I need an expert opinion


3 replies to this topic
  • wearalloyer

Posted May 18, 2009 - 02:56 PM

#1

Hi.This is my first post and i m new to yamaha's.Actually i made a fork rebuilt to my yz450 08' but i need new fork springs and shock sprints too.The point is that i m very confuse about spring rates.So i visited both Racetech.com and Mx-Tech.com calculators in order to find a solution.The results were almost the same but Mx-tech.com recommended for the tall riders to round up 1 full size on the rear spring.For example 5.1kg/mm rider would use 5.4 kg/mm spring rate.
I weight 220lbs w/o gears and i m 6.3.Anyway recommended springs are 0.48 and 5.8 and my question is....does 6.0 help me set up my sag at 100mm without my bike fell a lot stiffer?i dont know exactly the deference instead to use the 5.8 spring.
Also i need some advices about fork oil volume.How much outer chamber fork oil volume is do you use for 08 yz 450 forks and which height level have to prefer?

Sorry for my bad english

  • grayracer513

Posted May 18, 2009 - 03:15 PM

#2

Your English is much better than my Greek, believe me.

With the rear spring, remember that with the shock so far forward, it works at a very great disadvantage. The fork springs work one-to-one against the wheel, but in the rear, the wheel has a lot of leverage over the shock.

This leverage is about 3:1 (just roughly). So, if you take the 5.8 and divide by 3 to get the working spring pressure at the axle, it's 1.93 kg/mm, and the 6.0 is 2.0. Not so different anymore. Only 0.07 Kg, instead of 0.2. In lb/in, the working pressures are 67 and 69, so there's not a lot of difference.

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  • wearalloyer

Posted May 18, 2009 - 03:41 PM

#3

Your English is much better than my Greek, believe me.

With the rear spring, remember that with the shock so far forward, it works at a very great disadvantage. The fork springs work one-to-one against the wheel, but in the rear, the wheel has a lot of leverage over the shock.

This leverage is about 3:1 (just roughly). So, if you take the 5.8 and divide by 3 to get the working spring pressure at the axle, it's 1.93 kg/mm, and the 6.0 is 2.0. Not so different anymore. Only 0.07 Kg, instead of 0.2. In lb/in, the working pressures are 67 and 69, so there's not a lot of difference.


first of all...Thanks for the quick respond Gray.finally which is your advice 5.8 or 6.0?Does 6.0 help me set Sag at 100mm?And please give me some volume and level advice.My rider type is 50% enduro and 50% motocross.I m thinking to use 340cc and 145mm.if i was wrong please correct me.
Anyway your answer makes me sleep good tonight because i was confused a lot about that situation

  • grayracer513

Posted May 18, 2009 - 03:59 PM

#4

I'm not sure on the rear spring, but I tend to lean toward the advice given by MX-Tech when they differ with each other. If they say you should use a 6.0, then try it.

For the oil, on the inner chamber level, you need to understand that the level given, 145mm is not the amount of oil that will be in the cartridge when you have assembled it. There will be some excess that is forced out as you compress the rod the first time, so that the right level is "completely full". You need to be sure that it is at least 145-148mm as you start to put the base valve in. There is no harm in too much, just a little bigger mess. If there is not enough, there will be a problem with air and foam.

For the outer chamber, I say start with 340 and if it bottoms out too easy, or seems too soft, add 5cc/side until you like it better.





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