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Author Topic:   wheel rate
madnik18
Dirt Full Roller

Total posts: 40
posted August 12, 2005 09:46 AM  
i am trying to find a wheel rate and i am not for sure what to measure...it is not a swing arm ..the part i am confused about is do you measure from the spring to the housing too.. the spring mount is longer then most and my spring set father away...

thanks,

wfoondirt
Dirt Forum Champ
Total posts: 505
posted August 12, 2005 10:07 AM  
To find the motion ratio in ride (bump) you first need to find your instant center and then the svsa length (distance from ic to axle) then you will need the distance from the ic to the spring mount. The motion ratio is then (ic to spring mount/svsa)^2. From there you multiply the spring rate by the motion ratio to find the wheel rate in ride.

The basic formula to find motion ratio in roll is Rear motion ratio in ride * (shock width/track width)^2. Multiply that by the spring rate to find the wheel rate in roll.

Now if you start mounting springs on the arm or birdcages then the calculations get more complex.

xhubby
Dirt Freak

Total posts: 376
posted August 13, 2005 07:29 AM  
Wow! Reading that made me dizzy.LOL.

madnik18
Dirt Full Roller

Total posts: 40
posted August 13, 2005 11:29 AM  
ok thanks.. is there a book "finding wheel rate for dummies"...lol...
i am still stuck on the "IC to axle"..huhhh

thanks

[This message has been edited by madnik18 (edited August 13, 2005).]

Clyde Torkel
Dirt Full Roller

Total posts: 33
posted August 13, 2005 08:11 PM  
Wheel rate is a factor of the spring rate times the motion ratio in the suspension.
INDEPENDANT FRONT SUSPENSION (A-FRAME)
The formula is - the length of the lower a-frame (center of the ball joint to the center pivot inner pivot) divided into the length of the center of the spring mount from the inner mount of the A-frame squared times the spring rate. This will give the wheel rate
An example: Inner mount to center of ball joint - 18". Inner pivot to center of spring mount - 10". Spring rate - 975.
10_
18 =.555 .555x.555 = .308

.308 x 975 =300 WHEEL RATE

STRAIGHT AXLE WHEEL RATE (FRONT OR REAR)
Center of the spring width mount divided by the length of the tread with times the spring rate.
An example: tread center distance - 61". Spring center distance - 35". Spring rate - - 300.
35_
61 = .574 .574 x 300 = 172 WHEEL RATE

Knowing where wheel rates are allows you to know the effects of spring location changes on spring rates.

madnik18
Dirt Full Roller

Total posts: 40
posted August 14, 2005 10:25 PM  
tread center distance.. is that measured from center of tire to the center of the other tire?

spring center distance..center of spring to the other center of spring?

thank,

Clyde Torkel
Dirt Full Roller

Total posts: 33
posted August 15, 2005 06:26 AM  
Yes, center distance is center of tire to center of tire. Same for spring center distance. Sometimes it is difficult to find the center point exactly. You can measure, for instance, right edge to right edge of tire or spring. Measurement will be same as center measurement.

wfoondirt
Dirt Forum Champ
Total posts: 505
posted August 15, 2005 09:05 AM  
quote:
Originally posted by Clyde Torkel:
STRAIGHT AXLE WHEEL RATE (FRONT OR REAR)
Center of the spring width mount divided by the length of the tread with times the spring rate.

This is only accurate if the springs are mounted on top of the rearend. The fore/aft location of the springs can have a pretty big effect on wheel rate.

Mad-
To explain how to find the IC I will need to know what kind of rear suspension you have.

How detailed you get in you calculations is determined by how accurate you want the results to be.


Clyde Torkel
Dirt Full Roller

Total posts: 33
posted August 15, 2005 01:29 PM  
To find the rear wheel rate with the springs in front or behind the axle requires the formula used for the front motion ratio to be used on the rear to find the motion ratio. It would be done using the forward locating point of the arm where the spring rests,the center of the spring and to the center of the axle. Then the center distance of the springs can be used with the adjusted spring rate.

wfoondirt
Dirt Forum Champ
Total posts: 505
posted August 15, 2005 02:33 PM  
You are correct if the spring is mounted to the bar.
If the spring mounts to the rearend housing behind or infront you use the length of the svsa and the formula I referenced above.

In reality this is only scratching the surface of whats required to accurately find the rear wheel rate. No mention has been made of mounting angle, pinion angle chnage, staggering springs, top link compliance, etc etc. Depending on the type of suspension some of these factors have little or no effect and some may have a large effect. Without knowing the particulars of a suspension its impossible to tell which of these factors can be disreguarded because they have little effect on the results. Again it depends upon how accurate you want to be.

madnik18
Dirt Full Roller

Total posts: 40
posted August 15, 2005 10:22 PM  
this is what its all over... my buddy bought a car and i asked him what spring rate he was running he said somthing like a 350rr.. 375 lr.. my first thought was that is high he then told me that the spring are father away from the wheels.. then i got down and notice that they are not that far away from the wheel compared to mine..and mine is 175rr 200lr.. but they are far away from the housing.. the spring mount is longer then mine ..just wondering if that is the reason for the high rates?

its starting to make more since now.
thanks,

Clyde Torkel
Dirt Full Roller

Total posts: 33
posted August 16, 2005 03:34 PM  
WFOONDIRT, CHECK YOUR PM

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