Rear Suspension on FWD Drag Race Vehicle

[FFgeoff]

We are building the worst idea ever used in a drag race -- a FWD drag car. Last year the car went 8.8 @ 167 and now we are building a new chassis to the SFI 25.5 Spec.

The question we are contending with currently is how to setup the rear suspension. We are using a 4 link system, traditional setup with parallel lowers and V uppers. The car will have wheelie bars to squat on, however the question i have been losing sleep over, is what can the rear suspension do in a FWD application?

Where should the IC be placed and why? I have my own opinions and would love to hear others...

for some pics of the car check out

http://www.full-race.com/racecar/sounddeadening3.jpg

http://www.full-race.com/racecar/cage3.jpg

http://www.full-race.com/racecar/node1.jpg

 

[BillyShope]

About all it can do is hide! Seriously, on a FWD drag car, you want the wheelie bars as far behind the car as the sanctioning body will allow and the rear wheel rate as high as practicable. In other words, you want the wheelie bars to come into play immediately and you want essentially no load on the rear tires. To visualize what's happening, picture weightless wheelie bars extending 100 inches behind the car, where they're a thousandths of an inch from the track surface. The rear suspension is solid. (Yes, that takes a bit of imagination.) On acceleration, essentially all the vehicle weight would be on the front tires.

Perhaps that didn't help, but I often find an effect easier to understand if I take it to extremes.

 

[NormPeterson]

As far as the first quarter of a mile goes, I don't think that either the rear axle side view or front view IC's matter much. There's no rear axle torque for the SVIC/antisquat to work with, nor is there chassis roll under acceleration with a transverse engine/transaxle setup.

It seems to me that you want as soft a rear road wheel suspension as possible, so that the resulting squat will start loading the "wheelie bars" immediately [ed. perhaps something like "anti-weight transfer bar" describes their function here better]. And you'll likely want some means of easily adjusting rear ride height so that the wheels on the bars are not in contact with the ground at the starting line (I seem to recall that wheelie bars not being permitted to contact the ground with the car at rest being a rule; please correct me if I'm off base here).

Beyond that, I see a trade-off of sorts between the wheelie bar length and its weight and stiffness. There may be some advantage to wheelie bars that have a certain amount of flexibility (probably in some relation to the rear road wheel rate). After all, the lower you can get the sprung mass CG to drop the less weight transfer off your drive wheels occurs.

Thinking as I type, you might even want to work up a spring and shock arrangement for the wheelie bar to keep the impact of the wheelie bar touching down from upsetting front wheel traction. Circle track guys use a similar approach for their 3rd links for a similar reason (avoiding traction loss due to sudden changes in suspension linkage loading). With some sort of travel limiter built in to all of this, you'd be able to set the rear ride height for best overall aero and independently adjust the wheelie bar height. At 8.8 ET's already, all the small things matter.

Norm

 

[NormPeterson]

The second part of it would relate to how it behaves under braking. The thought of possible brake hop with skinny low-drag/low grip rear tires at 100+ mph isn't a pleasant one, so I don't think you want to build in very much anti-lift.

Norm

 

[BillyShope]

Norm, I'll stick with the high rate rear suspension. I'm assuming the front will rise on acceleration, meaning that the car will tend to pivot about the wheelie bars. A high rate rear suspension will then allow the rear tires to unload with a minimum of angular travel.

Also, I was thinking "feet," but typed "inches" when I described the wheelie bar location. I think I've seen FWD drag cars with the wheelie bars about 100 inches behind. This makes my "essentially all" more reasonable. I had originally typed that the wheelie bars were a mile behind, but thought that too much for even a powerful imagination to conceive.

But, as "FFgeoff" pointed out, this is the "worst idea ever used," so, unless you're either a masochist or are sponsored by a FWD dealer, it's best to forget the whole matter.

 

[NormPeterson]

"Worst idea" or not, FWD drag racing is most likely here to stay. And I can see that there could be a particular - some might call it perverse - sense of satisfaction to be had from success in this field of motorsports with half the "normal" number of cylinders and "wrong-wheel" drive. It's kind of an "in your face" attitude toward the established quarter mile fraternity. Close enough, Ffgeoff?

BillyShope - Anyway, we're both looking at taking the rear road wheels out of the picture as much as possible, just from different perspectives (and effective pivot points). Maybe we should be looking at more than just the springs, say rear road wheel springs that are soft with rear road wheel shocks that have very little bump damping but an extreme amount of rebound damping that will let the rear suspension compress easily while "fighting" its extension as much as possible.

Right about now I wish that I'd had a video camera when I attended an import car show and drag race event a couple of years ago, or at least had the presence of mind to pay attention to watching what the chassis were doing during a run rather than by how much anyone was ahead.

Norm

 

[BillyShope]

Don't think you need a video camera, Norm. Next time you drive a FWD car, just hit the brakes and notice what happens at the front end. If it drops, it means that it will rise on acceleration. And, I doubt if you'll find a production car that doesn't drop (or "dive," as it's usually called).

You can design anti-dive geometry into any balljoint equipped car, but it usually has a deleterious effect on ride. Ford, for instance, in the past, has opted for the ride benefits and foregone the opportunity to eliminate excessive brake dive. This was particularly noticeable in the behemoths of the seventies, which would almost scrape their front bumpers on heavy brake application.

This might seem off topic, but the geometry which achieves anti-dive is the same as that which will eliminate rise on acceleration. So, I have a very strong faith in my assumption that the front of a FWD drag car will rise on acceleration, thus causing a rotation about the wheelie bars, making a high rear wheel rate desirable.

 

[funnelguy]

Not a suggestion, just an idea for discussion. Why not limit droop with a purpose built device or internal limiters in the shocks. Spring preload coupled with softer rates?

 

[NormPeterson]

If it is indeed permitted for wheelie bar wheels to be in contact with the ground as you're waiting on the green light I have no problem with stiff rear springs. It's just my understanding that wheelie bars have to be clear of the pavement at the line. Can anyone clarify this point?

If you do need to have clearance, you'll want a little more than just the visible bare minimum on a perfectly flat surface to cover you for pavement unevenness. As the nose rises, the rear will have to squat until the wheelie bar comes into play. And I see that as increasing the amount of weight transfer off the front.

Norm

 

[NormPeterson]

Just to clarify . . .

And I see that as increasing the amount of weight transfer off the front relative to the wheelie bar in contact with the ground at rest case.

Norm

 

[BillyShope]

Now I see from whence you are coming. (My high school english teacher would be proud of me.) Yes, if you have to rotate the car a bit to bring the wheelie bars into contact with the strip surface, it would be best if the rear wheel rate was very low in jounce. Then, to avoid the down side of the low rate, limit straps could be used to give an essentially infinite rate in rebound. No, that isn't going to work, either. If the rear suspension has to be driven very far into jounce in order for the wheelie bars to make contact, that means the front is going to have to continue rising far enough to bring the rear axle back down and into contact with the straps. The "trick" would be to have a mechanism to take up the slack in the straps as the wheelie bars came down and then release the axle later on. Like any sanctioning body would allow that! At this point, I'm getting a good laugh out of all this. But, that's a good part of the enjoyment of bench racing.

 

[shanba]

Just throwing out some ideas. To get the weight off the rear tires onto the front and wheelie bars, you would want the sprung mass to rotate about the rear axle.

1) Front geometry with lots of anti-dive to promote lift under accel combined with soft spring/wheel rates to allow the front to lift, also promoting the wheel center to move toward the vehicle CG to minimize loss of load.
2) Rear geometry with anti-lift (promoting squat under accel) again to increase sprung mass rotation under accel to increase load on the wheelie bar.

It would take some study to see if this would move load to the front wheels, or if it is just moving rear axle weight onto the wheelie bar and not affecting front axle load. Also soft springs with adjustable dampers would bring damping forces into the picture and give more tuning. Maybe a spring system on the wheelie bar to the frame that does the same function as weight distribution bars for trailering?

Another idea would be to put the SVSA instantaneous center below the ground to create a moment (similar to jacking forces in front view with roll center below ground) that would load the front tires. I don't know how practical that would be or other side effects to doing this.

 

[BillyShope]

Unfortunately, the "anti-dive" refers to braking. When accelerating, the same geometry could be called "anti-rise." If you want the car to rise, you DON'T want anti-dive geometry.

As for the rear, it's going to squat and there's nothing you can do about it. It's the thrust loads of a RWD car, acting through the suspension links, that are used to minimize squat or rise.

In short, there's just not a whole lot that can be done to help the FWD drag car. The thrust force is acting forward, meaning the inertial force is acting in the opposite direction. This means, then, that the increased normal loading (weight transfer) is heading south and there's nothing to be done about it! Well, you can minimize it, of course. The center of gravity, for instance, should be kept as low as possible. I saw a FWD car with the rear end jacked way up. The reasoning was that this would tend to rotate the center of gravity over the front wheels. Doesn't take too much pencil and paper work to see that this is a losing proposition. (Although, I must admit, the numbers I used showed it came a whole lot closer to working than I would have imagined. But, it's still better to simply keep the center of gravity as low as possible...i.e., with a production based car.)

I would think this discussion of the problems with a FWD drag car is a whole lot more fun than trying to race one of the things. (Yeah, I'm an old man who's "stuck" back in the fifties. Although I was part of a factory racing team, I lost a personal interest in drag racing when they fired the flagmen and brought in the tree and bracket "racing.&quot

 

[NormPeterson]

Correct me if I'm totally out in left field here, but I don't think you want much "anti-rise", perhaps none at all, or maybe even "pro-rise". I see anti-rise as temporarily lifting load off the front wheels through the linkage geometry without the benefit of spring/shock extension over whatever time that takes. IOW, immediate, abrupt, upsetting to traction, and right at the instant of launch. It's probably analogous to rear brake hop from too much anti-lift at the rear.

Maybe somewhat stiffer springs and front shocks that don’t have too much damping in either direction is a better compromise. While you don’t want to use the front suspension linkage (think side view) to resist rise, neither do you want so much rise that the sprung mass CG rises appreciably after the bars have touched down, at least not in the lower gears.

At the rear, well, I’m still trying to think a little “out of the box”. While I’m also certain that any sanctioning body would take a dim view of any active means of unloading the rear road wheels (e.g. air chambers or hydraulic rams), it’s kind of where I was going with the soft bump damping and really stiff rebound damping on the rear shocks idea in my earlier post. Maybe even entailing some sort of position-sensitive design a la "Sensa-traks" but with a dragstrip-unique groove profile over the shock stroke.

Or perhaps a rotary friction shock could be developed (re-invented!) that utilizes a ratcheting mechanism that drives the moveable friction plate(s) to resist rebound only while letting the bump direction either overrun or drive a low-friction assembly. Adjust the rebound friction such that the spring only overcomes it by a little bit. The hydraulic equivalent would be something like 90/10 front shocks for RWD cars with the dampings reversed but could be made even more extreme. You'd have lots of damping against body rise/wheel droop but very little working against the opposite motion, so that might pass muster.

Norm
AARP-eligible kid from the sports car side of the tracks

 

[BillyShope]

Since I prefer a no squat/no rise setup for a RWD drag car, I believe I'd prefer the equivalent for a FWD car. It's just a matter of avoiding those nasty transients. While the area under the curve might be the same, we know it never works out to be equivalent. (Pardon the engineering shorthand.) And, we want to keep that CG low.

Of course, racers of RWD cars like to see the rear drop on acceleration. Most don't realize that, if the sprung mass is being sucked down, the unsprung's being lifted. Those that do sometimes opt for a rise setup. But, what goes up must come down, as some wise man said, and you're back to the nasty transients.

No, whether FWD or RWD, it's best if the car just proceeds in the X direction, with a minimum of bobbing up and down in Z.

 

[mburgess]

Would there be a benifit in using inboard mounted coil over shock absorbers with a rising rate linkage similar to what is used in an open wheel circuit car. Using a competition style shock from Ohlins or Penske would allow a great deal of control over spring rate & give independent control over compression & rebound dampening. A shock with alot of rebound damping would allow the back of the car to pack down under weight transfer. The import drag scene is fairly strong in Australia. I have even seen some front wheel drive drag cars with some small wheelie bars behind the front wheels.
Regards,
MB