LocostUSA.com

I was able to get a copy of the Brunton worksheet for GAZ coilovers. I don't think I fully understand it yet. Here are two screenshot graphics from the working copy I have.



The green fields are ones you can alter to suit. The documentation says the use 33.333 Droop% as a start, which isn't really explained. It also says the red cell droop number (D17) is the amount the suspension will compress with any given frequency. Why that is true isn't clear to me.

I chose Droop% = 40 because that's what gave me 2" droop and 3" bump, which is the standard Locost assumption. My coilover angle is 45 degrees. Looking at the part of the spreadsheet that gives "Spring corrected for tilt and motion ratio", i.e., the spring you would actually buy (?), I get 757 lb which is about twice what most guys seem to run. So, I'm not sure I'm understanding this worksheet correctly.

My weight estimate is pretty conservative, and likely on the high side. It includes 2 passengers, full interior, luggage, insulation and the spare tire and it's carrier. So, I'm going to go back and re-examine it and alter values if indicated. I've used 80% of estimated weight at the tire contact patch as the sprung weight figure. That's the 378 input.

The other confusing thing is the rightmost table, "Available Shock lengths and specs." For the coilover with 5" of travel (the amount of travel I'd need) it says Rideheight = 13.0. Is that the length of the the coilover at ride height? It would seem to fit with a 2" droop (extended) and a 3" bump (compression), but not explicitly clear here.

I'm confused.

Is anyone more familiar with this worksheet than I am?

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886

I haven't used this one before, but from what I'm seeing, I can't say that I'm much of a fan of it.

Droop is simply the amount that your suspension is compressed at rest. Another way to think about is the amount of travel you'll get from 1G worth of compressive force.

The 33% target that many people have historically used is for at the shock, not at the wheel. However, the 'req travel' shown is for the wheel, not the shock. Make it 6, and your 'droop %' to 33, it'll get your droop back to 2" and you should get the same spring results. The more important thing that those fields produce is your frequency in 'cpm' and 'Hz'. I can find it later, but there is a direct mathematical relationship between the sprung natural frequency and the amount of travel from 1G worth of compressive force.

The 'rideheight' column looks to be trying to tell you the effects of the shock travel on travel at the wheel, assuming that the spring is initially set to be fully extended at full shock extension. This makes the 'bump' travel (and ultimately 'droop %') in the calcs pretty meaningless, since the motion ratio, shock travel, and bump stop length, will be what actually determines that.

I'd set the 'droop %' to 50, and use the 'req travel' to adjust your outputs. Using that, try 'req travel' of 5 an 6 and see if that starts getting the spring rate closer to your expectation.

Also note that D2 is actually supposed to go to the lower ball joint, not the centerline of the wheel. There is some effect from the difference between the two, but unless you're running very short virtual swing arm lengths, it's relatively minor.

Sorry I haven't reposted those other files yet. Apparently it might be on an old computer I need to dig out of storage... Which is a good idea anyway, since there are a lot of files like those that should be transferred over so they don't get lost.

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-Justin

"Orville Wright did not have a pilots license." - Gordon MacKenzie

@Driven5
Thanks for those clarifications, Justin.

I had a formula from another source (Bob Bolles' book) that did have D2 as you've described it. I do have the horizontal distance from my lower ball joint center to the center of the lower coilover mount hole. So, that will be an easy re-calc, just add that distance to D1 yielding the new D2.

Thanks again,

_________________
Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886

I went back and took a hard look at my weight estimate and modified some items making it as realistic as I can at this point. Then, I took Justin's suggestions and applied those. The results are closer to what I'd expect based on what others have arrived at for lighter builds.

My build is going to have thermal insulation, rugs and a full interior plus the chassis (now completed) is significantly more robust and heavier than book chassis, so weightier.

Here are the new results. They just apply to the front coilover springs.

Cheers,

_________________
Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886