steering geometry

[Richard]

Can anyone tell me the correct fork rake for a 1973 125cc with 35mm forks? The forks and triple clamps I have on this bike were removed from a 1975 175cc (the ones with the tapered interference fit at the upper triple clamp). From measurments I've made, it looks like I have about a 30 degree rake with a 14 inch shock in the center lower shock mount. Can anyone tell me the stock rake for any year of the high frame breather bikes (1973 thru 1975)?

 

[Doug Wilford]

Richard;  If you are using all stock items the degree of rake is correct, these bikes handled like a dream.   If you are looking for quicker steering (which is not recommended) raise the ass end.  Doug

 

[Richard]

Doug, thanks for the responce. I'm sure you guys tried a lot of different shock lengths when developing the bikes, and the advice of someone with as many hours in the seat as you is apperciated!

My '73 125cc now has longer '75 forks and I'm not sure what the shock length should be to maintain the stock steering geometry. What shock length did the later '74-'75 bikes with 35mm fork tubes use?

 

[Chris Brown]

I'm pretty sure they all use 13.5" shocks. Even my 400 has 13.5" shocks even though they're laid down. It has the Marzocci forks with 10-11" travel. I think the rear gets about 9.
Chris

 

[Richard]

Thanks for the reply Chris, I also have a 250MC5 that I bought new and it uses 13.5 inch shocks. The '73 125 came with Girling shocks and I'm not certain but I think they were 12.9 inches long. Can anyone confirm or correct my recolection?

 

[Chris Brown]

Richard,
I'm not sure, but I thought the Girlings just had a shorter body as specified for those models. I'm still pretty sure all of the Pentons came with 13.5" shocks (and a lot of other models in that vintage.)
Chris

 

[Gavin Housh]

Richard.I also have a set of the 75 forks. They came stock with about 8in. of travel. To bring them in to the earlier 35mm rake and travel (vintage legal for AHRMA)you can have a piece of aluminum tube machined 1in. long with inside & outside diminsions approximating the same inside & outside dimensions as the rebound spring on the damping rod. Place this spacer under the rebound spring and reassemble the fork. You'll also have to cut an inch off the fork spring so you can get the fork caps on.I prefer to us another spring on top of the existing return spring rather than the aluminum spacer.What starts to happen with the spacer is it gets beat up when the forks top out leaving the ends rolled over and galled and eventually leaving small pieces of metal in your fork oil. I've been using a return spring from an external sring (72-74.5) maico fork placed under the stock ceriani return spring. With a little carefull grinding on the outside diameter of the maico spring it will fit right in . Use the fork tube to check if it fits, and dont take off too much you want it to fit close. This works very well and in effect gives you 1in. more negative spring, not a bad thing.As far as oil amounts I've been filling to the top of the damper with forks fully extened, start with 200cc and go from there. I have some damping rod modification specs. that some of us have been using out here on the west coast. If anyone is interested I could post them later. On a 73 125 you should use no longer than 13.5 in. rear shocks in the forward most position. Anything longer than that and you have too much chain slack. Even with this set up I've had to make some delrin guide blocks to keep the chain from sawing on the swingarm as well as the brake arm under the chain. I've even made a modern type chain roller near the rear sprocket to help take up some of the chain slack when the suspension is fully extended, it also helps to guide the chain straight onto the sprocket.