3rd link offset??

[JD'sTT55]

Like the title states I'm debating on how much to offset my 3rd link to the passenger side. I've read that offsetting it helps compensate for torque during acceleration but too much may cause it to do the opposite during braking. I realize there are variables for the exact location but what would be a good window to stay between?
Thanks in advance. Jeremy.

[exwestracer]

In a road racing application you will almost always see the top link centered in the chassis. The further you offset it, the more difference you will see in handling and acceleration between right and left hand corners. To get the link to make a considerable difference in acceleration would probably have a serious effect on handling.

[Randy67]

I've seen a couple Trans-Am racing rear suspensions with an offset upper link. Brian Hobaughs (very bad arse car) car has it offset as well: http://www.lateral-g.net/forums/showthread.php4?t=23997 You can see it in post #4 pretty good. I don't think Mike Maier would have done this if it didn't work well.

I have the upper link offset as well, it is just to the left of the housing (not going to try and weld it to cast iron). Once the car is going I will know how it works.

[Bryce]

In a road racing application you will almost always see the top link centered in the chassis. The further you offset it, the more difference you will see in handling and acceleration between right and left hand corners. To get the link to make a considerable difference in acceleration would probably have a serious effect on handling.

Basically these are my thoughts too and the reason I built a centered 3-link.



The jaguar (C-type?) used a offset upper link and was successful. My intermeccanica omega design by mark donahue and holman moody has an offset 3-link.

[exwestracer]

Basically these are my thoughts too and the reason I built a centered 3-link.

The jaguar (C-type?) used a offset upper link and was successful. My intermeccanica omega design by mark donahue and holman moody has an offset 3-link.

The Jags broke the top link mount often. May have been a structural issue, though. Sure, offsetting the top link will work, still not the best from a design standpoint...

[Norm Peterson]

Perhaps fortunately, the "penalty" for offsetting the upper link under braking is less severe than the benefit under acceleration. The effect is based on the amount of longitudinal force that can be developed at the rear wheels, which is going to be at least 55% of vehicle weight during acceleration but only 25% or so under braking. As the tires get grippier, the 55% acceleration term potentially goes up (assuming sufficient power is available) and the 25% braking term would need to go down to avoid overbraking the rear.

With respect to compliance steer effects during either acceleration or braking, I think offsetting the upper link would call for relatively more rigid lower link bushings or (preferably) rod ends.

There is no requirement for an offset upper to cancel 100% of the acceleration torque reaction . . .


Norm

[David Pozzi]

Near the end of the Trans-Am series, I looked over cars racing at Laguna Seca. Most of them used an offset upper link.

[exwestracer]

Near the end of the Trans-Am series, I looked over cars racing at Laguna Seca. Most of them used an offset upper link.

Well, tough to argue with that! David, did you happen to notice if they were all pretty much the same offset? I'm wondering if that had to do with the chassis mfr...?

[David Pozzi]

There was one MFR that built a lot of them. I can't remember the name. I took pics of a 3 link in a Corvette & posted them in this forum several years ago. The front end of the link had a sliding clamp on a roll cage tube. I've also driven a car with the top link offset & it worked well with no adverse effects. The offset was 8" to 10" I would guess.
David

[exwestracer]

There was one MFR that built a lot of them. I can't remember the name. I took pics of a 3 link in a Corvette & posted them in this forum several years ago. The front end of the link had a sliding clamp on a roll cage tube. I've also driven a car with the top link offset & it worked well with no adverse effects. The offset was 8" to 10" I would guess.
David

Possibly Pratt & Miller. They used a unique method of triangulating the windshield area. Kind of a small pic, but I believe I see the vertical tube where the upper link slider mounted. No arguing with their success...

[David Pozzi]

Here is the 3 link thread with pics I took: https://www.pro-touring.com/showthre...n-TA-race-cars

[MadBill]

Hi. I should probably have taken more time to search before making my first post, but since this thread topic exactly matches my question...

How does one determine the required offset for a 3 link in order to reduce/eliminate the unloading of the right tire under power? My gut feel is that it relates to axle ratio, rear track and angle of the (assumed upper) 3rd link, but...

BTW, this is for a road racer, and since virtually all of our courses are right hand, the resultant unloading of the left front under braking will be offset by trail braking-induced weight transfer.

<EDIT>
OK, I've already found more, e.g. Bryce's 65 Falcon thread, and I realize my problem with Billy Shope's calculator (I too got "NaN"s for results) was likely that I was targeting anti-squat values too low for it. [The input I'm getting is that for high powered cars like ours (2150# dry, 600 HP) is that 10-20% is in the ball park. We've already experienced wheel hop under braking with values ~ 35%] I guess I want to know if it can be calculated based only on the slope and location of the upper arm, irrespective of % A-S.

[exwestracer]

I can't see why the split would be anything other than proportional to the percentage of track width that you offset the link.

If you had a travel indicator on each rear shock, you could compare the load/unload travels on a good launch to figure out the rear roll moment. The weight transfer could be expressed as a percentage of total rear axle weight and used to figure out how much offset would just counteract it. I think.

[MadBill]

I S.W.A.G.ed that the way it worked was that the vertical component of the torque-induced load in the 3rd link acts to push down on the right side axle, e.g. if the link was at 30° and the axle torque reaction created say a 2,000 lb. tensile load, the vertical component would be 2000 x sin 30° or 1,000 lb., and so the added load at the right contact patch would be 1000 x (distance from link to left contact patch)/track width. In this scenario, less anti-squat would likely mean a lower angle for the third link and thus more offset required to get the same cancellation force..

[BrianP]

Billy Shope has a website for calculating linkage loads for 3 links with offset linkages.

go to page 18 at http://www.racetec.cc/shope/

[MadBill]

Per a post by Billy in the "3 Link for 65 Falcon" thread, that calculator doesn't work for less than 51% A-S and, depending on the resolution re my thread on calculating A-S, that's more than I want to use.

[MadBill]

Something Ray said a couple of posts back re checking shock travel lit a bulb!

How about this approach for determining percentage:
1. Set the car on stands with the rear coilovers removed and the wheels resting on weigh scales, simulating static ride height.
2. Lock the brakes to prevent wheel rotation and then with a long wrench or a lever bolted to the pinion flange, apply CW torque to the pinion gear.
3. Note the relative change of L & R scale readings from the unloaded case to determine offset effect and adjust link angle and/or lateral location to achieve desired result or to populate a table for future reference.

[exwestracer]

I don't think locking the chassis down on stands will give you an accurate result. In effect, you have eliminated the rear roll center and any possibility of torque roll, which is what you are trying to offset in the first place. Interesting concept... I wonder if there is a way to lock the drive wheels tightly enough and SAFELY "brake stand" the car on the scales?

[Norm Peterson]

I think you'd need a T-handle sort of wrench, so that there are no unbalanced forces being applied to the pinion. Otherwise the force that you apply at the end of whatever you're using for leverage will also contribute to the changes in wheel loads.

I also think you'd want to block the front tires instead of setting the rear brakes. Applying the rear brakes will "short-circuit" the axle torque right back through the axle tubes directly to the pumpkin to resist the pinion's attempt to rotate upward/downward. The torque would remain within the axle assembly, no external link or arm loads would occur from it, and the offset of the upper link or TA wouldn't matter.


Norm

[exwestracer]

I think you'd need a T-handle sort of wrench, so that there are no unbalanced forces being applied to the pinion. Otherwise the force that you apply at the end of whatever you're using for leverage will also contribute to the changes in wheel loads.

I also think you'd want to block the front tires instead of setting the rear brakes. Applying the rear brakes will "short-circuit" the axle torque right back through the axle tubes directly to the pumpkin to resist the pinion's attempt to rotate upward/downward. The torque would remain within the axle assembly, no external link or arm loads would occur from it, and the offset of the upper link or TA wouldn't matter.


Norm

DOH! Good point!