fasttrackfab
fasttrackfab is offline Semi-Pro

Join Date
    Jul 2009
Location
    port huron, michigan
Posts
    126   

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    To calculate motion ratio it is a simple formula:

    mr=(d1/d2)^2

    mr= motion ratio

    d1= distance from lower a arm pivot point on frame to centerline of ball joint
    d2= distance from lower a arm pivot point on frame to cneterline of lower shock mount

    a brief explanation of motion ratio:

    (K)
    A-----B-----P

    P is the pivot point, B is the spring mount, and A is the wheel. Here the motion ration (MR) is 0.5... imagine a spring K that is rated at 10 lb/in placed at B. If you want to move A 1 in vertically upward, B would only move 1in*MR = 0.5 in. Since K is 10 lb/in, and B has only moved 0.5 in, there's a force at B of 5 lb. If you balance the moments about P, you get 5(B)=X(A), and we know B = 0.5A, so you get 5(0.5A) = X(A). A's cancel and you get X=5(0.5)=2.5. Which is 10*MR*MR or 10*(MR)^2.


    now the problem you run into when moving the upper shock mount is changing the angle of the shock, ie.. the angle of the shock to the ground, moving the upper mount more outboard will stand take the shock angle closer to 90 versus moving the upper mount inboard lays the shock down and decreases the angle. What this does is the effects the spring rate needed for the same ride.

    To calculate the spring rate needed go to this site and use their calculator. It also explains where to take measurements from. Much easier than me typing it in here.

    http://www.f-o-a.com/calculator.html

    I am being very brief here. if you have more specific questions please ask.

    Last edited by fasttrackfab; 09-11-2010 at 06:39 AM.

wheel travel measurement

1. put quad on jack stands
2. take front wheel off
3. take shock off
4. remove spring from shock
5. put shock back on without spring
6. measure from the ground to a point on the spindle at full droop
7. cycle the suspension to the max upward postion in the travel
8. measure from the ground to the same point on the spindle
9. reassemble shock and spring, and reinstall

This will give you the max travel out of the suspension. The reason for removing the spring is just for ease of doing this measurement. If you can compress it with it still on the quad than leave it. I find it easier to remove though.

Measure the angle of the shock and full droop, I tend to always use this measurement for calculating spring rates.

Guys just be careful when you cut bump stops and such. Bump stops are actually an integral part of the shock if properly used. They have their own spring rate and can be had in either a liner rate or a progressive rate. You actually design the suspension to utilize the bump stops, they are not just there to stop the shock from bottoming out.

I will try and get the location of the upper mount done tomorrow but cannot promise anything.

Thanks for the pics Shorty, does the spring hit the upper A-Arm when the suspension is at full drop?

Also from the looks of this full drop is less than stock as the shocks are mounted so much higher? Looks like they are a good 3 inches above the stock mounts? Also is this a YFZ450 shock or what is the E2E on this?

More info on leverage ratio:
Called Wayne at PEP yesterday about some other stuff and ended up talking about this as he also runs a LT500 for his duner bike.

Stock LT500 is 2.4/1 with the OEM rims (2/3 rims)
LT500 with 3/3 rims (from FSS) Leverage ratio is 2.5/1

Stock YFZ450 is 1.95/1 with the OEM rims (1/4 rims)
YFZ 450 with 3/3 rims (from FSS) Leverage ratio is 2.25/1

Wayne said running the stock YFZ450 shock will work on the zilla I just need to be mindful of the compressed length of the shock when re-locating the upper perches so it does not bottom out on the frame rails.

Having to make new upper perches will post up when completed.

thanks for all the help

Ok got them welded up and cleaned up, shocks mounted and adjusted, should be painting tomorrow and then finally done and ready to ride.

Here are some shots of the work start to finish & how they look finished on the bike unadjusted.

I'm working on putting LTR-450 shocks on the front of mine. I have to re-do both the frame mounts & the A-arm mounts.

Looks good man planning on doing this on my build but I'm not a metal fabricator at all so I have to see if m buddy that does metal fab will help me with it. You might know someone with a set of 450 shocks for sale? Let me know what they might want.

did moving the mounts up hit the plastic when you put it on?

I made an aluminum support bar and used some energy suspension shock tower mounts to make a 1.25" spacer across the front fender mount, this raised the fenders enough to clear the new shock tower mounts.