Rear suspension design

[petawawarace]

Im asking here because you FB guys seem to have a lot of experience building cars similar to whatbim trying to build.

I've started drawing out the rear subframe for my car (bike engined with carbon tub). This is my first IRS car so I'm a little in the dark. I have a wheelbase of 102-103" I've taken some quick measurements off of a Formula Renault to get me in the ballpark, but I can't completely copy it because of differences in engines. I'm looking for answers to a few questions and possibly someone to run a few numbers thru a suspension program (or a link to a good free one). This is going to be a track day car, and if your worried about sharing information, you could PM me and I promise it will stay between us.

I have a pretty clean slate. With the engine I'm using, the side plates should be ~8" outside to outside. I also do have some axles and tripods that I'm planning to use. I'm building my own uprights, wishbones, etc

Questions:
#1. - how much camber gain should be designed into the rear suspension?
#2. - Where should the rear roll center be?
#3. - I'm going to make my own body work (diffuser too). I'm going to make the toe-bar part of the upper wishbone, but is there any other "aero" concerns I should know about??

Thanks again for any help!!!

[petawawarace]

Rough start

[S Lathrop]

Here is an early picture of the rear of a Citation. The suspension mounts to the rear drive housing.

Before I designed this car, I spent a season engineering a Stohr DSR. I learned a lot from that experience and this is what I came up with. I think Lee's approach to the rear end was fundamentally the best I have seen.

Because the CG of the bike engine is so much lower than the Renault was designed around, you will want to lower the roll center to about Swift FF levels, just below 1" with a ride height of about 1.7". I would duplicate the rest of the geometry of the car you have started with because that geometry mates to the front.

[petawawarace]

Thanks for those numbers!!! that's what I'm looking for. I'd love to get dimensions from a Pro Mazda car, as that's the chassis, however, no seems to be able to provide them. I found what looks like a decent free software and I'm going to give it a try and see what I come up with. I have Inventor, and I'm going to see if there is a way I can do it in and assembly.

[S Lathrop]

The rear of a late model V D is quite good. The roll center height will not work at all for you, though.

[petawawarace]

So I've found a setup that physically works with my design. It has a RC of 1 inch, scrub radius of ~1.5" and a Camber gain rate of ~.65 degree per inch of movement. Does this sound reasonable? I'm really not sure about the camber gain numbers. It has a Virtual swing arm length of ~74"

Also, Steve, you mentioned a ride height of 1.7", but you were meaning chassis ride height? Thats what I assumed, but the program I'm using describes the LCA mounting location as the ride height. The LCA mount is at 4.625", but the chassis will be lower than that.

[mousecatcher]

I'd love to get dimensions from a Pro Mazda car

http://starracecars.com/Resources/20...l_Brief_V2.pdf

Last page.

The numbers for the front have some major errors. I don't know if the numbers for the rear are similarly bad or not. But at least it's something you can plug into some software and see if it makes sense.

[petawawarace]

http://starracecars.com/Resources/20...l_Brief_V2.pdf

Last page.

The numbers for the front have some major errors. I don't know if the numbers for the rear are similarly bad or not. But at least it's something you can plug into some software and see if it makes sense.

Thanks, thats great info for sure!!! Ill take a better look at them and see. I just looked quickly, but what major errors did you notice?

[S Lathrop]

So I've found a setup that physically works with my design. It has a RC of 1 inch, scrub radius of ~1.5" and a Camber gain rate of ~.65 degree per inch of movement. Does this sound reasonable? I'm really not sure about the camber gain numbers. It has a Virtual swing arm length of ~74"

Also, Steve, you mentioned a ride height of 1.7", but you were meaning chassis ride height? Thats what I assumed, but the program I'm using describes the LCA mounting location as the ride height. The LCA mount is at 4.625", but the chassis will be lower than that.

The swing arm radius would be better at 100 to 150 inches. I look at roll center height, and lateral migration in roll and bump. In general I want the roll center to move as little as possible as the car rolls and bumps.

With a winged car you need to look at the geometry for ride heights between static ride height and down as much a 1 inch. You want all you variables acceptable within that range.

Your geometry program might have a variable for clearance, referring to ground clearance. That is what I am calling ride height. Many cars, mine included allow for adjustments for the suspension pickups. So you do not want to use a suspension pickup as a ride height measuring point unless it is unmovable.

When I use ride height I am referring to the plane of the bottom of the chassis projected to the center line of the front and rear axels. I use 1" front and 1.75 to 2 inches at the rear. I never measure the car from that point, it is just too much work, so I make various devices to measure ride height from some point on the front and rear of the chassis that is easy and accurate.