Why the Different rates???

Papakeith

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#1
For the past week or so I've been asking around for recommendations on the proper spring rate for me and my bike.
So far, from three tuners, I've received three different suggestions. They all seem to be fairly close to one another on the fork rates .52-.54.
On the shock rates, however, they range from 4.9- 6.0
Here's what I told them (more or less):
  • weight--------------320
  • skill level-----------intermediate play rider
  • jumping skill------limited, but want ability to catch more air w/o bottoming
  • terrain ridden----50 woods/gravel pits

Why is there so much of a difference of opinion between tuners?
 

marcusgunby

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#2
I would say the recommendation for a 4.9 rear spring is way off so if you ignore that you have similar rates.Ask 5 experts in any given field a question and you will get 5 different answers. :D different tuners will use different valving to suit the springs they say to use.For woods work i beleive they go for lighter damping than for mx where some use light springs/heavy damping.I like the 2nd option but i race mx so i havnt any experience of woods riders needs/likes.
 

Papakeith

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#3
Thanks Marcus,

I am still trying to get a grasp on the basics of suspension.
5.0-6.0 seems to be quite a large range.
I am going to over simplify this for my benefit.
My understanding is that the spring is chosen to support the rider weight, and the sprung weight of the bike with aprox 100mm of sag. Also thrown into the mix is the type of riding that will be done.
In my case valveing will be whatever clicking can be done to the stock systems.

Take a 5.0 rear spring. If I have done my math right it will take 279.4 pounds to compress the spring 1 inch. A 6.0 spring would need 335.2 pounds to compress that same inch. That is almost a 56 pound difference in just the first inch. If what I have typed above is correct(not terribly sure that it is), then after 2 inches of compression there will be over a 111 pound difference in supported weight by the springs.
As I understand spring theory, the next inch of travel takes double what the first inch took. So now at two inches of spring travel I have the bike and me being supported by the spring. This doesn't even include any weight being borne by the fork springs.
Am I anywhere close to how this actually works?
For a non mathematical type of guy these are way to many numbers and calculations.:confused:
Please feel free to correct any(or all) of this.
 

marcusgunby

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#4
dont forget about the linkage-the leverage ratio changes according to travel.The fact that the forks carry some weight makes calculations hard-how much weight do they carry??The linkage ratio isnt a constant(straight line)it stays fairly constant at first then really goes stiff on the last few inches of travel.Im not sure of the correct numbers but what i can say is on a cr i have a 4.7kg/mm springs thats close to correct for my weight (175lb)so a 5.0 will be way soft for you.The 6.0 sounds alot closer.The fact that most reccomend alot stiffer fork springs would tell us we need alot stiffer shock spring.
 

Papakeith

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#5
I had a question about linkage in my last post, but I edited it out. I couldn't put in words what I was thinking in my head without sounding like a complete spaz.:eek:
 

Papakeith

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#7
Thanks for the shot in the a$$. Jeremy. :) Since Marcus' last post I've been mulling things over but wasn't sure how to proceed. I guess the best way is just to keep asking.
Basically, it (my question that is) had to do with the ratio of wheel travel as compared to the amount of compressing that the spring did. My own math spurred the question. I am always hearing people quote the 4 inch sag rule.Then, I saw that within the first two inches of spring travel the shock spring was more than supporting the weight of me and the bike even without trying to figure out how much of a percentage of the total weight was being carried by the fork springs. I was trying to figure out how much wheel travel equaled one inch of spring travel.