Vintage Dirt Bike Q & A

I swapped an mr175 front end and the rear shocks(both serviced and filled with fresh oil) on my SL100 race bike. Seems kinda ok on normal vintage tracks but when i race the indoor stuff with small (15-20') doubles....it bottoms out bad, so bad both my foot pegs are bend down at terrible angles..on reinforced peg mounts. Anyone have any suggestions for some mods i can do to the front end to stiffen it up? Also, are there set of easily obtainable shocks that swap in place of the early elsinore rears with some sort of compresion dampening...and/or stiffer spring rates? Thanks for any help!!

If you're bending your footpegs you may need to take more drastic measures than the ones I'm suggesting, and it's quite likely one or both of the skinny 31mm MR175 fork tubes are bent, but here goes anyway...

For the forks, you could try preloading the fork springs with plastic sleeve bearings, available from MSC Direct:

http://www1.mscdirect.com/CGI/NNPDFF?PM ... &PMCTLG=00

I used the 3/4"-long #35443068 ones, but any of the 7/8" O.D. units on that catalog page should work. I also had some stiffer springs I got from Vintage Iron kicking around here somewhere; they were too stiff for me, but they might be perfect for your needs.

Increasing the fork oil level (start out maybe 5" or so below the tops of the fork tubes, with the forks fully bottomed and the springs out; this is about 170cc of oil) can create significant internal air pressure near the end of the stroke to resist bottoming; you'd have to experiment to find the best level. I used air fork caps (air pressure alone, no springs) in my CR125 forks for many years, so the forks can take the pressure - just make sure they never 'oil bottom', which could blow out the seals. Using thicker oil will add compression damping (good for preventing bottoming) and also rebound damping, which may or may not be a good thing for your situation.

The same things can be done with the shocks. I used PVC plumbing tubing to add preload to my CR springs at one point, and the shocks could be refilled with heavier oil (5W or 7W, depending on what's in there now) to increase damping. The heavy single springs used on the early CR250's might also be worth a try, if you could find a set.

You're probably aware of these things already, but they're easy to try and to reverse if they don't work. Obviously you can throw all sorts of time and money at the problem with Race Tech fork cartridge emulators, Works Performance shocks, damper rod/shock cartridge mods, 80's fork retrofits, long-travel swingarms, etc., but you might get lucky and these simpler things will be enough.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

I thought of preloading the forks, any idea on what the max pre-load spacer is before coil bind occurs?? Also, these forks have the steel peice c-cliped in the top. I assume i can just drill and tap that peice to add a schrade valve to it? I dont think ive bent the forks yet, but i race with a guy who has bent MR175 forks in the same application. I went on the high side with oil volume from the start but i will double check. How much do you want for those vintage iron springs Ray? I would be interested. Also, i never new the CR250's had heavier springs, i always(wrongly) assumed that all of those old elsinore shocks where identicle!! Course it doesnt help one of my shocks barf's oil every moto. This whole bike is a low buck fun racer, so i hate to go big bucks on the suspension like i do on my evo racers. I just want to give my lower back a break!

I really don't know how much preload space is available before coil bind. You could measure between turns and then multiply that times the number of turns, at least you'll have a ballpark figure.

I wasn't talking about puttting in air valves; I just meant the natural air compression that occurs when the oil level is intentionally run high (you probably knew that). I used the Goki air system, with a single Schrader valve assembly and hoses running to fittings in eack fork cap. It's pretty hard to get both forks identical using two Schrader valves.

I lost my original CR125 springs and told myself that I'd never let that happen again, so now apparently I've put away the Vintage Iron springs so well that I'll never find them. I had listed them on eBay a while back, but I can't find any record of actually having sold them so I believe they're still around here somewhere. I'm pretty sure Vintage Iron still sells them, but I'll keep looking.

I'm working on a source for decent shock seals, and I've also found some Honda gearbox oil seals that may work - they should be in this week. I had zero luck with local hydraulic shops, other than some cheesy snowplow seals I wouldn't put in a garden hose.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Ray,

The method i use to set identicle pressures in fork tubes is to use a regulator to fill them. I use a clip on fitting and just leave it on for a few seconds so i know pressure has equalised then move to the other. I am leary of anymore fittings than neccesary. Thanks for the tips!!!