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Can Opeler
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Discussion Starter #1
So Rally Bob is always talking about the problem of anti squat geometry. I can’t find any threads on here that explore fixing the problem after lowering a GT. (Mine is lowered 1.5”)

I’m having a lot of trouble at autocross spinning my inside? rear tire in corners when I’m trying to power out. It’s infuriating and costs me a lot of time. Rally bob dropped me a little hint that fixing the anti squat geometry would help my situation without the need for an LSD.

I know the basic idea how to fix it, but this thread will be about how to do it right and get the process and experience documented in one place.

Rally Bob; others? Give me some photos, measurements, etc. In return I’ll write an article about the process for the blitz, YouTube, etc.

I just bought a cheap welder I’m ready to get this fixed!!!

Here’s my current setup.

 

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I know you are an engineer, so I'm probably preaching to the choir. Suspension mounting points are critical in terms of transferring load to and from the chassis. More so, side to side, such as a panhard bar mount. Bit scary to hear someone say they bought a cheap welder for a racecar. Seen too many failures in paved oval applications with gumball sticky tires. I mean components literally ripped from underneath the car, right before they hit the wall.
 

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Opel Rallier since 1977
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Hopefully you can get something useful out of the ramblings below.....

With the car being lowered, then the angles of the trailing arms and torque tube have gone lower, which ought to make some level of reduction in anti-squat. (but I have not plotted it out to be sure of that...) So to get it back to where it was, then raise the front mounting point and lower the rear mounting point of the trailing arms, and/or raise the front attachment of the torque tube. The goal would be to increase the slopes/angles on both again. The angle change on the trailing arm looks to have been greater than on the torque tube

Having said all that.... I have to wonder if that is really the prime issue. Is this wheel slippage going on under certain situations? Example: Is this problem more pronounced on long constant radius turn, or when you are in a transient situation, like snapping the car from one direction to the other in a S-turn? The former problem indicates too stiff a rear roll stiffness, due to some combination of too much rear antiway bar and/or too much rear spring stiffness. The latter indicates too stiff of a shock rebound.

What size of rear anti-sway bar do you have right now? And what spring rate in the rear? Shocks?

Another matter to think about is the change in the car's roll axis. When you lowered the car, you lowered the front and rear roll centers, unless you made some other compensating changes When you lowered the rear by 1.5" the rear roll center dropped by by about 0.75". The next question is how much the front roll center changed. I think the front roll center would drop 1" with a 1.5" drop in the front suspension; that is based on a calculator I wrote for the 50 series chassis, so that 1" number is subject to confirmation with Kadett/GT info. So if this is right, that would slightly steepen the roll axis a bit and make the car a bit more 'tail-happy'.

Do you have any adjustment to the panhard rod's end height on the chassis end, so that you can adjust the rear roll center, and thus the roll axis angle? This seems like an easy mod to weld up and implement.

And FWIW.... one thing that I figured out about anti-squat in looking it for rally use and studying how to implement it with a 4 link rear suspension: It can be made to work well in cars that accelerate a lot. But as soon as you hit the brakes hard, it works against you. So great for circle track.... maybe not so useful for road race, autocross, or rally.

And you are welcome to a copy of the front roll center program (in Excel) if you want to play with that .... I make no guarantees, but it would be interesting to put in the Kadett/GT front suspension info and see what you get out of it for roll center changes versus front height changes.
 

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Opeler
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Kyler, I don't really autocross any more, did some 20+ years ago but this is exactly why I stopped running the rear sway bar. I only ran the front heavy sway bar as the GT rear end is pretty light and I found the rear sway bar caused exactly what you describe.....lifting the inside tire.

One day....a long time ago.....in a land far from where I currently live........I was having the inside tire lift issue and very nagging for sure. An easy experiment was to unbolt the rear sway bar links and I actually had better performance. My car was not lowered at that time however.
 

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Opeler
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Also, see this video:

and:


And an even better article. Note the recommendations for road racing:
 

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Opel Rallier since 1977
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Mike, your comments set things up for something I wanted to add about real suspension roll stiffness. The local circle dirt track cars in the tube chassis classes (like Late Model Modified and Sportsman, and perhaps others) have followed a trend in the recent years to make the rear roll stiffness very, very low. The rear axle essentially just 'flops around' while the front takes almost all of the body roll force. It looks goofy on a banked track but it works to keep the rear tires solidly planted. They are using anti-squat and all that, but the real roll stiffness is very low too.
 

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Opeler
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That makes sense Mark. I think there are two different physical actions going on: one being suspension squat and the other being body roll coupled with wheel lift. The two should be treated individually depending on whether the racing environment is straight ahead or autocrossing. I don't claim to be really smart on this, only know that I prefer the GT without the rear sway bar.
 

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Can Opeler
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Discussion Starter #8
That makes sense Mark. I think there are two different physical actions going on: one being suspension squat and the other being body roll coupled with wheel lift. The two should be treated individually depending on whether the racing environment is straight ahead or autocrossing. I don't claim to be really smart on this, only know that I prefer the GT without the rear sway bar.
I’m gathering some info to comment on the other points so far.

But I would bet that you are no longer lifting your rear wheel without the sway bar because you have severely limited your handling capacity by greatly increasing understeer.

I’ve never lifted a rear wheel at autocross before. Understeer still controls on every corner for me. I’ve experimented with running different combinations of sway bars. I can pull the most Gs with only rear, no front bar. The car is easier to rotate too but slaloms are impossible without a front bar. Front sway bar only is worse than nothing on my GT. It understeers like mad but feels tight because it doesn’t roll up front.

My rear is very stiff because I’m trying to get some ability to oversteer a bit. And I can on sweepers, but hairpins I just spin the inside tire and can’t push through the corner and the weight shift to the front when this happens adds understeer on top of it.

I’m trying to understand what anti-squat has to do with this. Rally Bob swears that the anti squat is the issue. To me it sounds like a roll center problem, but I don’t understand most of this stuff. I just go by feel and times.
 

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Can Opeler
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Discussion Starter #9
Here’s a quote from my Facebook post from Rally Bob.

“A safe bet is to change the pickup points by the same amount you lower the suspension. But this merely gets you back close to stock. Realistically you need to tweak the TT front mount and the front and rear trailing arm mounts.
With the right mods you can put down 150 HP without an LSD.
I did this mod to Roger Wilson’s 2.2 GT and he couldn’t believe the difference. Had to relearn driving the car, but it put the power down.


This was the old rear axle from my dad’s GT. But I ended up switching to a Toyota axle because of the power I’d be making, and gave the old one to Gary Farias. I built it back in ‘92, so yea, I’d do things differently now. But you get the idea.



The higher you can get the TT center mount “up” into the chassis, the better. Then I make adjustable upper/lower bump stops to move the actual tube higher.



It allows me to push the TT higher up front. Then I adjust the trailing arms to create a converging angle.”


End quoted material.
 

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Opel Rallier since 1977
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There you go; Bob has implemented what was mentioned in moving the torque tube front mount upwards, as well as increasing the angles of the trailing arms.

One way to think of anti-squat is to imagine the trailing arms and torque tubes being angled up quite a bit. With that being the case, it is not hard to imagine the rear wheels and axle trying to drive forward and jam itself under the body of the car. The twisting force on the axle is opposite to the rotation of the tires, and that will put a lifting force on the front of the torque tube, which can be amplified by the angles selected. When that happens, more weight is momentarily placed on the rear wheels.

The reaction force feels to be primarily at the front of the torque tube, and you will feel the car want to lift if you have enough anti-squat. On my rally Ascona, which was raised a bit from stock ride height for ground clerance over obstacles, that would have put in more anti-squat vs stock. In those unusual cases where you got decent traction on a stage road, you indeed could feel the car rise up when you got up on the torque curve of the engine, in 1st or 2nd gear in particular, where the drivetrain was putting out some good torque. From what I understand, that rise of the car under acceleration means greater than 100% anti-squat.

This may be a useful read for you to see some informed discussion of this:
 

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Opel Rallier since 1977
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BTW, hairpins in rallying have been problems for me too in all the rally cars I've had. So I am not 100% sure more anti-squat will do any good for that situation; the positive effects of anti-quat that I felt were never in hairpins. You ought to ask Bob what corner types were being improved with the anti-squat implementation he described.

FWIW, in rally, the things that helped in hairpins was limited slip and sometimes use of a hydraulic handbrake to lock or partially lock the rears and spin the car into the corner. (And incidentally, part of the reason to use the handbrake is that a decently tight limited slip can cause some severe understeer into a corner if the rear traction is momentarily better than the front traction. That may be more of a rally road problem than a pavement problem.)

The things that helped the most for getting the rear to step out going into corners in general (and thus to allow the power to be used for steering the car) was not roll stiffness in the rear, but more rear braking force. The change to 3/4" (19 mm) rear wheel cylinders was a big step in the right direction for that. The Opels have the typical factory brake bias towards the front and that promotes braking understeer quite a bit when entering corners hard. My son and I did the same on his '65 Barracuda, and it was quite an improvement in the corner entry and exit behavior of the car when driven hard.

Sand rails often use individual rear brake levers in the cockpit to help control the rear wheel slippage. That would keep your hands busy!

Do you use the same tires front and rear?
 

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I've been thinking a lot about this matter (always dangerous LOL!) and wanted to post some more..... FWIW.

Figuring out the IC (instantaneous center) through where the rear axle and trailing arms are transmitting force to the chassis is easy with a 3 or 4 link. The fact that that there are joints at both ends of all the arms means that the force through each will be transmitted only in a direct line through the 2 joints, for each arm. This makes the plotting of the IC easy. Here is the best explanation of how to plot the IC that I have found; see figure 10 and the surrounding text on pages 4 and 5.

Now the torque tube design is not so straight forward to figure the anti-squat effect. There is no joint at the back of the torque tube so any force through there can be transmitted both linearly through the tube's axis, and also in rotation around the axis of the rear axle. So there are 2 components of anti-squat force for the torque tube that need to be separated out.

I think you can do this:
  • Figure the linear force through the IC the same as with a 3 or 4 link, by plotting it from the trailing arm mounting point axis and the torque tube axis. This takes into account the torque tube's linear force component.
  • Realize that there is a 2nd anti-squat component from the rotation force on the torque tube.
How to figure the relative values of the linear and rotational anti-squat forces is not so straightforward. at least for my brain at this point in time. It would take some actual force computations to see the relative levels of these two forces. But one thing I am pretty sure of: If the torque area is long enough, then anti-squat from the linear forces will dominate over the rotational force.

So this leaves you with the situation that you should figure out how to put in some anti-squat by moving the torque tube mounting point up and some combination of moving the trailing arm points so that the trailing arms are sloped up rear-to-front, as Bob has showed you. A guess is that lowering the suspension as has been done took out most or all of the anti-squat that was in the system originally.

That is part of what Bob corrected with his described mods. But there is another important change to rear suspension operation that come with what he did, and which I think you need to consdier very strongly for what you want to do with your car's behavior. I'll post that next.
 

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Opel Rallier since 1977
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Let's think about how the suspension will work with Bob's mods as shown in this pix above, with the torque tube angled up more than stock and that combined with the trailng arm angles setup to induce some anti-squat. The rear of the chassis will rise with the anti-squat, and with the torque tube angled up at some number of degrees, when the chassis rises, the axle moves forward.

Now if you are cornering and allow some body roll to occur, by making sure the rear roll stiffness is not too high, then the inside rear wheel wil lmove forward more than the outside wheel. This angles the rear axle towards the outside of the car and induces oversteer (roll oversteer).

Now think about that: This sounds exactly like what you want to do! You have been trying to get oversteer in the car by adding rear roll stiffness. But you removed rear oversteer when you lowered the car and made the torque tube and trailing arm angles flatter. By stiffening the rear roll stiffness, it just prevents the car from rolling and getting any roll oversteer benefit that might still exist (or might not exist after lowering the car).

There is nothing new in this idea The circle track racers do this all the time. In fact they will often angle the trailing arms on the left side up more steeply than on the right side to get an even greater roll oversteer effect and then adjust around that setup.

So this whole proposition seems to be a win-win change for you. Get the torque tube and trailing arms angles setup right, to get increased anti-squat, and then try softening the rear roll stiffness to get even more rear roll oversteer back into the car.

And... when Bob says the driver of the car with his modded rear suspension had to re-learn how to drive the car..... I am willing to bet as much or more of that came from the increased rear roll oversteer than the anti-squat. Whether this happened when Mike disconnected his rear anti-sway bar on a stock height GT, IDK.

So get that welder busy! And, I would still mod the chassis mounting point of the panhard rod so you can move that up and down too, so as to adjust the rear roll center height. This is what the NASCAR guys are doing when they talk about adding or removing 'wedge' from the rear end. The chassis mount of the panhard rod (or 'track bar') is on a strong, threaded mount. When they insert a long speed wrench down through the rear windor and crank it a few turns one way or another during a pit stop, they are moving the height of the chassis mount of the track bar, and thus changing the rear roll center height.

I hope all of this is of some use.
 

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Opeler
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Sorry I don't have time right now to read all of the responses, but yes I've had the same issues autocrossing my GT. It is a combination of anti-squat and roll centers, or more specifically roll axis. When you lower the front of the GT the suspension geometry causes the front roll center to drop really low, mean while the rear roll center, as determined by where the pan hard rod crosses the differential, still remains high (probably 11" above the ground). This steep roll axis back to front makes the car tip like a tricycle when you corner hard and unloads the inside rear wheel. (If you look close at Bob's picture you'll notice he not only lowered the LCA mounting points on the axle, he also lowered the pan hard rod mount.)

Now I bet you also have more trouble with tight right hand turns than left hand turns. The torque from the drive shaft through the differential tries to lift the right tire and plant the left tire. (remember Newton's laws about forces) If you are running a torque arm instead of a torque tube you can move the torque arm towards the right to combat this force reaction. You really need to read some of Billy Shope stuff on suspension setup. Unfortunately it looks like his old website is down. Copy and paste this web address into the Waybackmachine internet archives to see if you can find it. www.shopeshop.org/contentsDrag.htm He had suspension setup calculators for various types of rear suspension that you could use to figure out your anti-squat as well as achieve equal tire loading on acceleration. Here are a couple of other links discussing is stuff. Link 1: Billy Shope;GM F body torque arm? - Don Terrill’s Speed-Talk
Link 2: anti-squat with a torque arm
Link 3: Just Suspensions By Billy Shope| Grassroots Motorsports forum |

Okay last thing on anti-squat, on a torque arm suspension the anti-squat is going to be determined by the length of the torque arm and the angle of the lower control arms, that's it. If you're running an Opel rear end you can't change your torque arm length so the only wheel you have to turn is the LCA angle. My suggestion is to get some cardboard, straws, and some brass paper pins and construct a model of your rear suspension, then play with it till you understand how it works and how the different links affect each other. Another hint: Lookup rear steer, or roll steer as related to a 3-link or torque arm suspension. (each LCA pushes or pulls the wheel it is attached to forward or back as it travels up and down in an arc) This is also contributing to the front end push.
 

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Opeler
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Here are some visual comparisons of front and rear body roll on my GT compared to some other cars with similar times on course. (It had a 7/8" front bar and no rear bar, running 23" dia Hoosiers road race slicks)
Screenshot_20200419-222856_Gallery.jpg Screenshot_20200421-125452_Chrome.jpg Screenshot_20200419-222551_Gallery.jpg Screenshot_20200419-223618_Gallery.jpg Screenshot_20200419-225256_Gallery.jpg Screenshot_20200419-225753_Gallery.jpg Screenshot_20200419-230334_Gallery.jpg
 

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Opeler
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The GT's wheel track is so much more narrower than modern cars. I'm planning to widen the suspension on my future build which will include the Steinmetz flares. Some good threads on custom changes to front suspension and alternate rear suspensions. Heliman has done some amazing work for example!
 

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The GT's wheel track is so much more narrower than modern cars. I'm planning to widen the suspension on my future build which will include the Steinmetz flares. Some good threads on custom changes to front suspension and alternate rear suspensions. Heliman has done some amazing work for example!
Wider Trak Isuzu Impulse Rear end housing, TT and axels for sale. Freight will run you more than the parts themselves though. Some modification of the spring buckets and/or shock mounts is req.
 

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Opel Rallier since 1977
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Just to follow up a bit on the roll-oversteer topic that Bigben has spoken about too. There was a dirt circle track, late-model race on TV a while ago, and decided to look to see how much this was happening in those cars. Dang! The left wheel was moving forward 8-10" when the cars rolled into the corners! That is some serious roll oversteer. If you get the chance to see this on NBCSports channel, watch for this. it is a huge part of how these cars go around the corners. (And I see the same thing on our local 0.4 mile dirt oval.)
 

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Opel Rallier since 1977
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Sorry I don't have time right now to read all of the responses, but yes I've had the same issues autocrossing my GT. It is a combination of anti-squat and roll centers, or more specifically roll axis. When you lower the front of the GT the suspension geometry causes the front roll center to drop really low, mean while the rear roll center, as determined by where the pan hard rod crosses the differential, still remains high (probably 11" above the ground). This steep roll axis back to front makes the car tip like a tricycle when you corner hard and unloads the inside rear wheel. (If you look close at Bob's picture you'll notice he not only lowered the LCA mounting points on the axle, he also lowered the pan hard rod mount.)

Now I bet you also have more trouble with tight right hand turns than left hand turns. The torque from the drive shaft through the differential tries to lift the right tire and plant the left tire. (remember Newton's laws about forces) If you are running a torque arm instead of a torque tube you can move the torque arm towards the right to combat this force reaction. You really need to read some of Billy Shope stuff on suspension setup. Unfortunately it looks like his old website is down. Copy and paste this web address into the Waybackmachine internet archives to see if you can find it. www.shopeshop.org/contentsDrag.htm He had suspension setup calculators for various types of rear suspension that you could use to figure out your anti-squat as well as achieve equal tire loading on acceleration. Here are a couple of other links discussing is stuff. Link 1: Billy Shope;GM F body torque arm? - Don Terrill’s Speed-Talk
Link 2: anti-squat with a torque arm
Link 3: Just Suspensions By Billy Shope| Grassroots Motorsports forum |

Okay last thing on anti-squat, on a torque arm suspension the anti-squat is going to be determined by the length of the torque arm and the angle of the lower control arms, that's it. If you're running an Opel rear end you can't change your torque arm length so the only wheel you have to turn is the LCA angle. My suggestion is to get some cardboard, straws, and some brass paper pins and construct a model of your rear suspension, then play with it till you understand how it works and how the different links affect each other. Another hint: Lookup rear steer, or roll steer as related to a 3-link or torque arm suspension. (each LCA pushes or pulls the wheel it is attached to forward or back as it travels up and down in an arc) This is also contributing to the front end push.
Mmmmm..... on the torque tube suspension, if the front of the torque tube is fixed reasonably firmly in place and not on a slider, then the angle of the torque tube works with the LCA angle to determine the IC for the linear force component. The IC diagram on the 2nd link above is for a torque arm with slider mount at front (free to move forward and back), so is not correct for the Opel setup, unless the biscuits at the front of the torque tube are very soft or broken, allowing the front of the torque tube to move freely fore and aft.

Now with Knorm's cars, and with solid joints on the LCA's then maybe it gets close to the IC computation shown in that 2nd link. But not with the stock LCA's and bushings. If any forward force is transmitted to the chassis via the torque tube, then it's angle is also part of determining the IC. With LCA bushings in the system, then you can generally locate the IC, but the exact IC can be vague and you end up building adjustments into the system.

Now your idea about moving the contact point of the torque arm is a pretty interesting one. Never read or thought of offsetting that contact point to the side....but it sure makes good sense.
 

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Here are some visual comparisons of front and rear body roll on my GT compared to some other cars with similar times on course. (It had a 7/8" front bar and no rear bar, running 23" dia Hoosiers road race slicks)
View attachment 429510 View attachment 429511 View attachment 429512 View attachment 429513 View attachment 429514 View attachment 429515 View attachment 429517
Seems like you have alot more roll in comparison ... Better track times with the rear sway bars omitted?
 
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