Shope asymmetrical 3-link

[dwilliams]

I've been looking at Bill Shope's asymmetrical 3-link stuff:

http://home.earthlink.net/~whshope/id16.html

Bill, I have a 1979 Mazda RX7 that I use for autocross and drag racing. I've also done a couple of long-track events with it. It has a conventional McPherson strut front end, slowly lowered, which puts the front roll center near ground level. I haven't measured everything exactly since the roll center moves about quite a bit on bump and roll.

The rear suspension is coils, a four link, and a Watts link. The two lower control arms are horizonal to the ground and parallel to the car centerline. The two upper arms are shorter and angled slightly inward toward the front.

The Watts link is unusual; it is mounted ahead of the axle and on the passenger side, with the pivot at axle height. This means the roll axis is skewed diagonally front to back. My best guess is that it was done for packaging reasons; the car was made in both left and right hand drive, so I doubt it was to offset the driver's weight. The offset doesn't seem to be an issue with the RX road racer types, and I can't feel anything odd while driving it.

The car has a 302 Ford, a C4 automatic, and the stock 3.91 Mazda axle. I have a GM rear end on hand to swap in. Since I will have to make new brackets, it would be an appropriate time to alter the rear end geometry.

The car weighs around 2500 pounds, and I have traction problems.

Q1: would there be any problems running the asymmetrical 3-link on an autocross or road course? Specifically, would brake hop be an issue?

Q2: your illustration shows a Panhard running from the body on the right to the axle on the left. Is this a generic locating device for purposes of your illustration, or do you have a specific recommendation for axle location?

Q3: On previous autocross cars I mounted the Panhard as low as possible to lower the rear roll center, which I thought would help keep some weight on the inside rear tire during turns. Is a low rear roll center appropriate for a drag car?
If not, what do you recommend? For that matter, would an asymmetrical roll axis be useful for a drag car?

[BillyShope]

Q1: would there be any problems running the asymmetrical 3-link on an autocross or road course? Specifically, would brake hop be an issue?

This setup was used on the early (pre IRS) C-Type Jaguars. Need I say more?

Q2: your illustration shows a Panhard running from the body on the right to the axle on the left. Is this a generic locating device for purposes of your illustration, or do you have a specific recommendation for axle location?

This was an illustration from "Race Car Vehicle Dynamics." You can use the mirror image if it's more convenient. Doesn't matter. In my opinion, the Watts is totally unnecessary. Just calculate the amount of sideways motion with a Panhard. The Watts could be justified with an offroad vehicle with yards of suspension travel, but, in your application, I think it would be a waste of effort and money.

Q3: On previous autocross cars I mounted the Panhard as low as possible to lower the rear roll center, which I thought would help keep some weight on the inside rear tire during turns. Is a low rear roll center appropriate for a drag car?
If not, what do you recommend? For that matter, would an asymmetrical roll axis be useful for a drag car?

The roll center height and roll stiffness combine to influence the fore and aft wheel load distribution in a turn. Low rate springs and a high roll center, in other words, might yield the same handling as high rate springs and a low roll center. For a drag car, it doesn't make any difference.

With an asymmetrical link arrangement, you are not relying on force transmission through the suspension springs for driveshaft torque cancellation, so an asymmetric roll axis would never be necessary.

Hope this helps.

[dwilliams]

>This setup was used on the early (pre IRS) C-Type Jaguars. Need I say more?

I think I get the hint!

> This was an illustration from "Race Car Vehicle Dynamics." You can use the mirror image if it's more convenient. Doesn't matter.

10-4.

> In my opinion, the Watts is totally unnecessary. Just calculate the amount of sideways motion with a Panhard.

I've always used a Panhard before. I didn't bother to calculate it; I used my square and a ruler and found less than 1/8" lateral motion. I figured that was just fine.

In this case the car already has a Watts, with big beefy brackets already attached to the chassis... but all the Watts linkage is right where the third link needs to go. Looks like I'll be doing more Panhard bracketry.

Thanks, Bill!

[PackardV8]

Hi, dwilliams,

The two things I didn't see are:

1. You seem to have a lot of experience in this area. Specifically what problem does your OEM suspension have you are trying to solve? Why cannot work with the current suspension get the job done?

2. Does Bill agree the assymetrical three-link is the only or best solution?

FWIW, I don't see a cost-benefit ratio here. As you said earlier, many/most Mazda road racers and drag racers get good results from the suspension you have. In many years of being around this stuff, I have seen more cars hurt/scrapped by cutting apart the suspension than from most any other modification. Is every other component/combination already maxed out? If not, save the rear suspension replacement for last.

thnx, jv.

[dwilliams]

Why cannot work with the current suspension get the job done?
...
If not, save the rear suspension replacement for last.

The RX7 has a 3.91 peg-leg rear end. The ratio seems to be close to optimal, but options for differentials are few and very expensive. It also has drum brakes and a unique Mazda wheel bolt pattern, not used by any other car, that limits wheel availability. I also have a cobbled-up driveshaft that's Ford on one end and Mazda on the other.

I already have a Corvette brake kit for the front, which takes small Chevy 5-on-4-3/4 wheels, cheaply available anywhere.

I already have a G-body 7.5" rear end, which is the exact same width as the Mazda. I have a Camaro rear disc brake setup to fit it, and it has the same bolt pattern as the new front hubs. Gear and differential selection for the 7.5 are fairly good, and on a modestly powered, lightweight car it should be strong enough.

Since I will have to make all new bracketry anyway, I am picking Bill's brain to see if there are any advantages to doing things a bit differently.

"Remember, every corner ends in a holeshot."