Sorting Out Steering Kickback

[Trackrash]

Yes, still parallel in your bottom picture. That is the point, of course that is in a perfect world.

Your bottom pic in your post above shows what happens when the tierod is not parallel to the lower control arm axis.

Regardless, the only way to check and minimize bumpsteer is by measuring the toe change.

FWIW, I spent months adjusting my tierod to be perfectly level to the ground on my 911 after I raised my spindles. It handled like a pig until I got out the strings and started actually measuring my toe change and adjusted my bumpsteer setup accordingly.

[Bravoaloha]

Originally Posted by Bill Verburg

You have multi interrelated issues
* you must address the front camber disparity
* the steering tie rod ideally wants to be horizontal when the car is on the ground, the reason is that wheel travel pulls the arm when in droop and pushes the arm in bounce, you want to minimize the magnitude of the push/pull which will be when the arm starts off horizontally when at static ground position, a contributing factor is to try to keep wheel travel to a minimum
*any bump steer issue is magnifored by any increase in scrub radius w/ stock hubs and 7ET23.3 a 911 starts w/ a lot of feedback caused by the s/r of 52.4mm, push the same wheel out another 3/8" and that increases to 61.925mm, so any feedback from the wheels is amplified by ~18%
*any time an aggressive bump steer kit is added you need to measure the bump steer to see if it helps or hurts, either can happen. To measure bump steer you need a flat, rigid plate bolted to the hub, a strut w/o a spring or internals and a dial indicator, attach the plate to the hub, move the wheel through it's arc while measuring the toe and recording it correlated to travel from the neutral condition seen when the tire is on the ground and full weight is on it. You can still do it w/ the spring and shock internals in situ but it's going to be harder. The stack of steering arm o/s's that you keep should be chosen after measuring the effects of as many of them as you have the patience for. W/ an extra dial indicator you can also measure camber and draw a camber curve for you setup.

Here are the curves for a stock 3.2 Carrera at stock ride height

Thanks Bill!

Does anyone have experience with RSR struts? Is it true they come de-cambered? My shop seems to think so. I have not been able to get in touch with Steve W. (he sold them to me) but I know him, how he operates and he knew what was in/what was going in my car. I can't imagine he would sell me a set of struts that wouldn't work with my car.

I will re-measure the hub distance and start playing with the bump/steer when I have a chance.

Thanks!

[Bravoaloha]

The more I think and look at this the more I am thinking it is a scrub angle issue. Look how far away from the center of the wheel the bottom of the strut is.



I can't jack the car up right now but I am thinking the car must have turbo hubs or something on it to push the wheel out that far.

[Bill Verburg]

Originally Posted by Bravoaloha

The more I think and look at this the more I am thinking it is a scrub angle issue. Look how far away from the center of the wheel the bottom of the strut is.



I can't jack the car up right now but I am thinking the car must have turbo hubs or something on it to push the wheel out that far.

scrub radius is defined here, though here it is called pivot radius


a stock 911 w/ 7ET23.3 Fuchs has a s/r of 52.4mm per the factory the back space of that wheel is 121.1mm, so if you can measure the b/s of your wheel you can then determine s/r.

b/s is defined here

[Bravoaloha]

Originally Posted by Bill Verburg

scrub radius is defined here, though here it is called pivot radius


a stock 911 w/ 7ET23.3 Fuchs has a s/r of 52.4mm per the factory the back space of that wheel is 121.1mm, so if you can measure the b/s of your wheel you can then determine s/r.

b/s is defined here

Wouldn't that only be true assuming I have stock hubs? I know the wheel is pushed out further than stock, and these are Fuchs off my Dad's '86 Carrera that had no issues. I think the ticket is to measure these hubs and compare them to my Dad's and go from there.

[Bill Verburg]

Originally Posted by Bravoaloha

Wouldn't that only be true assuming I have stock hubs? I know the wheel is pushed out further than stock, and these are Fuchs off my Dad's '86 Carrera that had no issues. I think the ticket is to measure these hubs and compare them to my Dad's and go from there.

You are right about that

I'm still confused by the hubs
AFAIK there are only 4 different hubs that have ever been used on any 911
early cars used 37 or 40mm hubs, for ~'69 thru 89 they used 47mm hubs, in 1980 930 got 68mm hubs but they can't be used on a n/b because they push the wheel way too far out for regular 911 front fenders, before 1980 930 used 911 hubs

[Bravoaloha]

Originally Posted by Bill Verburg

You are right about that

I'm still confused by the hubs
AFAIK there are only 4 different hubs that have ever been used on any 911
early cars used 37 or 40mm hubs, for ~'69 thru 89 they used 47mm hubs, in 1980 930 got 68mm hubs but they can't be used on a n/b because they push the wheel way too far out for regular 911 front fenders, before 1980 930 used 911 hubs

I need to do some measuring. I do know for a fact that the hubs stick out further on my car than on a normal Carrera. I don't know why, but I cant imagine it's anything but something wonky with the hubs (as opposed to a different suspension/chassis component).

[Bravoaloha]

I was hoping to find a hub issue, no such luck. I measured width from rotor to hub face (where wheel mounts), rotor width and back of rotor to chassis. Everything checks out compared to my Dad's car. I even measured the A-Arms to make sure mine hadn't been modified at some point.

Bill I keep referring to the following post.

Originally Posted by Bill Verburg

You have multi interrelated issues
* you must address the front camber disparity
* the steering tie rod ideally wants to be horizontal when the car is on the ground, the reason is that wheel travel pulls the arm when in droop and pushes the arm in bounce, you want to minimize the magnitude of the push/pull which will be when the arm starts off horizontally when at static ground position, a contributing factor is to try to keep wheel travel to a minimum
*any bump steer issue is magnifored by any increase in scrub radius w/ stock hubs and 7ET23.3 a 911 starts w/ a lot of feedback caused by the s/r of 52.4mm, push the same wheel out another 3/8" and that increases to 61.925mm, so any feedback from the wheels is amplified by ~18%
*any time an aggressive bump steer kit is added you need to measure the bump steer to see if it helps or hurts, either can happen. To measure bump steer you need a flat, rigid plate bolted to the hub, a strut w/o a spring or internals and a dial indicator, attach the plate to the hub, move the wheel through it's arc while measuring the toe and recording it correlated to travel from the neutral condition seen when the tire is on the ground and full weight is on it. You can still do it w/ the spring and shock internals in situ but it's going to be harder. The stack of steering arm o/s's that you keep should be chosen after measuring the effects of as many of them as you have the patience for. W/ an extra dial indicator you can also measure camber and draw a camber curve for you setup.

I feel that by measuring and confirming I have stock hubs (PN 911.341.605.4R) I have eliminated the possibility of a scrub angle issue.

I agree that the camber disparity needs to be addressed but I don't see how it would contribute to the kickback issue.

I feel similarly about bump/steer. As far as I know, my car does not display the symptoms of a bump/steer issue.

Am I wrong?

Thanks

[stownsen914]

Looks like you are running a pretty common, stock setup wrt hubs and wheels. 911 front suspension has scrub radius built into it by virtue of having strut suspension. You shouldn't have it any worse than other 911 drivers.

Given your setup, it doesn't necessarily look like bumpsteer should be a prominent issue either. Bumpsteer was my first thought in reading your first post. Trackrash's and Bill's comments are the generally accepted guidance, and you have the tie rod spacers which should help address bumpsteer. I'll only caution that bumpsteer, when you have it, can feel pretty unsettling and make the car feel unpredictable even when driving straight but especially when going over bumps. The car will dart around even with no steering wheel input. And the only way to really eliminate it is to measure it while exercising the suspension, and adjust the outer tie rod spacers accordingly.

Sorry if I missed it, but can you describe the kickback in more detail? E.g. over bumps, when you're turning, etc. Is there slop in your steering by chance?

[Tyson Schmidt]

Looking at that pic of your lower control arm and wheel, it definitely looks like you scrub radius is higher than stock.

If your wheels are 7” with ET 23, and your hubs are stock 47mm, then I would put money on your struts being decambered about 1 or 2 degrees.

[Bravoaloha]

Originally Posted by stownsen914

Looks like you are running a pretty common, stock setup wrt hubs and wheels. 911 front suspension has scrub radius built into it by virtue of having strut suspension. You shouldn't have it any worse than other 911 drivers.

Given your setup, it doesn't necessarily look like bumpsteer should be a prominent issue either. Bumpsteer was my first thought in reading your first post. Trackrash's and Bill's comments are the generally accepted guidance, and you have the tie rod spacers which should help address bumpsteer. I'll only caution that bumpsteer, when you have it, can feel pretty unsettling and make the car feel unpredictable even when driving straight but especially when going over bumps. The car will dart around even with no steering wheel input. And the only way to really eliminate it is to measure it while exercising the suspension, and adjust the outer tie rod spacers accordingly.

Sorry if I missed it, but can you describe the kickback in more detail? E.g. over bumps, when you're turning, etc. Is there slop in your steering by chance?

Thanks Stownsen. The car in a straight line generally feels ok. Not perfect, some darting but I attribute that to the alignment and tires (R888R's). Bumps in a straight line do not cause feedback but during cornering the wheel "kicks" into the corner when the inner tire hits a bump. (for example, coming through a right turn, right front tire hits a bump and the wheel will kick right and then return to previous level of driver input). Coming through the same turn, if the outside (left/loaded) tire hits a bump there is little to no feedback.

What is odd is that I don't think this "kickback" has a significant effect on the trajectory of the car. The car doesn't seem to really change much direction as a result. (or maybe it is just hard to tell) I do, however, think that it definitely unsettles the car and I also hate the way it feels.

Originally Posted by Tyson Schmidt

Looking at that pic of your lower control arm and wheel, it definitely looks like you scrub radius is higher than stock.

If your wheels are 7” with ET 23, and your hubs are stock 47mm, then I would put money on your struts being decambered about 1 or 2 degrees.

Thanks Tyson. I definitely think something is going on. I was really hoping to find I had some wonky hubs that were messing up my scrub angle. What throws me off about your hypothisis is that I have -1.4° of camber on the passenger side (Which is expected on a stock car). I don't have much of an explanation for the -.7° on the driver side other than the fact that the car had an impact there but the car was checked on a celette after and straightened out.

Measuring, comparing and finding very similar measurements on my dad's car still doesn't explain why the wheels I had fit on his car and stuck out on mine.

[Bravoaloha]

ttt