FZR bucket suspension

**speedpro** · 8th February 2008, 12:11

Little help and advice please. I’m putting together a FZR250 (3LN) chassis as a bucket. The front forks have about 150mm travel which is lots more than another bucket I rode in the weekend that handled superbly. Question is, should I restrict the travel to say about 75mm whilst I have the forks in pieces and, apart from fitting emulators, is there any other mods that would be worth doing? I was thinking of fitting say a 75mm spacer over the top-out spring and cutting a similar amount from the main spring preload spacer. I’m running 3LN wheels front and rear, 2.75” and 3.5” with a single large disc off the earlier model with a late model 4 pot calliper and slicks both ends.

Any thoughts on the rear shock would be appreciated as well. Stock currently but with the major weight reduction not right I would think.

**Robert Taylor** · 9th February 2008, 19:11

Originally Posted by speedpro

Little help and advice please. I’m putting together a FZR250 (3LN) chassis as a bucket. The front forks have about 150mm travel which is lots more than another bucket I rode in the weekend that handled superbly. Question is, should I restrict the travel to say about 75mm whilst I have the forks in pieces and, apart from fitting emulators, is there any other mods that would be worth doing? I was thinking of fitting say a 75mm spacer over the top-out spring and cutting a similar amount from the main spring preload spacer. I’m running 3LN wheels front and rear, 2.75” and 3.5” with a single large disc off the earlier model with a late model 4 pot calliper and slicks both ends.

Any thoughts on the rear shock would be appreciated as well. Stock currently but with the major weight reduction not right I would think.

Well, its certainly going to be compliant under brakes with the increased disposition of the fork bushings at full extension. I dont understand buckets so well but why so little travel? If you have a fork with adequate travel and good brake dive control that is going to allow weaker rebound damping in the rear shock to get better off corner traction. If you get to the bottom of fork travel too early whilst under brakes you are going to be waving the rear in the air. Id be in the first instance just respringing the current shock.

**SHELRACING** · 9th February 2008, 21:20

I'd say you were on the right track with your spacers. It's a bit of trial and error to get the spacers just right. I've done similar on mine then added and removed washers till I got it just right. Use 15w fork oil, make sure you measure it exactly for each leg. Where you going to race the bike Mt Welly. You may find your tyres a bit wide ( they'll take too long to warm up).

You're welcome to come round and look at the setup on my bikes if you want some ideas.

Cheers
Steve
SHEL Racing

**speedpro** · 10th February 2008, 20:15

Thanks for the replies and the offer.

I have tried all sorts of parts from several FZRs and most are bent, shagged, or just the wrong size. Anyway . . . . . I have ended up with a set of forks that the 2.75" FZR front fits into with just one of the real big discs off an early model. I have the linear wound springs fitted and the shorter damper rods. With the short spring spacers there is maybe 10mm preload which is supposed to be the way to go. The FZR calipers I have won't bolt up so I'm going to use the caliper and master cylinder off my old RS125 chassis and make a little adaptor plate. It seems that the RS caliper needs a 10mm spacer to line up on the disc anyway so very conveniant. The rear 3.5" already has a used 120X17 600 front slick fitted and it looks about right. The fast guys seem to be getting along OK with 2.5" and 3.5" wheels so I'm in the ballpark. Warmup on the JCR125 #23 doesn't seem to be a problem with this size tyres fitted. With Gary riding it though who would know.

The forks seem to have a reasonable amount of rebound damping but no discernable compression damping. I'm missing something as I can't see where the differance comes from unless there is a one-way valve I missed. The rear shock seems useable with noticeable rebound damping and the spring doesn't seem too bad. Makes me wonder how soft it was on the original FZR.

**FROSTY:Me** · 11th February 2008, 19:35

I'm missing something dude-How on earth do ya get a 250 to turn fast enough on Mt wellington ?

**speedpro** · 11th February 2008, 20:51

The Wellington guys manage OK. It is about 200mm longer than my RG/MB though and 100mm longer than the RS. I've lowered the whole bike but am thinking it may need to be raised again as being so wide the frame may cause ground clearance problems. If so I'll bring the back up first and see how it handles. JCR #23 drops in to a corner, flops would be a better word, but it's catchable on the throttle and possibly your knee if you are a bit slow. If it ends up like that it'll be fine. I'm looking at a way of shortening the frame and will optimise the rake if I do.

Assembled my drill press yesterday and made some swarf today, Geez it's a good feeling to be building something again.

**FROSTY:Me** · 11th February 2008, 21:02

fair nuff. arse ends gonna need to be pretty high though

**speedpro** · 12th February 2008, 07:05

Originally Posted by Sparky Bills

It will never make up for lack of talent....

I have no intention of letting you ride it

**speedpro** · 16th February 2008, 20:58

Originally Posted by speedpro

The forks seem to have a reasonable amount of rebound damping but no discernable compression damping. I'm missing something as I can't see where the differance comes from unless there is a one-way valve I missed.

Found the one-way valve. In the bottom of the staunchion there is a small housing with a nylon washer which seals round the damper rod as the fork moves. The washer is free to move slightly within it's housing and has a clearance of a couple of mm round the outside. On the top of the washer are 4 raised bumps which stop it seating and making a seal with the effect that on the compression stroke it allows oil to bypass into the lower staunchion below the damper rod piston. The 4 large holes in the bottom of the damper rod also allow oil to move from below the staunchion as it moves down, up through the damper rod to the space above the damper rod piston inside the staunchion. End result - lots of places for the oil to flow and not much damping.

On the rebound the washer seats and seals trapping oil between it and the damper rod piston inside the staunchion. The only place for the oil to go is through one little hole in the damper rod, causing rebound damping.

I've blocked two of the holes in the bottom of the damper rod and will see what effect I can notice. I'll get it going before I alter it any more.

**Robert Taylor** · 18th February 2008, 08:51

Originally Posted by speedpro

Found the one-way valve. In the bottom of the staunchion there is a small housing with a nylon washer which seals round the damper rod as the fork moves. The washer is free to move slightly within it's housing and has a clearance of a couple of mm round the outside. On the top of the washer are 4 raised bumps which stop it seating and making a seal with the effect that on the compression stroke it allows oil to bypass into the lower staunchion below the damper rod piston. The 4 large holes in the bottom of the damper rod also allow oil to move from below the staunchion as it moves down, up through the damper rod to the space above the damper rod piston inside the staunchion. End result - lots of places for the oil to flow and not much damping.

On the rebound the washer seats and seals trapping oil between it and the damper rod piston inside the staunchion. The only place for the oil to go is through one little hole in the damper rod, causing rebound damping.

I've blocked two of the holes in the bottom of the damper rod and will see what effect I can notice. I'll get it going before I alter it any more.

Careful, you are by nature playing with progressive rather than speed sensitive damping. Its going to be very easy to introduce harshness as the damping holes choke off at higher fork velocities. There is also so much uncontrolled bleed due to loose tolerancing and a little bit of wear that not all the rebound flow is going through that tiny hole.

**speedpro** · 18th February 2008, 18:01

Originally Posted by Robert Taylor

Careful, you are by nature playing with progressive rather than speed sensitive damping. Its going to be very easy to introduce harshness as the damping holes choke off at higher fork velocities. There is also so much uncontrolled bleed due to loose tolerancing and a little bit of wear that not all the rebound flow is going through that tiny hole.

Ta. already thought about that, hence not changing anything else till I ride it. I haven't altered the rebound I don't think. The little rebound damping hole is so small in comparison to the 4 (now 2) holes at the bottom of the damper rod that I can't see how it could materially affect the rebound damping by blocking up just the two holes.

I'm having trouble getting my head round how the damping won't be speed sensitive. You are talking about the speed that the fork either compresses or rebounds?? Surely for any given increase in fork velocity there will be a requirement to move damping oil at a higher velocity through the damping holes. As the hole size is fixed surely that will result in a rise in resistance as velocity increases? Or is that the "progression" you talk of? If so, speed sensitivity would be gained by . . . . ?

I can see this being as complex as modifying a 2-stroke!

**Robert Taylor** · 19th February 2008, 16:30

Originally Posted by speedpro

Ta. already thought about that, hence not changing anything else till I ride it. I haven't altered the rebound I don't think. The little rebound damping hole is so small in comparison to the 4 (now 2) holes at the bottom of the damper rod that I can't see how it could materially affect the rebound damping by blocking up just the two holes.

I'm having trouble getting my head round how the damping won't be speed sensitive. You are talking about the speed that the fork either compresses or rebounds?? Surely for any given increase in fork velocity there will be a requirement to move damping oil at a higher velocity through the damping holes. As the hole size is fixed surely that will result in a rise in resistance as velocity increases? Or is that the "progression" you talk of? If so, speed sensitivity would be gained by . . . . ?

I can see this being as complex as modifying a 2-stroke!

The problem with damper rods is that those holes are way too big at low fork shaft velocities so you may as well have a sponge pudding for a suspension medium. But at higher velocities they max out / choke off in flow rate meaning the forks will jarr badly ( felt through the handlebars ) over an abrupt bump. An emulator has a small bypass hole to control low speed, it will actually give it some low speed damping and ride height control that many people will otherwise apply a dark age fix to...overfirm springs or preloading the hell out of it. ( We are in the 21st century for gods sake! )

When an abrupt bump is ridden over a spring preloaded poppet lifts on the emulator to flow the required amount of oil. The harder the ''hit'' the more that poppet opens, so it is speed sensitive.

Its way less complex than a 2 stroke. But the sad thing is that accountants still dictate that many bikes are made with damper rod forks and a lot of the rubbish coming out of China will continue to be made in this way.