Front wheel chatter?

[bucketracer]

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Front Shocks

Pulled the fork legs from the bike, and tried compressing them by hand. They pushed in Ok but bounced right back, just as RT predicted blowing through the rebound damping (20wt oil) as if it wasn’t there.

Stripping them down, I found the style of damper rod was the kind that has a piston ring with the damper shaft passing down through a washer trapped in the end of the fork tube. At the top were two small (2.5mm?) rebound damping holes and at the bottom two larger (5.5mm) compression holes and a small rebound cushion spring.

The piston rings did not touch the fork tubes, I guessed they had collapsed or worn down to the point that they were no longer sealing and I figured this was the problem with the fork legs blowing back up through the rebound damping.

I made new rings from glass filled Teflon that sealed the piston rod and fork tube without dragging. I drilled another pair of 6mm compression damping holes in the bottom of the damper rod and blocked with weld the lower of the two rebound holes at the top of the damper rod.

I was very happy with the new Gold Valves from RT, and they were a very close fit in the fork tubes. I was happy because I know that some damper rods just rely on a close fitting piston without any piston ring to seal the rebound damper. The fork springs will hold the Gold Valves firmly in place on top of the damper rod and they would act like pistons. I was happy because I now had a piston and a good piston ring on each damper rod to seal the rebound damping.

I assembled the whole shebang and filled each forkleg with a 130ml of 10wt and gave them the hand compression test. And wouldn’t you know it, apart from the last 20mm there was no discernable rebound damping. They just bounced right back up and then slowed for the last 20mm.

I don’t understand it! I expected to see more visible rebound damping in the mid stroke. I am sure the Gold Valves are doing a good job of the compression damping but I can’t see that I have made any real improvement to the rebound damping.

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[cowpoos]

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Front Shocks

Pulled the fork legs from the bike, and tried compressing them by hand. They pushed in Ok but bounced right back, just as RT predicted blowing through the rebound damping (20wt oil) as if it wasn’t there.

Stripping them down, I found the style of damper rod was the kind that has a piston ring with the damper shaft passing down through a washer trapped in the end of the fork tube. At the top were two small (2.5mm?) rebound damping holes and at the bottom two larger (5.5mm) compression holes and a small rebound cushion spring.

The piston rings did not touch the fork tubes, I guessed they had collapsed or worn down to the point that they were no longer sealing and I figured this was the problem with the fork legs blowing back up through the rebound damping.

I made new rings from glass filled Teflon that sealed the piston rod and fork tube without dragging. I drilled another pair of 6mm compression damping holes in the bottom of the damper rod and blocked with weld the lower of the two rebound holes at the top of the damper rod.

I was very happy with the new Gold Valves from RT as they were a very close fit in the fork tubes. I was happy because I know that some damper rods just rely on a close fitting piston without any piston ring to seal the rebound damper. The fork springs will hold the Gold Valves firmly in place on top of the damper rod and they would act like pistons. I was happy because I now had a piston and a good piston ring on each damper rod to seal the rebound damping.

I assembled the whole shebang and filled each forkleg with a 130ml of 10wt and gave them the hand compression test. And wouldn’t you know it, apart from the last 20mm there was no discernable rebound damping. They just bounced right back up and then slowed for the last 20mm.

I don’t understand it! I expected to see more visible rebound damping in the mid stroke. I am sure the Gold Valves are doing a good job of the compression damping but I can’t see if I have made any real improvement to the rebound damping or not.

.

mid and higher stroke movements [high speed damping] should be ALOT faster...and your last 20mm you speak of is resemabling low speed damping...

[Robert Taylor]

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Front Shocks

Pulled the fork legs from the bike, and tried compressing them by hand. They pushed in Ok but bounced right back, just as RT predicted blowing through the rebound damping (20wt oil) as if it wasn’t there.

Stripping them down, I found the style of damper rod was the kind that has a piston ring with the damper shaft passing down through a washer trapped in the end of the fork tube. At the top were two small (2.5mm?) rebound damping holes and at the bottom two larger (5.5mm) compression holes and a small rebound cushion spring.

The piston rings did not touch the fork tubes, I guessed they had collapsed or worn down to the point that they were no longer sealing and I figured this was the problem with the fork legs blowing back up through the rebound damping.

I made new rings from glass filled Teflon that sealed the piston rod and fork tube without dragging. I drilled another pair of 6mm compression damping holes in the bottom of the damper rod and blocked with weld the lower of the two rebound holes at the top of the damper rod.

I was very happy with the new Gold Valves from RT, and they were a very close fit in the fork tubes. I was happy because I know that some damper rods just rely on a close fitting piston without any piston ring to seal the rebound damper. The fork springs will hold the Gold Valves firmly in place on top of the damper rod and they would act like pistons. I was happy because I now had a piston and a good piston ring on each damper rod to seal the rebound damping.

I assembled the whole shebang and filled each forkleg with a 130ml of 10wt and gave them the hand compression test. And wouldn’t you know it, apart from the last 20mm there was no discernable rebound damping. They just bounced right back up and then slowed for the last 20mm.

I don’t understand it! I expected to see more visible rebound damping in the mid stroke. I am sure the Gold Valves are doing a good job of the compression damping but I can’t see that I have made any real improvement to the rebound damping.

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The emulator controls compression only. I will post later on what you will need to do to sort out the rebound issue you have.

[bucketracer]

The emulator controls compression only. I will post later on what you will need to do to sort out the rebound issue you have.

Thanks RT, yes I knew the emulator was compression only, I thought I had the rebound sorted too, but no, and now need help with the rebound side. Many thanks RT for taking an interest in this.

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[Robert Taylor]

Thanks RT, yes I knew the emulator was compression only, I thought I had the rebound sorted too, but no, and now need help with the rebound side. Many thanks RT for taking an interest in this.

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Ok here goes. With damper rod forks rebound is controlled ( or not controlled so well! ) by;

1) One or two tiny holes drilled underneath the ''hat'' of the damper rod. Very often we have to bronze up those holes and drill one only smaller hole or none at all.

2) Look down inside the fork once you have installed and tightened the lockbolt for the damper rod. All too often you will find that it is not concentric with the centreline of the fork tube, it will be skewed to one side. That means that the piston ring will not seal around its entire circumference ( even though youve made some lovely new ones! ) and will create a lot of uncontrolled spillage that will negate any rebound damping.
What you have to do is to lightly machine the bottom of the damper rod to true it up, but that itself is no guarantee. You need to clock and reclock until you can get the rod sitting as concentrically as possible. Very often the rods themselves are not straight and if you do have to end up bronzing up the stock rebound holes be very very mindful of heat disortion!

3) At the very bottom of the chrome fork tubes should be some encapsulated nylon rings or shim steel check plates whose id is slightly larger than the od of the lower reaches of the damper rods. These provide a form of sealing, if they are damaged in any way again your rebound control will be non existent

4) Oil viscosity selection will then be dependent on how much uncontrolled bleed you have minimised and the size of the rebound holes. If you have done a good job with this you will be able to run a relatively light viscosity, that pays a further dividend in that lighter viscosity oils are less variable in performance according to ambient temperature shift.

On another thread I was bagged by a customer for not being totally specific about oil viscosity to go with the emulators we sent him. The reality is that given the variables above every job is different. I can safely say we fit way more emulators than anyone else in the country and with a model we havent done before we do one fork first and try our best guess with oil viscosity. If it hasnt got the rebound feel we want we either change the viscosity or we pull the rod out again and tune the size of the return hole. Thats a pain in the butt but the only way of ensuring a good job.

Hope this all helps!

If you can forward a fax number on Monday I can send you some flow path diagrams that will help you understand damper rod function a little better.

[Robert Taylor]

mid and higher stroke movements [high speed damping] should be ALOT faster...and your last 20mm you speak of is resemabling low speed damping...

Further, bronze up those 2 x 2.5mm rebound holes and drill one new one starting at 1.5mm, allied with all of the other things to sort in my previous post. Start with 10 weight as you have it to hand. Rebound return speed should be relatively fast BUT CONTROLLED with no aftercycle or solid thud back up to full re-extension. Hard to describe in words!
Its arguably worse having too much rebound, that is more likely to cause crashing as the forks wont follow depressions as readily, allowing the tyre to stay interlocked with the pavement.
Please do let me know how you get on.

[bucketracer]

Further, bronze up those 2 x 2.5mm rebound holes and drill one new one starting at 1.5mm, allied with all of the other things to sort in my previous post. Start with 10 weight as you have it to hand. Rebound return speed should be relatively fast BUT CONTROLLED with no aftercycle or solid thud back up to full re-extension. Hard to describe in words!
Its arguably worse having too much rebound, that is more likely to cause crashing as the forks wont follow depressions as readily, allowing the tyre to stay interlocked with the pavement.
Please do let me know how you get on.

Thanks RT, I will forward the fax number Monday. Your last two posts make sense and give me plenty to work on. I feel sure the problem is in there somewhere amongst the things you describe.

Today I have been looking at a friends older Honda RS125 and can see the rebound of those forks is much more controlled, so now I have an idea of what I am looking for.

You have given me plenty to go on with, and will start by looking at the sealing of the check plates and the concentricity of the damper rods, many thanks for your input.

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[bucketracer]

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Pic-1 is pretty much what my setup looks like now, I suspect there is a problem with sealing of the damper rod.

I am thinking of converting the damper rod check plate thats in the bottom of the fork tube to the better looking foot valve assembly you see on some damper rods.

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[Robert Taylor]

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Pic-1 is pretty much what my setup looks like now, I suspect there is a problem with sealing of the damper rod.

I am thinking of converting the damper rod check plate thats in the bottom of the fork tube to the better looking foot valve assembly you see on some damper rods.

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Yes you are on the right track!

[bucketracer]

An update of where I am at with my suspension problem. I fitted the Gold Valves and took the bike for a spin they worked a treat. I could attack rippled sections much more aggressively and the front end of the bike no longer jarred on the bumps. And I expect more improvement of the ride is possible with tuning of the spring adjustment on the valve.

But the front-end chatter is still there, no surprises though as that is related to the rebound damping which was non-existent. So I pulled one fork leg off, took the Gold Valve out and started trying heavier grades of oil starting at 20w and working up. Even with 85wt. gearbox oil the fork behaved like a pogo stick with no discernible rebound damping.

So I striped it down and welded up (Tig) the two 2.5mm rebound holes in the damper rod. Straightened the damper rod, trimmed both ends up in the lathe and checked that the piston end of the damper rod was central in the fork leg when every thing was re-assembled. With a 1mm rebound hole and 10wt Oil the fork leg was no longer a pogo stick and the rebound was visible controlled. Modified the other leg and reassembled it all back in the bike.

Bounced the bike up and down a few times and realised I had made a mistake. What was visibly controlled before had now become slow and sluggish. I had not figured on the weight of the bike. The fork springs were now working against the damping and the bikes weight. I will have to use thinner oil or increase the rebound damper hole size. I think I will go with thinner oil first off and take it for another test ride as soon as I can.

Using an old set of bathroom scales and a bit of arithmatic I figured that I have a set of 54lb/inch and a set of 71lb/inch springs so will try them both in due course.

Its a lot of work but I think I am getting there.

[CookMySock]

Very interesting to read about your project. I'm tempted to fit emulators to our road bikes..

Just for interest, I purchased a GT250 Hyo for my daughter, and at speeds over 90k the front got twitchy and darty and there was obviously something scarily amiss. We replaced the rear tyre (it wasn't WOFable) and discovered it had no balance weights on it. New tyre and re-balance, and the rear wheel took 55 grams to balance it. Bike now rides mint. Dunno if theres anything relevant there for you.

Steve

[Robert Taylor]

An update of where I am at with my suspension problem. I fitted the Gold Valves and took the bike for a spin they worked a treat. I could attack rippled sections much more aggressively and the front end of the bike no longer jarred on the bumps. And I expect more improvement of the ride is possible with tuning of the spring adjustment on the valve.

But the front-end chatter is still there, no surprises though as that is related to the rebound damping which was non-existent. So I pulled one fork leg off, took the Gold Valve out and started trying heavier grades of oil starting at 20w and working up. Even with 85wt. gearbox oil the fork behaved like a pogo stick with no discernible rebound damping.

So I striped it down and welded up (Tig) the two 2.5mm rebound holes in the damper rod. Straightened the damper rod, trimmed both ends up in the lathe and checked that the piston end of the damper rod was central in the fork leg when every thing was re-assembled. With a 1mm rebound hole and 10wt Oil the fork leg was no longer a pogo stick and the rebound was visible controlled. Modified the other leg and reassembled it all back in the bike.

Bounced the bike up and down a few times and realised I had made a mistake. What was visibly controlled before had now become slow and sluggish. I had not figured on the weight of the bike. The fork springs were now working against the damping and the bikes weight. I will have to use thinner oil or increase the rebound damper hole size. I think I will go with thinner oil first off and take it for another test ride as soon as I can.

Using an old set of bathroom scales and a bit of arithmatic I figured that I have a set of 54lb/inch and a set of 71lb/inch springs so will try them both in due course.

Its a lot of work but I think I am getting there.

Its a real positive if you can go to a thinner viscosity oil as its less sensitive to changes in ambient temperature, which is essentially what forks run close to. I do suspect though that you may end up having to process at least one small rebound bleed hole.

[bucketracer]

Its a real positive if you can go to a thinner viscosity oil as its less sensitive to changes in ambient temperature, which is essentially what forks run close to. I do suspect though that you may end up having to process at least one small rebound bleed hole.

Thanks RT for the reply and all of your help with this.

The forks presently have a 1mm rebound hole and there will be a chance to run the bike again this weekend. I have an oil mixing chart and will take a range of oils with me so I can progresivly try thinner oils.

As it is I am expecting the forks to packdown over bumps and will thin the oil. If it still packs down I will open the 1mm rebound hole up 0.25 (or less) at a time. I know its an area thing and that 1.25mm has more than 0.25 extra flow of the 1mm hole.

When the rebound is sorted then I can get onto adjusting the Gold Valves for compression damping. Like diving under brakes, jolting over bumps and dipping the front when throttling off into corners.

There is so much juggling that it makes my head spin. Still I am feeling more confident with it.

Thanks again RT.

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[Robert Taylor]

Thanks RT for the reply and all of your help with this.

The forks presently have a 1mm rebound hole and there will be a chance to run the bike again this weekend. I have an oil mixing chart and will take a range of oils with me so I can progresivly try thinner oils.

As it is I am expecting the forks to packdown over bumps and will thin the oil. If it still packs down I will open the 1mm rebound hole up 0.25 (or less) at a time. I know its an area thing and that 1.25mm has more than 0.25 extra flow of the 1mm hole.

When the rebound is sorted then I can get onto adjusting the Gold Valves for compression damping. Like diving under brakes, jolting over bumps and dipping the front when throttling off into corners.

There is so much juggling that it makes my head spin. Still I am feeling more confident with it.

Thanks again RT.

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Yes the reality is that there are so many variables and so many issues to correct with the fork internals. Most people would have thrown their toys out of the cot by now and unfairly maligned the product. But perseverence always rewards!

[bucketracer]

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A friend took my bike out today. The first thing he did was sit astride it and bounce the front up and down. He frowned then said “It’s a bit slow coming backup isn’t it?”

He took it out and blasted around for a few laps and his first comment when he came in was “You sure have to get your knee down fast when the front lets go”.

I am not sure whether I am more depressed because I still haven’t got the front-end right or because the answer is to “Just get your knee down fast” and steer it with the throttle.

Then the clutch shit itself.

F..k-IT I am off to take a Valium and have a lie down.

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