Selecting spring rate

[2strok4fun]

I have some questions regarding spring rate selection.

My buddy just aquired a 1990 RMX 250 (with my help and another board member)

My question is this:

How can I determine spring rate for him? He is a big boy at 210 lbs. The bike feels good for me at 155 lbs.

I know the optimum would be to send the forks and shock for a complete setup from MX tech, but in the interest of getting him riding with the minimal investment, can only the shock spring be changed to get race sag set?
OR
would the forks have to get springs as well?

Current set up is
forks .37
shock 5.2

I am a rookie when it comes to suspension setup (my own incluced) so any help is appreciated.

 

[Barbarian]

If he has never ridden before u might want to leave it alone for awhile so the bike will have lower seat height. Then I would try .42kg/mm fork springs and maybe a 5.6 shock spring. If you are going to increase the shock spring rate that much I think you should increase the fork spring rate also.

 

[Layton]

When I had my RMX I just put stock RM springs in it.

The RMX's are very soft unless you are around 150 lbs. The stock RM springs are naturally a little stiffer and worked out great for me.

Hope this helps.

 

[placelast]

Go to the Race-Tech.com website. You have to go thru their series of questions; just enter jibberish if you do not want to give out pers. info., but do be exact when it comes to weight, type of riding, and skill. Go beyond the Gold Valve promo, scroll around and you will find the stock and recommended rate (if you want to buy another brand) and their part number.

Just winging it the numbers mentioned above seem about right: .43s or so up front and a 5.6 out back should work for him but do verify with the website. I do not recommend he ride it with such light springs as it might headshake like a wet dog.

For about 1/2 price of new, Steve @ Motowest has a collection of used rear springs, or just about any other suspension shop. Just don't get pink or fucia (no joke, he has those colors). I believe the '89/'90 YZ/KX/RM uses the same fork springs (and KYB fork parts) as the '89/'90 RMX; check the online White Bros. catalog to be sure before shopping for used.

 

[2strok4fun]

Hmmm, Placelast, it seems you should know this bike :)

Thanx for the tip on race tech Ill do that. Excuse my ignorance where is Motowest?

BTW, I swapped out the piston on Sat and put a few heat/cool cycles. For reference I took compression before (155psi) and after (198psi) The piston appeared to be in good shape with no evidence of overheat, just blow by, the ring end gap was over spec. Pilot still feels too rich but Ill sort that out after Don gets more time on the top end.

 

[2strok4fun]

DOH! Motowest, it didnt click. That is the Gas Gas dealer a few doors down from Zracing. I found it on Google.

RT sugests .455 front and 5.6 rear.

Funny thing tho, I did it for my KTM and the generater sugusted a 1006.00 for the rear over the stock 8.0, seems a *little* stiff :think:

 

[Barbarian]

.455 seems a bit stiff to me also. Maybe for a 4-stroke but an RMX??