Suspension spreadsheet & dimensions

[Guest Seefo]

Hi all,

 

(I hope I am posting this in the right area...) We are building an SC300 to race (road race) In chumpcar/WRL and possibly in NASA PT. As part of that, I took some measurements of suspension (motion rato, sway bar, corner weights, etc). to put together and calculate our roll stiffness and front roll couple (FRC). Since there is a lot of similarities between our cars, I wanted to share this for anyone with similar goals and/or get feedback from everyone.

 

This spreadsheet could help you figure out your "gixxer_drew" setup for example.

 

The motion ratios I calculated are:

 

Front Shock: .619

Front Sway Bar: .752

Rear Shock: .721

Rear Sway: .535

 

How To use it:

-The blue section covers springs. The first two columns are the front and rear spring rate. Put your new spring rates in those columns (Highlighted in black). Then look at your "T Roll stiffness" and "FRC" (Highlighted in Red). these are the values you compare with a different setup.

https://drive.google.com/uc?export=download&id=0B4yHVDDToOBzeXhuY0pBd2N5WVE

 

-You can see the Natural Frequency (highlighted in green) of your springs here. These are affected by corner weights which are located in the upper blue section. I am assuming the car has a equal weight distribution on both sides, which is not really a bad assumption. Our car as 51% left/right and 50% cross weight.

https://drive.google.com/uc?export=download&id=0B4yHVDDToOBzelo1dHBpSDI5REE

 

-The Orange section covers sway bars. These can be a bit more complicated since most of the sway bars I find are tubular. In that case you need the sway bar thickness, wall thickness, and arm length. I haven't found the wall thickness for most sway bars, so you are on your own there. Arm length is usually the same as OEM, but sometimes they are different.

https://drive.google.com/uc?export=download&id=0B4yHVDDToOBzR0ExWFRudFk0bW8

Limitations:

-The sway bar measurements, particularly bar length may not be accurate.

-The sway bars on these cars are not straight so each bend will have some bending deflection and will affect the overall stiffness. I don't take these into account, but I am planning to fix that at a later date.

-Lastly, as I mentioned above, sway bar wall thickness has not been measured and is based off an old thread I found on the thickness of the supra sway bars, it may not be correct. I am waiting on a spare so I can drill and measure it.

-Motion ratio inaccuracy can have a drastic effect, I measured them 3 times at a minimum of 60mm of motion so I think they are pretty close, but .01-.02 mistake front and rear is not minor. (so .59 and .74 would definitely be a big difference in the balance). You can see my measurements in the red section underneath the actual spreadsheet.

-Rear motion ratio probably doesn't apply to a Supra. If anyone wants to measure their motion ratios, and share them that would be greatly appreciated!

 

Realistically, this spreadsheet should be used based on a good before setup, rather than for absolute measurements. In other words, I suggest you plug in some numbers that you liked before, then increase the spring rate/sway bar to compare a new setup you want to try.

[Chris Wilson]

Tubular anti roll bar wall thickness has a surprisingly small effect on torsional stiffness, in relation to OD, so I would say your spread sheet is still useful if just the OD is known. (Skin effect). Of course anti roll bar stiffness effectively changes dynamically as the link rod to arm angle changes with motion.

 

My calculations for front and rear spring ratios concur pretty much with your own. Thanks for sharing. You will find roll centre heights and position vary alarmingly with lowered ride height, especially at the rear, probably due to compromises dictated by Toyota wanting a reasonable rear load area. The top and bottom arms are really too close together for good rear suspension kinematics. Good luck with the build.

[Guest Seefo]

Tubular anti roll bar wall thickness has a surprisingly small effect on torsional stiffness, in relation to OD, so I would say your spread sheet is still useful if just the OD is known. (Skin effect). Of course anti roll bar stiffness effectively changes dynamically as the link rod to arm angle changes with motion.

 

My calculations for front and rear spring ratios concur pretty much with your own. Thanks for sharing. You will find roll centre heights and position vary alarmingly with lowered ride height, especially at the rear, probably due to compromises dictated by Toyota wanting a reasonable rear load area. The top and bottom arms are really too close together for good rear suspension kinematics. Good luck with the build.

 

I agree about the sway bar stiffness. Agree about the dynamic change also. On my other car, I set the angle (adjustable endlinks) to be pretty close to 90* when compressed, when looking from the side of car. As long as your suspension travel is relatively low (as you would see on most race cars), you can stay pretty consistent. Unfortunately, for some of the series we run, adjustable end-links are a points mod and probably not worth it.

 

Interesting info on the roll centre. This is my second time trying to model that, so I am not sure how accurate my numbers will be, but I will try to post them.