "short stop" if the damper hits the top edge of the tapered cup.

 

[arcticXS]

when doing forks I have found it best to bottom the tube in the lower without a spring, THEN tighten the Allen bolt, this assures the tapered cup is centered with the damper. if it isn't centered you can get friction rubbing of the damper on the side of the cup and even a "short stop" if the damper hits the top edge of the tapered cup.

Thanks Gary!
That is actually a useful tip! Straight and centered it will be!

 

[gggGary]

Not saying it's better or stiffer but it's a bit longer.



from an XS650 motor..... a rummage through some motor shop scrap bins might find you just what you are looking for.

 

[arcticXS]

Not saying it's better or stiffer but it's a bit longer.

View attachment 60801

from am XS650 motor..... a rummage through some motor shop scrap bins might find you just what you are looking for.

Another good tip Especially as I am scrapping the stock valve springs anyway, due to a MrRiggs Shell #1

 

[5twins]

Yes, the '77 forks had no top out springs. Adding a '78 and later one will lower the forks a little. The set I made up came out about 11mm shorter. The stock top out spring is about 22mm long. This would seem to indicate it gets about half compressed when everything is assembled.

I like the '77 damper rods. I think they were made to a bit higher quality standard than the later rods. The ones I've encountered have been polished from the factory and have beveled oil holes. An engineer buddy of mine told me beveled holes flow better/more.

 

[arcticXS]

You're learning. But you're not taking into account (at least you haven't mentioned) one rather important thing. I'm going to wait and see if you catch it.

Well, I do not claim to have all answers to all questions, but I do have a solid mechanical engineering background, and a genuine interest in achieving actual performance improvements in all aspects of a motorbike.

I am not sure what you are implying, but I do appreciate that a longer top out spring will, to some degree influence the sag/pre-load equation of the fork. Since the fork spring and the top out spring are creating opposing forces. And yes a lowered front may reduce ground clearance, that's why I have long Ohlins shocks. If anything, a slightly lowered fork will be more rigid than stock, due to more overlap between tube and slider. 125 mm fork stroke is where most modern sportbikes and standards are at, and should be sufficient as long as the damping is working as intended. I do have emulators installed already.

My reply to XSLeo's and your posts may come across as a bit harsh for some, but I posted a simple, straight forward question, and got 2 rather smug replies, before gggGary chimed in with constructive replies. I much prefer the latter variety.

I also like this statement (copied from the bottom of Gary's posts):
"The goal of the forum is sharing our experiences and what we've learned,

not dismissing the ideas of others"

 

[arcticXS]

Getting slightly annoyed here.
As 5twins, as well as I stated, the 1977 XS HAD NO TOP OUT SPRINGS INSTALLED FROM THE FACTORY (Not only did I use all upper case, I also typed it very S-L-O-W-L-Y, so the message is clear for everyone)

So please explain how your reply "Essentially, no. It won't change a thing" applies when I install a spring where there previously was none?
On a bike where the fork is topping out with a most annoying "clunk".

I am fully aware that many, if not most, forum members owned their first XS in the 60's, 70's and 80's, while I got mine in 2010. But I have been owning, riding and working on bikes for over 35 years, and really do not appreciate unjustified "I know better than you" or "I rode bikes when you were in diapers" type BS. Nuff said.

 

[gggGary]

Don't look under the bridges....

 

[5twins]

I think adding the top out springs to a '77 fork without them is a good idea. Apparently, so did Yamaha as they put them in all the later 650 forks. I also like the idea of lowering the forks internally rather than just sliding them up in the trees. The amount you can slide them up is very limited. At full compression, there's only about 7/8" between the slider and the lower tree. Adding the dust wiper/cover uses up 5/8" of that. That means raising the tubes much more than 1/4" in the trees can result in the lowers striking the bottom tree on full fork compression. Lower the forks internally and you don't have to worry about this.

The stock travel for the 35mm forks is given as 150mm in the shop manuals. Lowering them 20 to 25mm using longer top out springs should put you right in the 125mm travel range you noted is common on many modern bikes. With the proper springs and sorted damping, I think that would be fine.

 

[arcticXS]

5twins, as you pointed out, sliding the tubes up in the trees is a BAD idea, and I have not even considered that. Still, I see a lot of "builds" with forks like that. I just wonder what people are thinking sometimes. Anyway, it is good to see that some people agree with my ideas

 

[5twins]

People aren't thinking, that's the problem. I see all those builds you mentioned as well, the ones with the forks slid up an inch or two in the trees. They'll learn the hard way after hitting some big bumps and feeling the large jolt from their fork lowers slamming into the lower tree, lol. If they survive that lesson and ride, that is.

I routinely raise the 35mm forks in the trees on these bikes, but only about 1/4" because I know the limit. This is an old road racer's trick. It lowers the front end slightly which is supposed to improve steering feel, input, and handling. It's also supposed to more firmly plant the front tire to the road. Not sure 1/4" is doing much but it hasn't had detrimental effects either. The additional lowering through the use of longer top out springs sounds good. That will give me some more fork configurations to play with. Those inner valve springs look perfect for the job. They look like they may be stiffer than the originals as well.

 

[5twins]

My "modded" '77 forks consist of '77 tubes, lowers, and damper rods combined with later model top out springs, fork springs, and the 3-way adjustable top caps. The addition of the top out springs and later top caps add some additional preload to the springs over stock but they're still too soft for my liking. I need to experiment with adding even a bit more preload. I'm hoping maybe another inch or so will do it. I'm going to need to fully compress a fork spring and measure it to see just what my limit on preload is before the spring becomes coil bound.