Topic: Shortening damper rods without shortening fork length ?

[spotty]

I've just got a set of XR forks which are 35mm by 960mm , I want to use them on the chopper but with stock stroke I'll probably ground the exhaust out under braking/pothole navigating.
So I've been told to get the damper rods shortened but looking into this most people seem to also be going for shorter fork overall.
Has anyone ( chopper owners, I'm looking at you 😁 ) done this sort of surgery and not lost fork length ?
It also just occurred to me that I could just use 750 damper rods ? Again, does anyone know if they are interchangeable.
I will be using the XR lowers as they have a mount for a real brake caliper, to which a bracket can be attached to use a four piston GSX-R caliper on a VFR disc the same as on the dirtbike.

[bryanj]

What bike are you modifying?

[Alan F.]

The damper piston ring from the sohc cb750s 51437-329-010 is used on a few 70s XL/XR models, it looks to me like you can use 750 dampers.
https://www.cmsnl.com/honda-cb750k8-four-1978-usa_model7244/ring-piston_51437329010/

[spotty]

What bike are you modifying?

its a CB750 chopper with a very raked neck hence the need for long forks, i think they work out to about 12" over stock

[spotty]

The damper piston ring from the sohc cb750s 51437-329-010 is used on a few 70s XL/XR models, it looks to me like you can use 750 dampers.
https://www.cmsnl.com/honda-cb750k8-four-1978-usa_model7244/ring-piston_51437329010/

cheers, that gives me a starting point once i have everything cleaned and ready for building

[HondaMan]

12" is a lot! That's springer territory! 

[newday777]

12" is a lot! That's springer territory! 

Won't the sliders bind up at that extra length and angle to get the frame setting level and the tubes bend from the weight exertion forces?

[HondaMan]

12" is a lot! That's springer territory! 

Won't the sliders bind up at that extra length and angle to get the frame setting level and the tubes bend from the weight exertion forces?

Usually. That's why the springers get used...

[scottly]

What bike are you modifying?

its a CB750 chopper with a very raked neck hence the need for long forks, i think they work out to about 12" over stock

More like 8" over stock.

[Don R]

 I have a raked frame, but I made my forks shorter. I guess I'm weird that way. It's a Turbo'd engine so there aren't any exhaust pipes to drag.

[Medyo Bastos]

I run 8 over with external springs



8 over on the one on the left too

[jakec]

I had a 550 with stock rake, with 10 over tubes (the kickstand was like 18” long). My gf has a 550 with a rake meant for probably 4 over, and she has 8 over tubes. These are stock 550 front ends I’m talking about. And then I now have a 750 with a sportster front end (also Japanese made, similar construction to the cbs, funny how they make the important parts in Japan on those bikes), 10 over. I don’t think the length affects the damping.

Not sure if you’re trying to use xr forks just because you already have them, but you can buy extended fork tubes for like $250.

[Medyo Bastos]

I've just got a set of XR forks which are 35mm by 960mm , I want to use them on the chopper but with stock stroke I'll probably ground the exhaust out under braking/pothole navigating.
So I've been told to get the damper rods shortened but looking into this most people seem to also be going for shorter fork overall.
Has anyone ( chopper owners, I'm looking at you ) done this sort of surgery and not lost fork length ?
It also just occurred to me that I could just use 750 damper rods ? Again, does anyone know if they are interchangeable.
I will be using the XR lowers as they have a mount for a real brake caliper, to which a bracket can be attached to use a four piston GSX-R caliper on a VFR disc the same as on the dirtbike.

Im not understanding why you want to shorten the dampers? Longer fork tubes come with spacers. You reassemble with stock dampers, stock fork springs and the supplied spacers above the springs. No need to modify dampers. Fork travel remains the same! My chops have just enough ground clearance to go over speed bumps. You have to learn to countershift your body when you have to maneuver around obstacles. Choppers don’t handle the same, but you learn to ride them. Give a shout if you need help, and visit hondachopper.com there is invaluable information in the garage section


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[spotty]

Not sure if you’re trying to use xr forks just because you already have them, but you can buy extended fork tubes for like $250.

i did see the 12" overs on fleabay but then i'd be stuck with using the cr4p stock brakes, using the XR tubes and sliders lets me mount a proper caliper which i'll use with a VFR disc

[spotty]

I've just got a set of XR forks which are 35mm by 960mm , I want to use them on the chopper but with stock stroke I'll probably ground the exhaust out under braking/pothole navigating.
So I've been told to get the damper rods shortened but looking into this most people seem to also be going for shorter fork overall.
Has anyone ( chopper owners, I'm looking at you ) done this sort of surgery and not lost fork length ?
It also just occurred to me that I could just use 750 damper rods ? Again, does anyone know if they are interchangeable.
I will be using the XR lowers as they have a mount for a real brake caliper, to which a bracket can be attached to use a four piston GSX-R caliper on a VFR disc the same as on the dirtbike.

Im not understanding why you want to shorten the dampers? Longer fork tubes come with spacers. You reassemble with stock dampers, stock fork springs and the supplied spacers above the springs. No need to modify dampers. Fork travel remains the same! My chops have just enough ground clearance to go over speed bumps. You have to learn to countershift your body when you have to maneuver around obstacles. Choppers don’t handle the same, but you learn to ride them. Give a shout if you need help, and visit hondachopper.com there is invaluable information in the garage section


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if i'm using XR forks they will have maybe 10" of travel, the frame will be sitting pretty low so i'll end up smashing the exhaust everytime the forks bottom out on a pothole or speedhump, i'm probably going to use stock 750 dampers which should sort it.

[scottly]

If you use shorter damper rods, won't it prevent the forks from extending fully, effectively shortening them?

[Alan F.]

Yes it should effectively shorten the forks a little.
Stock travel is what 5.5"?
XR forks being 12" over and originally having 10" of travel, subtract 4.5" for the difference the shorter damper rods make, and you're at 7.5" over at full extension, which is much closer to everyone's suggestions below.

Am I thinking about this correctly? 

I'm going to be modding forks for my RE Interceptor, but I'm going the opposite direction, adding springs and dampers from RE's Himalayan into my stock length forks to increase fork travel a bit without increasing fork length beyond what is absolutely required for that increased travel.

[Medyo Bastos]

I agree Scottly, it would make sense to stiffen up the springs allowing less rebound. Its done with dragbikes all the time


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[scottly]

960mm=37.7852 inches. Stock 750 forks are about 30 inches.

[Alan F.]

You'd definitely need to add preload spacers, length equal to the difference between long and short dampers, and that's if you use the XR springs.

If you want to use stock 750 springs, subtract the 750 spring free length from the XR spring length and add that to the difference between damper lengths, and add that on top of the stock XR preload spacers.

[spotty]

You'd definitely need to add preload spacers, length equal to the difference between long and short dampers, and that's if you use the XR springs.

If you want to use stock 750 springs, subtract the 750 spring free length from the XR spring length and add that to the difference between damper lengths, and add that on top of the stock XR preload spacers.

already thought of that, i'm going to use solid alloy rod rather than tube so that i don't end up with a huge empty area for air to compress in

[Alan F.]

already thought of that, i'm going to use solid alloy rod rather than tube so that i don't end up with a huge empty area for air to compress in

Nice one.

I might be tempted to just fill the forks a few inches from the top with cheap ATF and call it a day.

[Medyo Bastos]

pvc works well as spacers


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[Medyo Bastos]

Use the same amount of fork oil as before. I use 6.5oz of atf regardless of fork length


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