Car pulls to the left under wot

[Mag]

I have a MK4 with a 427W 535 hp 545 torque with a Moser rear axle with 3link and 3.55 gears. The alignment is good. Car tracks straight as an arrow at highway speeds and under moderate acceleration. But when I mash it it pulls to the left. Even if there is no wheel spin. Wonder if anyone has had this issue and how they solved it. I do have Breeze adjustable rear control arms set at factory length. Everything else in the suspension is stock Factory 5.

[FLPBFoot]

Have you had a 4 wheel alignment? My alignment shop wouldn't do the front until I fixed my bad thrust angle issue in the rear. With the FFR cast AL arms I had a 3/8" offset, not sure what the degrees were but they wouldn't touch it until fixed. I went with the Breeze adjustable rear control arms and they made sure the rear thrust angle was dead on straight then did the front.

[BEAR-AvHistory]

When you say it pulls to the left is it the front tires pulling left or the cars rear tires hooking left?

[GoDadGo]

Questions:
What time of rear differential do you have within your 3-Link solid axle?
...A. Clutch Pack
...B. Locking Type
...C. Torsen / Limited Slip
...D. TSD Style / True Track
...E. Full Spool / Locked Live Axle
The differential type has a lot to do with how straight a car is able to launch.

[NAZ]

Many things will cause a car to launch to one side or the other, more than there is space here to cover in detail. Start with the basic chassis set-up and ensure you haven’t built-in the problem. A few things to check first:

1. Ensure the rear axle is square in the chassis (thrust angle) and centered which will require you to establish a chassis centerline and locate the absolute lateral center on the axle. Both these centerlines should have been established and clearly marked with witness points during the build.

2. Ensure roll steer is at a minimum or under acceleration the car will tend to pull to one side as the torque on the chassis lifts the left LCA front pivot and lowers the right LCA pivot. If your LCAs are perfectly parallel to the ground roll steer will be insignificant to non-existent, but if the LCAs are at an angle to the ground, there will be some amount of roll steer and the steeper the angle to more roll steer.

3. Ensure the rear ride height is the same left to right (I use +/- 1/16” as a tolerance on my race car).

4. Make sure both rear tires are set to the same air pressure.

[Mag]

When you say it pulls to the left is it the front tires pulling left or the cars rear tires hooking left?
Really can't say. Car goes straight at speed and under say up to 3/4 acceleration, but when I really get into it the car heads left and I have to steer it to
the right to keep it straight. Could be the rear thrust angle is off. Would the car track straight cruising at a steady speed if the thrust angle was off?

[Mag]

Have you had a 4 wheel alignment? My alignment shop wouldn't do the front until I fixed my bad thrust angle issue in the rear. With the FFR cast AL arms I had a 3/8" offset, not sure what the degrees were but they wouldn't touch it until fixed. I went with the Breeze adjustable rear control arms and they made sure the rear thrust angle was dead on straight then did the front.

I had a 4 wheel alignment when I finished building the car with the aluminum FFR rear control arms. Then the car went in for paint for about a year and then I installed the Breeze adjustable control arms at exactly the same length as
the aluminum FFR arms, But I didn't have it realigned again. Maybe I should bring it back for another alignment with the new arms. I can't say if the car pulled to the left with the old control arms because the engine was still new
and I never ran it wot.

[Mag]

Questions:
What time of rear differential do you have within your 3-Link solid axle?
...A. Clutch Pack
...B. Locking Type
...C. Tornsen / Limited Slip
...D. TSD Style / True Track
...E. Full Spool / Locked Live Axle
The differential type has a lot to do with how straight a car is able to launch.

Moser solid with 3 link. Clutch pack.

[GoDadGo]

Moser solid with 3 link. Clutch pack.

I'm running a TSD style unit (Eaton Tru Trac) in my 3-Link so both tires pull pretty close to the same.
https://youtu.be/lZmsY2YvVsc

Does your car leave the same or similar length tire marks when wheelspin occurs?

Clutch Pack Posi's sometimes have a bias to the right rear as the axle torques a tad counterclockwise as power is applied.

Torsen Type:
https://youtu.be/JEiSTzK-A2A

Clutch Type Posi:
https://youtu.be/WeLm7wHvdxQ

Here's a video about the different types of differentials.

https://youtu.be/R5v6hXYcEz0

Finally, I Think The Naz Man Can Give You The Best Recommendations To Remedy Your Issue!

[CraigS]

I used to get that once in a while. I found that the bolts that attach the bracket to the axle had worked loose. See the red arrow.
https://live.staticflickr.com/65535/51191207886_6b136eff09_z.jpg (https://flic.kr/p/2kZAjCw)3 link bracket (https://flic.kr/p/2kZAjCw) by craig stuard (https://www.flickr.com/photos/152454123@N04/), on Flickr
I was tightening the side bolts w/ a breaker bar and cheater pipe in they would still work loose, so I finally welded them to the axle. It was usually the right side. Car would pull one way on accel and the opposite way on decel.

[Mag]

I used to get that once in a while. I found that the bolts that attach the bracket to the axle had worked loose. See the red arrow.
https://live.staticflickr.com/65535/51191207886_6b136eff09_z.jpg (https://flic.kr/p/2kZAjCw)3 link bracket (https://flic.kr/p/2kZAjCw) by craig stuard (https://www.flickr.com/photos/152454123@N04/), on Flickr
I was tightening the side bolts w/ a breaker bar and cheater pipe in they would still work loose, so I finally welded them to the axle. It was usually the right side. Car would pull one way on accel and the opposite way on decel.

Thanks for the tip, I'll check mine.

[Mag]

I'm running a TSD style unit (Eaton Tru Trac) in my 3-Link so both tires pull pretty close to the same.
https://youtu.be/lZmsY2YvVsc

Does your car leave the same or similar length tire marks when wheelspin occurs?

Clutch Pack Posi's sometimes have a bias to the right rear as the axle torques a tad counterclockwise as power is applied.

Torsen Type:
https://youtu.be/JEiSTzK-A2A

Clutch Type Posi:
https://youtu.be/WeLm7wHvdxQ

Here's a video about the different types of differentials.

https://youtu.be/R5v6hXYcEz0

Finally, I Think The Naz Man Can Give You The Best Recommendations To Remedy Your Issue!

I was planning on getting a Torsen when I put the car away for the winter. The True Trac is a Torsen type diff isn't it?

[GoDadGo]

I was planning on getting a Torsen when I put the car away for the winter. The True Trac is a Torsen type diff isn't it?

The True-Trac and Torsen work very similarly using worm gears to equalize the torque; however, I do not know which is better.
I went with the True Trac style because they only used that system and the clutch type for the Dana 44 rear, which is what is in my car.
Sorry that I don't know the answer as to which one is the better option.

[Mag]

I used to get that once in a while. I found that the bolts that attach the bracket to the axle had worked loose. See the red arrow.
https://live.staticflickr.com/65535/51191207886_6b136eff09_z.jpg (https://flic.kr/p/2kZAjCw)3 link bracket (https://flic.kr/p/2kZAjCw) by craig stuard (https://www.flickr.com/photos/152454123@N04/), on Flickr
I was tightening the side bolts w/ a breaker bar and cheater pipe in they would still work loose, so I finally welded them to the axle. It was usually the right side. Car would pull one way on accel and the opposite way on decel.

Checked the bolts and they weren't "loose", but they definitely weren't as tight as they were when I installed them. Re torqued them and gonna keep an eye on them. I don't think they were loose enough to cause my problem but
I will test it later today. Thanks

[GoDadGo]

Here's a great video that I came across this morning.

https://youtu.be/1a9JQC6fgXw

I didn't realize that so many types existed.

[GThompson]

“Right Hand Rule”. Physics. When you apply a torque, there is a force along the axis in a direction depending on the direction of rotation of the shaft. With the shaft spinning forward as in the rear axle, the force goes to the left which pushes the axle to the right and turns the car left.

[BEAR-AvHistory]

“Right Hand Rule”. Physics. When you apply a torque, there is a force along the axis in a direction depending on the direction of rotation of the shaft. With the shaft spinning forward as in the rear axle, the force goes to the left which pushes the axle to the right and turns the car left.

Yeah, this is what I was thinking about when I asked which end of the car was driving the car left. Did not know there was a "rule":D

[GoDadGo]

Hopefully Mag's car isn't torquing as much as this 1960 Bubble Topper:

https://youtu.be/NfKrdKLwRTY

[J R Jones]

Mag, I have pondered the side mounted upper drag link on similar rear suspensions. Can that lead to asymmetrical dynamics under high load?
If the LR tire surges forward, the car will steer left.

Once upon a time I was doing I/O boat drive development and I wanted to see what was happening under the boat. We had an underwater high speed camera test barge but travel and set-up was big complication.
I bought a video fish locator and mounted it under the hull. It wasn't the perfect solution but information was gathered. My question for you is can a Go-Pro be mounted to record your rear axle during launches?
A boom mounted camera to the side of the left and right rears can be compared.
Mounted aft of the axle looking forward under the trunk might show something.

What does FFR tech say about this reaction at your power level?
jim

[Avalanche325]

It will not be the type of differential that is the issue, or every car with a clutch pack differential would be doing it. It is also not just normal torque vectoring, or every car in the world would do it. It is not normal operation of the differential or just physics. You have a problem that needs to be corrected.

I would start with doing a bushing inspection. You will need to pull the arms one end at a time and get a good look at both sides of the bushing. A slight bit of squish at the rear can make the car change direction. Then do a full nut and bolt check. Then I would be measuring to make sure the thrust angle is good. Also check your wheel bearings. See if they have and fore-aft wiggle. Your symptoms sound a little drastic for that but worth a look.

[Avalanche325]

If the LR tire surges forward, the car will steer left.


You got this in while I was typing. That is what I am thinking with the bushing check.

[CraigS]

Mine weren't loose either. But I could see the tinyest indication that the bracket was moving in relation to the axle by the paint right at the edge. Problem is that they are 7/16 (? it's been >8 yrs since) and the holes are the usual 7/16 + a bunch of clearance and that clearance was all it took.

[J R Jones]

Mag, Since Craig has mentioned 7/16 bolt diameter and hole tolerance, I will offer that clamp load increases with bolt torque, thread count, bolt grade and bolt diameter, you have choices.

Assembly torque in lb ft:
7/16 grade 5 course is 37, fine is 41. Grade 8 course is 52, fine is 58
1/2 grade 5 course is 57, fine is 64 Grade 8 course is 80 fine is 90.

Clamp load range in bolts above is 6777 lb to 14396 lb.

REF: https://www.fastenal.com/content/feds/pdf/Torque-Tension%20Chart%20for%20A307%20Gr5%20Gr8%20Gr9.pdf

I assume that you are clamping a steel rod end ball or a steel bushing.
jim

[J R Jones]

Mag, My 818 uses a (confusing) mix of SAE and metric fasteners. I suggested upgrading your 7/16 to 1/2 for more clamp load, that may not be possible with your existing parts.
Slightly bigger and stronger than 7/16 is an M12 fastener. It might fit.
The M12 X 1.75 in 12.9 grade gets almost to 14,000 lb clamp load:

https://crafter.fastenal.com/static-assets/pdfs/Torque-Tension_Chart_for_Metric_Fasteners.pdf

BTW one other Forum thread here discussed handling and his alignment shop data sheet recorded wheelbase difference of 0.677 inch. Oddly the aligned thrust angle was OK. If that error is also on your car, it may contribute to your acceleration steering issue.
jim

[CobraboyDR]

“Right Hand Rule”. Physics. When you apply a torque, there is a force along the axis in a direction depending on the direction of rotation of the shaft. With the shaft spinning forward as in the rear axle, the force goes to the left which pushes the axle to the right and turns the car left.^^^This^^^


Torque.

A light car with torquey motor will add rotational force on the chassis and it'll want to wander.

[NAZ]

Jim, offset wheelbase has nothing to do with launching straight. Drag racers commonly offset the wheelbase the NHRA max allowable 1" to game the lights. You'll find that when you check wheelbase after an alignment it is common to have some wheelbase offset even on a street car as when you adjust caster you are affecting the wheelbase on one side or the other.

Everybody looks for a single cause without actually ensuring the car was initially set-up to launch straight. You can't build a solid house without a solid foundation. Same with a chassis set-up, you have to first tend to the fundamentals. There is typically more than one thing that causes a car to track to one side or the other and even the driver has more affect on this than most realize. To find the root cause requires a systematic troubleshooting approach and a good understanding of chassis set-up. This basic fact is why folks get frustrated and give up then take the car to a pro. There are many things that influence how hard a car hooks and how straight it launches and you can really screw up handling by indiscriminately tweaking on the chassis.

My car hooks hard and launches straight and pulls more than 2.5-Gs off the line. There is no drama, simply acceleration. That's not by accident -- it's science.

[J R Jones]

Interesting, two differing opinions in succession. My experience is in road racing and building street performance cars; those are the only cars I have measured wheelbase on. I have not seen a R/L wheelbase difference in a car except a wrecked one. Other than road racing circuits with high banked turns, where we weight jacked, our chassis and alignment were always symmetrical. In the aforementioned case at 0.677, I speculate that non-symmetrical track bars may contribute to the eror.

[NAZ]

Jim, I agree with your thoughts on the .677" WB offset but based on my experience I would suspect there is accumulation of error in the fabrication, set-up, and measurement that contributes to that total. As an engineer, you are familiar with tolerance stack up and building a chassis (even in a jig) you will see a some variance around a nominal dimension. I'm sure you've dictated +/- tolerances specs on things you have designed or modified in your professional life. And if a guy gets a bit sloppy measuring, or takes the TLAR (that looks about right) approach, that stack up can start getting larger and to the point it causes performance issues.

All the more reason to go back to the chassis set-up and verify things are as close to target as possible when a performance issue emerges before you start tweaking on things.

BTW, good to have you on the forum as we can benefit from your experience.

[J R Jones]

NAZ Thank you.
Yes the design layout establishes a primary datum and dimensions from that. I do not know what that is in a FFR design, but my guess it would be a control arm pivot or track bar mount.
Mag has big power and torque, but not as big as yours, even so, acceleration is normal until he does a hard launch. That suggests a dynamic misalignment, but not a yield.
I heard it said the some years ago road racing motorcycles at 200+HP were winding up the chassis accelerating off turns. That is not so bad especially if the power stays on, but if the rider snaps-off the throttle, what does that stored energy do?
I see you guys are running different chassis and you have a four link set-up. which I prefer. Could be the difference.

[seagull81]

I don't see anyone talking about corner weight. It can also affect the launch. Make sure your shock settings are equal front and back.