Topic: Anyone here that has done a modern upsidedown fork conversion?

[Brent C.]

Anyone here that has done a modern upsidedown fork conversion? With modern brakes dual or single on a SOHC?? I am really toying with the idea. I know you would need a set of custom triple tree clamps, but I have a buddy that owns a custom fab shop and makes stuff for offroad bikes and motocross all the time. I reall think it would be a good looking custom piece. I would do a rear disk coversion later as well. What do you think??

[Geeto67]

I can't think of anybody who has done an upsidedown fork conversion but there are a couple of people here who have done modern sport bike style conversions using CBR and FZR 600 parts. I personally have done a conversion on a kawasaki h1 involving FZR600 front forks which required swapping stems on the triple clamps, and I know it is possible to do with cbr clamps and the cb750 stem.

One thing you might want to look into is the cbr wheel swap the DOHC supersport guys have figured out with their stock forks. You might be able to do something simialr on a SOHC (I know some of the DOHCs use 35mm forks).

I think the cbr rear wheel is probably your best bet because it uses the same axle diameter as the cb750 so it is just a matter of getting everything spaced correctly.

[Brent C.]

Ya, I was thinking of going that way, but wanna keep the spokes....so I might just get some super moto wheels/hubs and a modern motocross front fork and trim the springs and keepers for a shorter travel length. And then have a rear made out of a motocross rear hub and make my own spacers, so I can keep the spokes.

Am I crazy??

[Geeto67]

if you want to keep the spokes then yes I think you are crazy (ok, not really). Half of the advantages of swapping to modern rims is a significant weight reduction, espically since you are reducing rotational mass.

Personally I think if you are going to upgrade then actually improve the function of things, don't just do it for looks. Modern motocross brakes will not be up to the task of slowing 700+ lbs of bike and rider since most of them are designed to slow much less weight and to be as light as possible. The forks are pretty strong themselves but who knows what will happen when you respring them for 700+lbs of bike and rider (you can;t get away with just cutting the springs).

If it weren't for the cost I would suggest looking at harley front ends but you really aren't saving much weight although you can run modern rotors and calipers to improve the braking.

There are motocross upsidedown forks for 35mm that may work in the trees and allow you to keep your wheel but they are really not meant for bikes as heavy as ours (and they cost a fortune).

Outside of ducati I can't think of another bike that uses spoked rims with upsidedown forks (sportclassics) and ducati parts are expensive. Disco Volante had spoke hubs specifically machined to work with Fazer forks, but they are not upside down. However, being they are yamaha you might find some other front ends (yzf-r6 maybe?) that could work with those hubs with some spacing - at least you can run modern thickness brake rotors which opens up a world of caliper possibilities. It will still cost money however and it might even be cheaper to have a machine shop make you a spoke wheel hub for a specific front end.

The bottom line is I don't see any INEXPENSIVE way of getting spoke wheels to work with an upsidedown fork. I'm sure there are solutions but unless you work in a machine shop they are all going to cost a fortune.

Personally I think you have underrated how much thinking and calculation has to go into swaps like this. You are usually decreasing front end size and changing fork size which means your rake and trail will change. You need to figure out your new rake and trail and if it will become dangerous or not, ideally you want them to be close to stock as possible. And that is just the tip of the iceberg.

Edit: a while ago I saw some custom bikes at a show that had lowrider style wire wheels (and only one disc instead of two). That might be an option also but I don;t imagine they would be cheap either.

[Brent C.]

Oh I think a good set of hubs and DID rims will be plenty strong...as a motocross bike can land a 30 foot plus flat landing 20 times in one moto. and if I used a set of oversized supermoto Front brakes that stop near 300lbs XR650R's from triple didget speeds in an eyepoping short distance....will be more than adequate.

Like these bad boys from Magura.



http://www.spieglerusa.com/cfm/super_moto.cfm

[rhinoracer]

Yes I have, and I kept the spoke wheels, put dual R-1 discs w/4 piston calipers. Bike is all in pieces right now because of engine issues.

[FunJimmy]

Brent

Search for fork upgrades and you will find all sorts of info on it.

I'm currently grafting a 2000 Yamaha R6 front end to my CB550 and it may be easier than first thought. The Yamaha steer tube is the same diameter for the lower bearing as the CB550, but the upper steer tube diameter is larger and overall length a little longer. I’m going to use the same taper roller bearings on the top and bottom, but the top bearing will be installed in the head tube using a bearing cup just like a bicycle headset. That will allow me to use the Yamaha triples with alloy steer tube without having to cut the length down.

Interestingly the 1999-2001 R6 triples have almost the exact same offset as the CB550 triples. This should keep the factory geometry as close to the same as possible.

While you are at the lathe making up the upper bearing cup, machine two bearing carrier/disc spacers to sandwich the front hub. These will be the diameter of the CB front hub/disc mating surface and as thick as required to mount 1998 Honda CBR600 F3 rotors.

Calipers are the only thing I’m still working out. Yamaha R6 calipers are too wide and hit the spokes of the CB wheel. I’m looking at using other calipers or machining the mounting ears of the R6 calipers a little, combined with possibly milling a little off the inside of the caliper in efforts to clear the spokes. Must be careful not to breach the piston bores when milling the backs of the calipers though.

Any thoughts?

October 15th 2007: Update to this project at http://www.sohc4.us/forums/index.php?topic=27159.0

FunJimmy

[Geeto67]

Oh I think a good set of hubs and DID rims will be plenty strong...as a motocross bike can land a 30 foot plus flat landing 20 times in one moto. and if I used a set of oversized supermoto Front brakes that stop near 300lbs XR650R's from triple didget speeds in an eyepoping short distance....will be more than adequate.

Like these bad boys from Magura.



http://www.spieglerusa.com/cfm/super_moto.cfm

I don't doubt the wheel is strong - the forks on the other hand....well let's put it this way, while they withstand taking jumps with proper springs, you are going to need to triple the spring strength (can't think of the proper term) which can play all sorts of funky internally with the upsidedown fork housing. You could find that ti will work or you could end up cracking the housing (espically if you have shortened it too). Cutting the springs just to make them shorter however is not going to work for a 500lbs motorcycle no matter how much you want it to (and you will make them softer to boot)

As for the brakes, well maybe but you are now in the custom, let's spend a ton of cash catagory with those magura brakes and you still need a spoke rim to attach them to. Also you may be decreasing your wheel size and adding around 200lbs which can have a significant effect on braking. Also motoX bikes are usually working in low traction situations (dirt, gravel, etc) and usually require more feel than overall stopping power.

It is better to try to work with whole systems rather than mix and match components, so try to get matching forks, brakes, wheel and adapt the whole system to your bike. Also take measurements, you may find a supermoto bike that has forks close to the stock travel of a cb750.

I think you are better off just looking for sportbike front end swaps, which are easier, and canning the spoke rims.

anyway, you are in custom territory and I wish you luck. The secret here is to do a lot of research and take a lot of notes. When I was doing the fzr600 swap I carried a tape measure and calipers with me everywhere and measured every bike I came across.  

[Geeto67]

Yes I have, and I kept the spoke wheels, put dual R-1 discs w/4 piston calipers. Bike is all in pieces right now because of engine issues.

I would love to know how you did this. Where did you get your rims? Did you turn down your brake rotors to work with the calipers?

Calipers are the only thing I’m still working out. Yamaha R6 calipers are too wide and hit the spokes of the CB wheel. I’m looking at using other calipers or machining the mounting ears of the R6 calipers a little, combined with possibly milling a little off the inside of the caliper in efforts to clear the spokes. Must be careful not to breach the piston bores when milling the backs of the calipers though.

Any thoughts?

FunJimmy

yeah, measure your rotor thickness and make sure it will work with the tolerances of the r6 calipers. If you are out of the range or at the top of the range for the r6 rotor thickness you could have a potentially bad situation when the disc heats up. You can turn older rotors down some but this is something you have to be careful with: I had a set of FZR600 floating calipers that someone had turned down to use with different calipers and the rotors cracked when they heated up, on the otherhand kawi triple guys turn down stock h1 and h2 rotors all the time to work with ex500 calipers. Rotor thickness is something most people usually don't think about when they do these kind of things.

[FunJimmy]

Geeto67

Cutting the spring will increase the spring rate.

Fewer spring windings to distribute the forces make the spring rate higher.

[Brent C.]

Oh I think a good set of hubs and DID rims will be plenty strong...as a motocross bike can land a 30 foot plus flat landing 20 times in one moto. and if I used a set of oversized supermoto Front brakes that stop near 300lbs XR650R's from triple didget speeds in an eyepoping short distance....will be more than adequate.

Like these bad boys from Magura.



http://www.spieglerusa.com/cfm/super_moto.cfm

I don't doubt the wheel is strong - the forks on the other hand....well let's put it this way, while they withstand taking jumps with proper springs, you are going to need to triple the spring strength (can't think of the proper term) which can play all sorts of funky internally with the upsidedown fork housing. You could find that ti will work or you could end up cracking the housing (espically if you have shortened it too). Cutting the springs just to make them shorter however is not going to work for a 500lbs motorcycle no matter how much you want it to (and you will make them softer to boot)

As for the brakes, well maybe but you are now in the custom, let's spend a ton of cash catagory with those magura brakes and you still need a spoke rim to attach them to. Also you may be decreasing your wheel size and adding around 200lbs which can have a significant effect on braking. Also motoX bikes are usually working in low traction situations (dirt, gravel, etc) and usually require more feel than overall stopping power.

It is better to try to work with whole systems rather than mix and match components, so try to get matching forks, brakes, wheel and adapt the whole system to your bike. Also take measurements, you may find a supermoto bike that has forks close to the stock travel of a cb750.

I think you are better off just looking for sportbike front end swaps, which are easier, and canning the spoke rims.

anyway, you are in custom territory and I wish you luck. The secret here is to do a lot of research and take a lot of notes. When I was doing the fzr600 swap I carried a tape measure and calipers with me everywhere and measured every bike I came across.  

SuperMoto is on pavement with dirtbikes with EXTREME braking taking place. And you can get anyspring rate you want in a motocross fork. But I like the other Idea of the R6 front end with my spoked wheels. I think I just may go that route!!!

[Geeto67]

Geeto67

Cutting the spring will increase the spring rate.

Fewer spring windings to distribute the forces make the spring rate higher.

increases the rate but decreases the weight it can support. Back when I used to cut springs to lower cars the car's springs rebounded faster but were also easier compressed (since the weight was now evenly distributed over less sping surface area as well). It also wore them out faster. The end result is a car that whose suspension was less firm overall. I'm sure the heat generated in cutting the springs wasn't helpful to them either. 

[rhinoracer]

No, I took the complete frontend of the R1, turned the top of the post to fit the stock CB bearing.

Turned the stock hub to fit custom bearings to Yam spindle (doesn't have a speedo as of yet, haven't decided if going digital or retrofitting the stock unit). Drilled out the discs from the carriers and made custom spacers to fit the lighter R1 discs.

Turning radius is increased because the fatter forks will hit the gas tank so you'll have to modify the stop.

Maybe there's an easier way like FunJimmy's R6, don't know. I didn't research, just bought the frontend and started working on it. Got lucky in that sorta fits.

[babyfood1217]

Hey Rhino, is there any way you could get a pic or two up of the conversion you did/are doing?  I really enjoy the looks of a modern fork on these older bikes, and IMO the sport classic DUC's all got the look down to a T.  -Chris

[wardmoto]

I am surprised Greg @ www.ohiocaferacers.com didn't chime in...probably busy hammering tanks.  I snipped a few photos for you guys...A 350 twin with inverted forks in place that apply themselves to the original post, the 350 that I saw a year ago and my 350F that his 350 inspired. 

[rhinoracer]

Hey Rhino, is there any way you could get a pic or two up of the conversion you did/are doing?  I really enjoy the looks of a modern fork on these older bikes, and IMO the sport classic DUC's all got the look down to a T.  -Chris

Bike's all in pieces at the moment, stripped it to powdercoar the frame, rebuild the engine, paint the gastank and rechrome the rims.

AS soon as it's back together I'll post some pics.

I agree with you about the Sport classics, that's what started my inverted fork project.

[bill440cars]

       Interested in hearing more on this one. Like wardmoto, I'm surprised that Greg (ohiocafedacer) hasn't been here yet. He used a GSXR setup on his 350. Closest that I've come to doing it is the 82' VT750C front end that I'm figuring on using on my 77' CB550K project. It's not an "upsidedown" job but, it does have 39mm tubes, dual disc brakes, air assist and a factory built in fork brace. Doesn't have spoke wheels though. Looking forward to seeing some pictures of rhinoracer's bike and any others who do anything like this.

                                                            Later on, Bill    

[babyfood1217]

Can't wait to see them rhino.  And on the topic of OHIO's bikes, again, great look IMO.  I'm sure some of us recognize the name Tim Shutters.  I wish I could sit down and pick his brain, because what he did to this '78 750F is amzing.  Spokers would be ideal, but still a super clean punctuation to a nice project.

http://www.robswheels.com/tims_other1.htm

-Chris

[Jinxracing]

Geeto67

Cutting the spring will increase the spring rate.

Fewer spring windings to distribute the forces make the spring rate higher.

increases the rate but decreases the weight it can support. Back when I used to cut springs to lower cars the car's springs rebounded faster but were also easier compressed (since the weight was now evenly distributed over less sping surface area as well). It also wore them out faster. The end result is a car that whose suspension was less firm overall. I'm sure the heat generated in cutting the springs wasn't helpful to them either. 

Geeto, I'm not sure I follow. Spring rate is measured in lbs. required to compress the spring a predetermined distance...how is that different from the weight that it can support? I think the problems you had with them wearing out faster may have been completely due to heating the steel when cutting. If spring steel is cut in a way that doesn't cause it to lose its temper, there shouldn't be any accellerated weakening of the material. Also, spring surface area shouldn't have anything to do with it, it's the diameter of the wire, diameter of the spring, and number of active coils that matter.

Here's an article: http://www.tuninglinx.com/html/spring-rate.html

Jinx

[Geeto67]

One of the things that article mentions is what effect sping rate (you did read page 2 right?) and one of those things is the number of coils, which is what you are reducing when you cut srpings.

3. Number of Active Coils (length / height of the coil spring): Determination of the active coil number varies according to spring design. Total coil number -2 for springs with both ends closed. Count the total coils -1 for springs with one end closed and one end open.

As the number of active coil decrease, the spring rate increases:

http://www.tuninglinx.com/html/springs.html

So why did I say it makes the suspensions softer in cars when you cut them? well that has a lot to do with heating up the sping to cut it (which if you do it by torch or by cutoff wheel is going to happen no matter what) and also to do with leverage than with the rate of the sping. Think about a traditional american car A- arm front suspension. The spindle is supported by two levers that press the spring against the frame. AS the suspension compresses the angle of the levers change. Now in a stock suspension when you compress the suspension an inch the car is preloaded beyond the static weight of the car, where as if you cut the spings 1 inch instead, at the same angle of leverage the sping is not preloaded. So in effect by lowering you are changing the overall geometry and the forces involved as well. That is why the cars rebounded well (increased spring rate) but were easier to compress (increased force acting on the spring by changing the angle of the lever).

The shocks also played a significant role, since they are not extending to their almost full length so you are actually working with less fluid dampning, which would also make the cars feel bouncy.

[Jinxracing]

One of the things that article mentions is what effect sping rate (you did read page 2 right?) and one of those things is the number of coils, which is what you are reducing when you cut srpings.

3. Number of Active Coils (length / height of the coil spring): Determination of the active coil number varies according to spring design. Total coil number -2 for springs with both ends closed. Count the total coils -1 for springs with one end closed and one end open.

As the number of active coil decrease, the spring rate increases:

http://www.tuninglinx.com/html/springs.html

So why did I say it makes the suspensions softer in cars when you cut them? well that has a lot to do with heating up the sping to cut it (which if you do it by torch or by cutoff wheel is going to happen no matter what) and also to do with leverage than with the rate of the sping. Think about a traditional american car A- arm front suspension. The spindle is supported by two levers that press the spring against the frame. AS the suspension compresses the angle of the levers change. Now in a stock suspension when you compress the suspension an inch the car is preloaded beyond the static weight of the car, where as if you cut the spings 1 inch instead, at the same angle of leverage the sping is not preloaded. So in effect by lowering you are changing the overall geometry and the forces involved as well. That is why the cars rebounded well (increased spring rate) but were easier to compress (increased force acting on the spring by changing the angle of the lever).

The shocks also played a significant role, since they are not extending to their almost full length so you are actually working with less fluid dampning, which would also make the cars feel bouncy.

Geeto,

I was agreeing with FunJimmy that spring rate does increase when you remove coils...perhaps I didn't get that across clearly in my previous post (and yes I did read page 2). 

I think that if you're careful, work with a wet rag wrapped around the spring, and maybe even keep the majority of the spring submerged in water, it's possible to cut a spring with a cutoff wheel without affecting the temper of the spring steel for much of its wire length (an inch or two of wire length at most).

Your theory on why the springs in your car gave a softer ride when cut makes some sense, but that's a function of that car's suspension geometry giving it more mechanical advantage at the lower ride height. Since a motorcycle fork travels in a linear motion, the suspension won't be affected by those forces. It's an apples-and-oranges comparison when dealing with motorcycle forks vs. A-arm automobile suspensions.

As far as I know, cutting the springs in a motorcycle fork will do two things: increase spring rate and reduce available suspension travel.

Jinx

[Joksa]

Not a Honda, but nice conversion I found
http://www.triumphrat.net/club-cafe/62841-ohlins-front-fork-completed.html

[Jim F]

I know most of you have seen the up dates on my up-to-date braking.
but I don't think Brent has so here ya go.
I like the vintage look, aluminum rims with spokes but with up dated brakes.
my take is this
RC51 rotors with 6 piston ZX6 calipers mounted on a GL1000 front end
of course if i clamp down on these and me being BIG-JIM then you know whats going to happen

[Wayne_Coots]

I just got a 77 cb750 and am looking into what would be required to pull off somethinglike this. Here is a link to what I have in mind. It was done by a company in Japan called Super Build Maximum.

http://www5.ocn.ne.jp/~sbmr0201/

Here is a video of their work. The sound is great!

http://www.youtube.com/watch?v=vLUCjKaTnWg

[XN]

You may be able to swap a VTX inverted fork setup on your bike. FYI the S models have spoke rims, but you may look kinda dumb with a front tire bigger than the rear... + it would not be cheap. There are plenty of cheap stock VTX tripple trees out there to buy....