Front fork tuning/modifications

Has anyone tried to tune their front forks using PVC spacers to increase the spring preload? Where should I start (ie. 1cm, 2cm etc...) and what materials did you use to create the spacers? I've procured a nice piece of aluminum 1/16" wall and would like it to be the final insert, relying upon the cheaper PVC to make all the rudimenary adjustments. Comments and experiences or links are appreciated.

 

I certainly have. But I started with aftermarket springs. Right now the spacer is only 2cm longer than stock.

But I also made other changes to the front end as well (stiction tuned, emulators).

 

This is a question of how much do you want to spend and what type of improvement you want to make.

If you are serious about improving the front end a suspension tech will tell you not to use spacers and to instead create smaller oil ports for the fluid to move through. Spacers reduce travel and make the intial edge of the bump harder than it should be. Essentially you narrow the feel of the suspension and loose travel but gain some stiffness for use under heavy braking.

Injuhneer sounds like he went that route with emulators and stiction tuned.

Personally I've not added much of a spacer... but I have put a bit on. I also run progressives - which again a suspension snob scolded me for... rightfully so. They feel better to me but then again I've not brought a full suspension race package and can only dream of what type of system would be the ideal solution.

I think most likely you fall into the same catagory as myself - I want to reduce dive and not pay too much money. I'd say that a spacer would probably be the cheapest solution and I personally think it feels ok.

If you REALLY wanted to improve the suspension you'd upgrade to cartridges (temperature effect on the oil is dimenished, etc)... but then that's a whole chunk of change I don't think would be worth it for a street bike.

Woot ( never really answering anyones questions... )

 

I've got progressive springs in my forks, and can STRONGLY reccomend them if you haven't already.

As for spacers, I'm using PVC in there right now, with a thick metal washer at each end for strength. My stock forks didn't have any spacers in there at all, the progressive's reccomended something like 2" (don't remember) - but it was too short. I cut mine so the spacer would be even with the top of the tube at full extension - which worked out to be something like 5".
Seems to work nicely.

 

NOTE: I'm trying to relay what I've been told/taught by a professional suspension tech. I myself am not anywhere near a suspension person.

The basic idea Sno is that by adding a bigger spacer on top of the spring you add preload. This preload compresses the spring a bit - when you hit a bump the spring is already compressed some so it is harder.

As the spring is compressed you do lose some travel. On the other hand, the forks are stiffer so don't tend to dive as much.

The arguement against that is that the springs will dive to the same depth in the end - but the unspaced springs will dive a longer distance to get to the same depth. Meaning that the unspaced springs will feel softer and give the same clearance in the end.

The actual construction - the pvc pipe is inserted in place of the spacer in the forks. It fits inside the fork tube and decreases the space the spring has to operate in - creating preload. On fancy forks you can get screws that adjust the preload.

The real solution is to have a smaller valve for the oil to squeeze through - this gives you the same travel for the forks, but gives more resistance to diving.

 

I've heard of the emulators and don't want to spend the money just yet. Progressive is just over the pass here so a trip up to the show room might be in order. I've measured the internal ID on my tubes and I can't find PVC schedule 40 in the right size to fit. I'll need to turn it down just about 2mm to get it in there. I bought the washers already and didn't think about re-enforcement top and bottom on the spacer, good idea. Valving is black magic to me and I don't have the first idea how to change it. I looked up this topic on google last night and got some education, so far, I'm not doing too bad but I do want to do it right. Thanks for the posts fellas, I'll let you know where I land.

 

On valving - the area of circle is the square of the radius (times pi).

Meaning - making the hole a tiny bit smaller makes it's area much smaller. The smaller the hole the more it impedies the foil of the oil. What some people do is cover in the original hole, and create a new hole.

As to exactly what sized hole - well a good suspension person could use some crazy formula and solve it for you.

I think progressives are a start - certainly going from old oil and springs to new oil and new springs will be a welcomed change. (Some people change their fork oil every year!)

 

I'll see if I've enough left over next month for springs. I'll have to soldier on with what I have for now. I'm playing with my FZR's forks too. Similar situation with a similar cure according the the sites I visited last night. Wish I had a few more dollars just lying around.

 

I replaced my springs with a set from Progressive Suspension. I cut down the original spacers to 1.5". It was still a bit soft, so I replaced the oil with AW32 hydraulic oil, and put in the longer spacer sections left over from the originals. The front end is transformed. The old springs would bottom out and lose half of their travel just putting the bike on both its wheels. Along with the braided brake lines, it feels like a completely different bike.

 

Well first off it seems to me that if you say redrill the diffusion holes you are doing it backwards. The first step in reducing dive is to use a heavier oil. Instead of using expensive fork oil just use cheap ATF but get the heaviest you can find. And if thats not to your liking add in some lucas transmission thickener. It all works the same.

Then after all that and your not satisfied, redrill your valve head.

 

Well now, just got back from an evening cruise on the FZR. The 21mm spacers are perfect. Was a royal pain trying to put everything back in the forks but it worked out beautifully. I am running 20 weight oil and like the characteristics. Don't think I need to tweak any more. Thanks guys.

 

Let's see...

The longer the spacer the more preload.
The more preload the less sag (loaded ride height).
The travel remains the same but the spring is further into its progression.
Travel is only limited if the spacer is too long and the spring reaches coil bind.
Lowering blocks lower the bike and reduce travel because they go between the end of the fork inner and the damper rod.