Modern USD dirt bike forks - on the street?

[tree fiddy]

So I've been seeing some cafe racer/flat tracker bikes lately that have modern dirtbike front forks on them. some research shows that the modern japanese showa forks on new dirtbikes are ~46mm (compared to 43mm gsxr/r6/etc forks). They look a lot like the gsxr forks (both made by showa).

Another thing I like about them is the single disc brake (less cluttered cosmetically), and their compatibility with a ton of cool spoked wheels.

Is the internal construction on these forks similar to the street going showa forks? Will i need to totally re-spring and valve them to make it work ok on a street bike? are the lengths similar to vintage bike forks?

example of bike with rmz450 forks:

http://www.bikeexif.com/yamaha-xs650-2

 

[neevo]

As you know the stantion diameter is wider on the dirtbikes. I would also imagine the travel would be too (considerably if I look at my KTM vs GSXR forks). The effort involved in revalving and springing would make me think a good set of USD road forks would be a more suitable option for a Cafe conversion. However they should be good for something like a tracker.

 

[tree fiddy]

I'm noticing that in stock form (on a dirt bike) there is a lot of exposed tube (since they have a lot of travel). on the custom build I posted the link too, there isn't nearly as much exposed tube. so maybe they cut the spring down or something to lower them?

I'm going to do more research on what guys run on supermoto bikes to see if that helps. It seems like they're the same 'cartridge type' fork that is used on street bikes. so if the length can be adjusted for, they might be a good setup.

 

[neevo]

You would want Enduro bike forks then. They are mostly cartridge forks. All the MX bikes have dual chamber as they are better at taking big hits (eg jumps).

 

[neevo]

Just checked the link and that bike is running modified RMZ forks. That would be a bit if work, cutting the tubes, adjusting the rebound rod and god knows what else.

Are you looking for a really tall front end? If not grab some R1/GSXR forks, much less work and I would say much cheaper too.

 

[AlphaDogChoppers]

so maybe they cut the spring down or something to lower them?

One of the most common ways that novice builders completely fuck up a set of forks. There is more to fork lowering than just cutting down the springs, unless you don't care about wallowing, front end dive, bottoming out, and overall bad handling.

 

[tree fiddy]

So how does one "properly" lower a set of forks (aside from sliding them up in the triple tree)?

It seems like these dirtbike forks may be more trouble than they're worth, but they do have enough advantages for me to want to know more about them.

 

[AlphaDogChoppers]

For every inch of preload that you remove from the main springs, you have to add an inch of preload to the rebound spring on the underside of the damping rod. This way, your springs are still under a preload, rather than just being saggy and soft. You will still have some semblance of suspension function, although you will loses some travel which may require just a touch more preload than you had before to prevent bottoming. You also have to consider the amount of oil in the forks. When lowered, there is less room in the forks for oil. You need some air space so that when the forks do bottom out, they are not bottoming out on the oil. In a lot of fork setups, I use MORE oil than the book calls for. This makes the air space smaller, so when the forks are compressed, the pressure inside the fork increases more quickly. This makes the forks more progressive in their spring action, the air being equivalent to a spring. When you lower the forks, you may have to reduce the amount of oil a little. When using longer fork tubes, like with a chopper, we add quite a bit more oil because there is so much more room.

More oil does not change the damping. Damping is adjusted by oil viscosity. The oil quantity controls the progressiveness of the spring action.

 

[tree fiddy]

Cool, that makes sense. Are the rebound & main springs readily available in alternate lengths?

 

[AlphaDogChoppers]

Progressive is an excellent source for fork springs.
Sometimes you can improvise, too. There is often a spacer at the top of the fork which you can shorten or remove, and you can add a spacer to the rebound spring.

http://www.progressivesuspension.com/

 

[clem]

I would tend to believe that just the weight differences between a street bike and a motocross bike would call for re-valving and springs. Two totally different fork setups. Those RMZ forks were designed to take hits from a suprecross track straight off the showroom floor. Now don't get me wrong, there are only a select few that could actually take a stocker and race it competively. They are semi plush with the ability to stiffen up to reduce bottoming but thats for a 230lb bike. A heavier weighted streetbike would blow through the stock suspension like a pogo stick. I could take my 2001 CR250 by the subframe and lift it up on a stand, I couldn't do that with my CB160 unless I wanted an ER visit. Without the ranting, you are correct that it would take more time and money than it is worth in the long run.