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Technical question on spring rates

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I have an 08 YZ250F and am trying to get the suspension setup for my son. I have adjusted the settings and the bike seems to track pretty well but I am not sure if it can be better.

Details:

Motocross

140lbs

B level rider

Race tech web sites says the following for his weight:

Front fork- .414kg/mm (stock is .440kg/mm) off by .026kg/mm

Race tech available rates are .40,.42,.44

Rear shock - 4.64kg/mm (stock is 4.9kg/mm) off by .26kg/mm

Race tech available rates are 4.6,4.8,5.0

Question: Can the front and rear adjusters compensate for being off by one spring rate on the front forks and also the rear shock? or will getting the proper spring rates as suggested by race tech make that much of a difference for a B level rider?

Thanks

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Springs hold the weight of the bike and rider at the proper attitude. Damping controls the springs. So, no - you cannot add or subtract spring rate with the damping adjusters.

Is he 140 in gear, or street clothes? What kind of sag/pre-load numbers do you have with the stock shock spring?

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Question: Can the front and rear adjusters compensate for being off by one spring rate on the front forks and also the rear shock?

no

springs are for the weight of the rider and valving is for his/her ability

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RKS1 - he is ~140 in gear but is 16 yrs old and growing as we speak. I will have to recheck the sag from last fall since he has put on a little weight but I believe it was not quite 4 inches.

I just was not sure how much difference one spring will make? and if it is worth the investment?

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those rates are big enough difference to be worth changing to.

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IMO, I'd leave them alone and save the money for all the things you will need for a teenage racer soon enough. You need to round up to the next size, meaning .42 and 4.8. Those are very close to the stock springs, and he will grow into them. There is a plus or minus 5% on most springs, you could even end up with a stiffer rear spring then you already have. Set the sag, then check the static sag and let us know. If it's too far off, I suggest getting to know a good tuner in your area, they can test yours and make a better decision, and often will have a used one thats perfect. They always have plenty of soft/stock springs, because most of us have not seen 140#s for a very long time.

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i am 165 and get good sag numbers with a std spring...you could leave it but the sag numbers will be off, if he is fast thats probably ok, if he is b class(i dont know what that means), it maybe too stiff.

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i am 165 and get good sag numbers with a std spring...you could leave it but the sag numbers will be off, if he is fast thats probably ok, if he is b class(i dont know what that means), it maybe too stiff.

A= expert

B= intermediate

C= novice

:lol:

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I was reading in a different post that the 08 has a 5.2 not a 4.8 shock spring, so I went on Yamaha parts diagrams to check and sure enough ...5.2 up from 4.8 on the 07. With that said, the calculator says he should have a 4.64 now the difference is .52

I will check the sags and repost.

BTW...thanks for the inputs! and yes he is fast and at amateur level which is "b" in our race club.

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Actually - the 08 YZ250F comes with 5.3kg spring in the rear. (Check pg 7-11 of your owner's service manual. You'll also see that is is etched "5.3" on the bottom of the spring when you take it off the shock.)

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