Asymmetric 3link braking stability

[BillyShope]

As many of you know, I'm a strong advocate of an asymmetric rear suspension for certain applications. Such a suspension, when used with a beam axle rear wheel drive car, is capable of providing, during forward acceleration, equal rear tire loading with no squat or rise. One form of such a suspension, an asymmetric 3link, was used in the Jaguar C-Type, a Le Mans winner (with, of course, a Panhard). What is not often mentioned is that the designer of the car had second thoughts about the design, after the Le Mans win, when further testing revealed certain braking stability problems during "panic" braking. Panic braking involves complete lock-up of the wheels, a condition not encountered in the Le Mans race. It is desirable, then, to provide equal tire loading in BOTH forward and rearward acceleration modes. The normal asmmetric 3link has two symmetrically placed lower links and a third link above and offset to the passenger side (US). Braking symmetry can be achieved with the addition of a fourth link. This can restore link symmetry for equal rear tire loading during braking and yet provide equal tire loading during forward acceleration IF that additional link can only act in compression. The first Ramcharger dragrace car accomplished this with a telescoping link which "bottomed out," but a simpler design would use a front bracket shaped, when viewed from the right side of the car, as a Greek upper case gamma (mirror image of the number seven). This bracket would have two pivot points. One, located at the bottom tip of the vertical section, would attach to the front of the link. The other, at the "corner" of the gamma, would attach to the chassis. During braking, the upper surface of the horizontal portion of the gamma would press against a hard rubber pad situated between the bracket and the chassis. During forward acceleration, the bracket would "swing free," and the link would be unable to carry a tensile load. Thus, the desired situation of equal tire loading, in both braking and forward acceleration, would be achieved.

Another interesting asymmetric arrangement would consist, essentially, of a single "ladder" offset to the passenger side. Again, braking symmetry would require, for instance, a link, mounted offset to the driver side and pointed at the rear tire patch, with a bracket which, in this case, would have the appearance of the number "7" when viewed from the passenger side. This link would be unable to carry compressive loads.

For simplicity, it is assumed that the links exist in planes parallel to the XZ plane.

[mitch_04]

This is interesting, but it is missing something... pictures!

[BillyShope]

Sorry. When I went from Vista to Windows7, I lost my CAD software. Would appreciate it if someone who follows all the verbiage would supply some artwork, but I realize that's a lot to ask.

[rustomatic]

When you can fit a useful, centered upper (single) link under an MG GT, why deal with the potential issues of an offset link? There's also the torque arm...

[BillyShope]

If you're satisfied with the unequal loading of the rear tires during forward acceleration, then there's certainly no reason to go asymmetric. Jaguar, with their C-Type, was not satisfied with a symmetrical 3link; Ford, with their Mustang...recognizing the "potential issue" (singular) of braking stability with full wheel lockup...WAS satisfied. It's simply a matter of priorities. I should explain that, with full wheel lockup, consideration of slip angles and steering input "goes out the window" and everything reduces to sliding friction.

Also, when you consider that the three links are similar to the three legs of a stool, there are an infinite number of combinations which could provide the desired asymmetry. Plus, as you point out (and, as I mentioned in my original post), there are other suspension designs...such as the torque arm and the single ladder bar...which would do the job.

[Ron Sutton]

Billy,



I read something of yours awhile back where you were changing the angle of the lower link & upper links to cancel the torque rotation on launch in drag cars. Is that something you still do & recommend?

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[BillyShope]

Billy,

I read something of yours awhile back where you were changing the angle of the lower link & upper links to cancel the torque rotation on launch in drag cars. Is that something you still do & recommend?

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Yes, the cancellation of the driveshaft torque is the only reason for asymmetry. That driveshaft torque causes all kinds of safety and performance problems. Since the link loads are proportional to the driveshaft torque, the proper setup cancels all values of driveshaft torque. This thread addresses an undesirable "side effect" of asymmetry, which can also be totally eliminated (original post).

[Ron Sutton]

Yes, the cancellation of the driveshaft torque is the only reason for asymmetry. That driveshaft torque causes all kinds of safety and performance problems. Since the link loads are proportional to the driveshaft torque, the proper setup cancels all values of driveshaft torque. This thread addresses an undesirable "side effect" of asymmetry, which can also be totally eliminated (original post).

Hey Billy,

Thanks for the reply. I'm pretty familiar with all the variations of 3-links. As you know, the term asymmetric can be vague, or interpreted a few ways. In your first post on this thread, it sounds like you're referring to an offset 3-link, with parallel lower links & the top link offset to the passenger side.

Is that correct?

Or are you referring to the asymmetric 3-link you recommend in which you have the links at different angles to each other?

[BillyShope]

Hey Billy,

Thanks for the reply. I'm pretty familiar with all the variations of 3-links. As you know, the term asymmetric can be vague, or interpreted a few ways. In your first post on this thread, it sounds like you're referring to an offset 3-link, with parallel lower links & the top link offset to the passenger side.

Is that correct?

Or are you referring to the asymmetric 3-link you recommend in which you have the links at different angles to each other?

I'm generally concerned with any form of asymmetry which provides the necessary cancellation, but, yes, I was referring to the normal 3link in the first post, as you described it.

[Ron Sutton]

I'm generally concerned with any form of asymmetry which provides the necessary cancellation, but, yes, I was referring to the normal 3link in the first post, as you described it.

Thanks for clarifying which version you're discussing here.

[ace_xp2]

I was going to use a 90/10 shock setup with a short internal bump stop for the braking linkage and then a circle track style pull bar for the accelerative one.
But, I've notice that it seems a lot of modern high performance stuff seems to shy away from anti squat of any amount, though this is just going off of arm angles, and independent suspensions, so it's only of so much use. However, Koenigseggs triplex damper setup is specifically described as letting them do away with anti-squats negatives while resisting squat (it's basically a left to right connected damper so that it only works in bump).
Now I'm thinking more along the lines of a low/no anti-squat arrangement, and some way to either increase spring rate when it's time to get serious or some similar damping arrangement to deal with those forces.

[Ron Sutton]

I think you may have misinterpreted the Koenigseggs set-up. They are most definitely using anti-squat, they've just "dampened" it with the 3rd shock.

[David Pozzi]

Billy, The Hotchkis 3 link for first & second gen Camaros has an offset upper link & it has been working very well. We have not encountered any rear lockup problems, however the upper link offset is not extreme. I think it's around 5" - 6" to the right of center. I suggested Hotchkis try it, I'd been wanting to do it for years. I've seen more offset used in 90's Trans-Am cars. Nearly every TA car used it near the end of the series.

[Ron Sutton]

Hey David,

If you don't mind me asking, how much anti-squat have you been running ?

[David Pozzi]

Around 50% Anti-Squat. Maybe a bit below that like 45%. I'd have to look it up. It is adjustable, both ends of the upper link, & also the lower links have a few holes.

[ace_xp2]

I think you may have misinterpreted the Koenigseggs set-up. They are most definitely using anti-squat, they've just "dampened" it with the 3rd shock.

None may be an overstatement,but it doesn't sound like they're using much, He even makes reference to being able to use less and seeing it as an advantage here:
http://www.youtube.com/watch?v=bbgjRBT4ltM
And here's a pic of the carrera gt rear suspension mounts (This really only lets you see that it doesn't look like a ton, how much is difficult to guess.)

And venom gt (not that this is a design masterpiece, rather that it's an example of a car that sure seems to have no a/s, and manages to put down a lot of power apparently without any obvious handling snafus being brought up):

Ortiz talks about the complications that antis can bring here:
http://www.theoryinpracticeengineeri...er/2004_12.pdf
I bring this up because my calculations put my offset link just about over top the lower link to cancel out torque reaction at 100% anti squat, which is about as far over as it can go without tire contact. Now based on the info I've found I'm thinking of anti squat in the low tens to low twenties, and while there would (I think) still be some torque cancellation at this rate, I question how noticeable it would be at that range.

[Ron Sutton]

Around 50% Anti-Squat. Maybe a bit below that like 45%. I'd have to look it up. It is adjustable, both ends of the upper link, & also the lower links have a few holes.

Hi Dave,

I have ran a wide variety of cars anywhere from 30% to 80% anti-squat, but always with the I/C far forward creating a long "swing arm." Within this range, we too have not encountered any uneven braking leading to a yaw condition under brake lock conditions. The cars track true if locked up in a straight line.

I design & build offset 3-links so the amount of top link offset is adjustable ... in the range of 8-12% of trackwidth. (I.E. 5.0-7.5" on a 62.5" track width) I make it adjustable, because it is challenging to accurately predict the frictional drag of various rear ends & gear assemblies. But 8-12% tends to cover the range pretty well.

[David Pozzi]

Here is an old 3 link thread with Trans-Am pics: https://www.pro-touring.com/threads/...=7228#post7228

[feeble]

Would it be correct to say that a car with ABS and an offset 3 link would have less of an issue with braking stability due to the offset link? And as a corollary, cars without ABS would be better off with a centered upper link, at least with regards to braking stability?

At my skill level, (a novice with no experience), I doubt I will be able to notice a difference between centered and offset, so for packaging reasons, I'm centering mine.

[Ron Sutton]

Would it be correct to say that a car with ABS and an offset 3 link would have less of an issue with braking stability due to the offset link? And as a corollary, cars without ABS would be better off with a centered upper link, at least with regards to braking stability?

At my skill level, (a novice with no experience), I doubt I will be able to notice a difference between centered and offset, so for packaging reasons, I'm centering mine.

Shawn,
What Dave & I are saying, is that we have not experienced any braking stability issues with offset 3-links.
Dave said his anti-squat is around 45-50%. I have ran cars from 30%-80%.


Billy,
Do you think anti-squat plays a role in whether or not a track car will experience braking issues?


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