Q about control arm design (non Z related)

[J__]

ok this question if for all you engineering and race junkies out there. my buddy is building his own rwd kit car and is required to fabricate both the front and rear control arms (upper and lower). now there isnt much posted info or forums about it, majority of the info is on the chassis and frame but not the suspension. the question is that is there a certain length the control arm has to be to be ideal? for example a 10 inch control arm opposed to a 15 or 20 inch one. if the width of the front were 40 inches wheel to wheel (wheel base remaining the same), how would it differ between a 20inch wide subframe and 2 10inch control arms subframcompared to a 10inch wide e and 2 15inch control arms. besides the fact that thewill longer control arm allow for more suspension travel, i want to knw if there is an advantage to performance and handling between one another. also what's the effects of having a wider front than rear wheel base, narrower front than rear, and same front and rear? tia

[JMortensen]

The longer the control arms, the less camber change you'll get. Make them as long as you can.

 

Wheelbase is front to back. Wider front track gives better front grip. Wider rear track gives better rear grip, generally speaking.

[boodlefoof]

It sounds like you're talking about an unequal length A-arm setup, and that the car uses an independent rear suspension as well, is that right?

 

If so, there is no real "best" length for the control arms... jmortensen is right that you typically should make the lowers as long as you can fit. The upper arms should usually be slightly shorter (around 2/3 the length of the lowers) to get the best results. Your friend should really invest in a computer modeling program before cutting and welding anything. I used Suspension Analyzer from Performance Trends to model my suspension (front and rear). They actually have a free 10 day demo you can use to try it out. The demo allows full functionality.

 

With an unequal length A-arm setup, there are a lot of variables you want to keep in mind... and sometimes getting them all within the acceptable range simultaneously isn't easy.

 

You will want to at least:

- maintain a low roll center height (RCH)

- minimize lateral movement of the roll center in roll

- design the RCH to move up and down relatively the same amount as the car body in bump and lift to keep your roll axis constant

- keep the swing arm length acceptable (not too short)

- build in anti-dive/anti-squat appropriate to the car's weight, spring rate and wheelbase

- consider how bumpsteer will be affected by your control arm design

- consider the caster appropriate for the car's purpose

- consider the camber curve appropriate for the car's purpose

 

Really, you should design from your steering knuckle/hub inward. Once you have the wheel/tire/knuckle combo set, you will know what your A-arm mounting points are at the knuckle. From there, work back to where your frame mounts should be.

 

What kind of kit is this? Most kits I've seen have a suspension setup already designed to sell with the car.

[tube80z]

There's a lot of good advice above. It's really hard to give any specifics for an open ended question, with little info on how the car will be used.

 

A few things to think about. Most of the software out there doesn't calculate the RC correctly. When you enter numbers for roll it simply moves the car on the centerline and calculates how the RC moves. There's a good reason for this but it can lead to misleading results. RCs move in three dimensions and most of the software doesn't take that into account. And none of the packages that I've seen (cheap ones anyway) allow for compliance in the bushings or the linkage. If you do come up with a design it might be interesting to see what happens on the computer if you move things around an eigth of an inch or so. Sometimes the results are interesting and will lead you to make parts much stiffer in some dimensions.

 

The best advice I could give is don't get to wrapped around a particular RC height or trying to control its movement too precisely. As stated above try and determine what the tires need from a camber point of view and use that to let things fall where they may. It may make more sense to look at changes to the slope of the curves that are generated. In my very limited experience keeping this more or less constant has made my car a little easier to drive.

 

In general a above ground RCs will have car that reacts quicker and heats the tires more from the lateral scrub. You will have a net lift when you corner with a suspension like this. An underground RC will make a car react slower, which sometimes can be good, put less heat into the tires, and will lower the ride height as it corners.

 

I'd try and keep my RC in an envelope that doesn't move more than 20% of the track width, doesn't cross the ground or a wheel during any high force manuvers. Some argue this is BS but again in my limited experience this has helped some stabilit problems I had.

 

It would be best to create a design that allows you to at least change RC height as this is a key tuning tool.

 

Hope that helps,

 

Cary

[J__]

wow! thats ALOT more good info than i was expecting. thnx for all of it. but i have to admit its alot more advanced than both our knwldge is capable of at this moment. im a film student and he's a autobody man, started off working on my Z and decided to build something fun^^ my buddy always loved the lotus super7. there's lots of kits out there thats pretty much the exact car but different name. lo-cost, etc etc. but generally the same car. the kits range from $3000 for the bare frame to $25,000 for a turn key and go kit. there's a wealth of info and blueprints available for the frame online so we downloaded some, picked the best one and drew out the dimensions on a 4x8 foot piece of particle board. put in one the garage floor and started cutting and welding square tubing. the frame took us 3 days and $250 to complete.... non bad from $3000 from the few available manufacturers of these kits. now the suspension... majority of these kits use miata suspension parts, which pretty much includes just the hubs, and steering columns, and whole drivetrain if ur goin with the 4 banger. Now, if you decide to go 4 wheel independent suspension, then u use both front and rear miata hubs? but if u want solid axle in the rear (easier) then u can go front miata, and rear toyota corolla (ae86) rear. With the hubs and miss parts taken care of, now comes the control arms. You can purchase pre-made control arms that everyone uses but its $750 for the front only..... it LOOKS farely easy to make, pretty much like the picture i posted in the other thread asking for the threaded tierod. Although theres lots of info on the frame of this car, there isnt much offered in regards to the dimensions and design of the control arms...... pretty much just pictures. But i'll try to get a hold of that program and play around with it, also do some more research. more info and comments are definately appreciated! tia!