Loose steering at high speeds

[dhanisch]

Hey everyone,

I know I've been missing for a few years now, but my GTM just got shot for That Racing Channel, and during the ride along, the videographer noticed the instability at higher speeds.

When I last raced in the half mile, I hit 170.19mph, but at the top end, the car just seems to want to float back and forth on deceleration. You can see it in the latter part of the video here:

https://youtu.be/plbCISyAQBo

There's also another video of a fellow member spinning out at 160 (if I remember correctly).

I tightened up the linkage going to the steering rack, which helped in the slack area, but still feel the car wanting to go back and forth at higher speeds. The big fear is it being a growing oscillation that causes me to lose control.

Would a different steering rack help? Think it's an alignment issue? This happened even after getting new tires, just for the sake of clarifying.

Any advice would be greatly appreciated. I want to turn up the boost and shoot for higher numbers, but this is keeping me from doing so.

Thanks,

[VRaptor SpeedWorks, LLC]

Sounds like an alignment issue to me....my best guess would be that you don't have enough toe-in in the rear or something related to bump steer in the front?

[beeman]

How much caster are you running?

[Al_C]

I had a similar problem in a roadster: loose steering above 70 mph. Nowhere near where you are, speed-wise, but disconcerting nevertheless. First thing was to ensure everything from the tie-rod ends to the rack was tight - which apparently you have done. The problem was a poor alignment job (your typical tire/alignment chain operation). My opinion: find somebody who knows how to align to specs and/or preps race cars and your problem will disappear.

[crash]

Sounds like an alignment issue to me....my best guess would be that you don't have enough toe-in in the rear or something related to bump steer in the front?

Yes both of those could be an issue, but so could having too much toe in at the rear. This causes the car to "crab walk". Remember that it sounds like it is only under braking for you so it could very well be that the DYNAMIC toe, in the front or rear, goes to toe out which will cause instability. There is really so much happening with weight transfer, changes in suspension geometry, and ALWAYS differing surface conditions that it is nearly impossible to get any car to handle 100% stable in all conditions. If you went 170 and did not have any issues at that speed without putting very specific settings on the car, you are lucky. One of the other things to look at is what type of rod ends you are using. I replace all of those with aircraft rod ends because of the higher durability and the increase in precision which makes for much better repeatability when making suspension adjustments. Going fast is not all that hard. Going fast relatively safely and relatively predictably over and over again is another thing all together. Lots of details need to be addressed...IMHO.

[beeman]

Agree with all the above. Please post a photo of your alignment #s from when you had your car on an actual alignment rack, unless you have very CAPABLE alignment equipment and knowledge in your shop.

[dhanisch]

Thank you guys for the suggestions. I haven't had the car on an alignment rack since I bought it. I'll take it in to have it checked and aligned to see if that helps. The car tracks straight otherwise, so I didn't think it to be an alignment issue.

I appreciate it!

[dhanisch]

Yes both of those could be an issue, but so could having too much toe in at the rear. This causes the car to "crab walk". Remember that it sounds like it is only under braking for you so it could very well be that the DYNAMIC toe, in the front or rear, goes to toe out which will cause instability. There is really so much happening with weight transfer, changes in suspension geometry, and ALWAYS differing surface conditions that it is nearly impossible to get any car to handle 100% stable in all conditions. If you went 170 and did not have any issues at that speed without putting very specific settings on the car, you are lucky. One of the other things to look at is what type of rod ends you are using. I replace all of those with aircraft rod ends because of the higher durability and the increase in precision which makes for much better repeatability when making suspension adjustments. Going fast is not all that hard. Going fast relatively safely and relatively predictably over and over again is another thing all together. Lots of details need to be addressed...IMHO.


You know, now that you mention that, someone pointed out on one of my dyno videos that the rear wheels toe in under power.

Here is a video of that:

https://youtu.be/Dc_St0N7qSk

I imagine I need to replace the bushings if that is occurring?

[cob427sc]

From my own experiences on the GTM and another car, I think crash has it right. The rear alignment is critical and being off slightly or if a link or joint is loose or worn will cause unnerving movements. Definitely get someone who knows how to do a full 4 whell alignment to specs and if he does race setups, even better.

[dhanisch]

In regards to the alignment specs, are the ones in the manual still applicable?

[beeman]

It's worth mentioning that the suspension mounts with cam bolts, if the car has previously been driven hard or they weren't torqued correctly, the alignment could have changed significantly even if it was set up right initially.

[cob427sc]

Beeman - good point. On my gtm after really torqueing everything down correctly, at about 500 miles and some hard driving I had one link loosen up at the cam bolt. Just roatated around about 1/4 turn. It was noticeable just looking at the car from the rear. I ended up having the alignment redone and for better or worse, added loctite to all the bolts and threads.

[VRaptor SpeedWorks, LLC]

Beeman - good point. On my gtm after really torqueing everything down correctly, at about 500 miles and some hard driving I had one link loosen up at the cam bolt. Just roatated around about 1/4 turn. It was noticeable just looking at the car from the rear. I ended up having the alignment redone and for better or worse, added loctite to all the bolts and threads.

That is not a loctite-fixable issue. The cam bolts on a Gen I chassis will physically not ever tighten up no matter how much you torque them. The thickness of the suspension mounts on the GTM are different than on the Corvette.....so you actually run out of travel in the cam bolt before it gets tight. All you're doing when you tighten the nut is you are tightening the cam washer against the step in the cam bolt.

http://vraptorspeedworks.com/cam-bolt-washers-36/

[Shoeless]

I'm on board with crash, this looks to be a dynamic toe situation, if the rest of your alignment checks out. You can see it in the dyno video and the same thing happens when you let off and brake from your highspeed run. It's more pronounced and only happens for a very short time on the dyno, but look at the front of your car when the operator let off the gas, your front suspension goes from rebound into compression. The rear goes from compression to rebound. This is likely where you issue is.

Are you running any kind of bump steer kit on the front and the rear?

I literally just went through setting up and checking bump steer on my GTM this past weekend. I have the FF front bump steer kit and an aftermarket corvette bump steer kit on the rear. No matter what I did, neither side was 100% the same as the other, so I set it as close as I could get it for the minimal amount of bump steer. The rears were a lot closer left and right than the front left and right and I'm thinking of adding bushings to raise my steering rack to help on the front. So understanding what your suspension is doing in the dynamic state, is going to clue you in to fixing the issue you are dealing with.

[dhanisch]

I'm on board with crash, this looks to be a dynamic toe situation, if the rest of your alignment checks out. You can see it in the dyno video and the same thing happens when you let off and brake from your highspeed run. It's more pronounced and only happens for a very short time on the dyno, but look at the front of your car when the operator let off the gas, your front suspension goes from rebound into compression. The rear goes from compression to rebound. This is likely where you issue is.

Are you running any kind of bump steer kit on the front and the rear?

I literally just went through setting up and checking bump steer on my GTM this past weekend. I have the FF front bump steer kit and an aftermarket corvette bump steer kit on the rear. No matter what I did, neither side was 100% the same as the other, so I set it as close as I could get it for the minimal amount of bump steer. The rears were a lot closer left and right than the front left and right and I'm thinking of adding bushings to raise my steering rack to help on the front. So understanding what your suspension is doing in the dynamic state, is going to clue you in to fixing the issue you are dealing with.


Looking at the car, it appears to have the front bump steer kit installed as it looks just as the FF instructions show. I did not install it though. I have Laurence Chapman's old car, bought it from him in 2013. I don't believe I have anything in the rear.

I have ordered the washers from Shane and also have a Delrin bushing kit coming. Before I take it in for the alignment, I want to get everything refreshed. I'm also going to look into replacing the hubs as well just for good measure.

[VRaptor SpeedWorks, LLC]

In the video, it looks like most of that "flex" is probably in the suspension bushings. Probably the best solution there would be to go to some polyurethane replacement bushings.

[Shoeless]

I ordered and installed these in my GTM. Kind of a PITA to install, but I wasn't going to put original bushings in my GTM.

https://www.corvettepartsandaccessories.com/products/c5-corvette-pfadt-control-arm-and-sleeve-bushing-kit

[dhanisch]

In the video, it looks like most of that "flex" is probably in the suspension bushings. Probably the best solution there would be to go to some polyurethane replacement bushings.

I went with Delrin as they are self lubricating to some degree and are a harder durometer than poly bushings. Biggest complaints are shorter life than poly and rougher ride, but the car gets limited miles as it is and I don't mind a rough ride. I also didn't want to deal with the poly squeaks. I know there are Teflon tape tricks and whatnot, but I didn't mind the extra cost. Plus when I get them, I can take measurements and just turn the delrin replacements on my lathe.

[crash]

The problem with the Delrin is that it does not allow ANY flex. When you are trying to get the maximum caster in the front you need this flex in the rubber or urethane bushings in order to get the suspension arms where they need to be. Especially if you are using something like these...

http://myraceshop.com/Alignment_Shims.html

The only way you will get the arms to where you want them to be and have a relatively solid mount is to use what I use on the FFR PDG GTM race car...

https://www.lgmotorsports.com/lg-motorsports-monoball-control-arm-kit.html

Yes they are expensive. Try sourcing the parts and making the bits you can't find and this doesn't look so bad.

You guys are getting me to reveal all my secrets here! LOL!

[beeman]

I would have thought that the monoballs (steel cylinder within a steel cylinder) would not be compliant at all?
Pretty much any serious C5 track car is running the monoballs.

[crash]

I would have thought that the monoballs (steel cylinder within a steel cylinder) would not be compliant at all?
Pretty much any serious C5 track car is running the monoballs.

What you need to understand is that with the Delrin it is a solid bushing with a hole through the middle. You will not get any bolt misalignment within that A arm mount. The monoball will rotate so that the bolt can be misaligned in the arm and line up to the chassis mounting holes.

In truthfulness, I don't see how the Delrin could ever even work on a GTM as the bolts have to go through the arm mounts at an angle AT THE LEAST when the suspension is going through travel. Delrin would be fine if the chassis mounts were parallel to the arm mounts, but in a GTM this is almost never the case, and certainly not when you try and dial in the required amount of front caster.

My recommendation is that the most solid you would want to go, without going to solid monoball, is to use the urethane, but know that getting the bolts through the holes will be difficult at best.

[dhanisch]

The problem with the Delrin is that it does not allow ANY flex. When you are trying to get the maximum caster in the front you need this flex in the rubber or urethane bushings in order to get the suspension arms where they need to be. Especially if you are using something like these...

http://myraceshop.com/Alignment_Shims.html

The only way you will get the arms to where you want them to be and have a relatively solid mount is to use what I use on the FFR PDG GTM race car...

https://www.lgmotorsports.com/lg-motorsports-monoball-control-arm-kit.html

Yes they are expensive. Try sourcing the parts and making the bits you can't find and this doesn't look so bad.

You guys are getting me to reveal all my secrets here! LOL!

I was talking to a local builder (Cordes Performance) and he recommended the LG bearings too. I may go that route, but not sure yet.

What would you think of delrin in the rear, urethane in the front?

I have added a Haas VF2 and a large lathe, CNC plasma table, and various other pieces of machinery to my shop at my house, so I very well could start custom machining parts for the GTM (which is the general idea), but in areas of going 170mph+, I don't think that's a good place to start doing DIY stuff, lol.

[dhanisch]

What you need to understand is that with the Delrin it is a solid bushing with a hole through the middle. You will not get any bolt misalignment within that A arm mount. The monoball will rotate so that the bolt can be misaligned in the arm and line up to the chassis mounting holes.

In truthfulness, I don't see how the Delrin could ever even work on a GTM as the bolts have to go through the arm mounts at an angle AT THE LEAST when the suspension is going through travel. Delrin would be fine if the chassis mounts were parallel to the arm mounts, but in a GTM this is almost never the case, and certainly not when you try and dial in the required amount of front caster.

My recommendation is that the most solid you would want to go, without going to solid monoball, is to use the urethane, but know that getting the bolts through the holes will be difficult at best.

Well it may be a expensive learning experience. I appreciate the feedback. Unfortunately the delrin bushings are already shipped, so I'm stuck with them unless I can throw them up on Ebay.

[beeman]

I was equating delrin with urethane, oops.
I have the urethane bushings on my C5 track car, I really like them. One trick I came up with on the fly, out of desperation, to push out the old bushings is get a hole saw that matches the diameter of the female part/control arm (cheap harbor freight) and grind the teeth off, then push the bushing into the hole saw with a vice and socket on the bushing.

[Ajzride]

I found the best way to get old bushings out is to cut them using a handheld coping saw. I tried to press some out on a Jaguar XKR that broke a 5 ton press, then I tried burning them out, that was an even bigger mess. Using the coping saw you just cut a straight line from the center to the edge (be very careful and go slow when you get to the end near the control arm), and it allows the bushing to collapse just enough to push right out with no effort.

[Shoeless]

I was able to push my old bushings out with I believe the ball joint tool kit from Auto Zone. Had to burn the front top ones out, but since I was powder coating my suspension, the mess didn't hurt me.

[dhanisch]

Quick update. Thank you everyone on the feedback. It appears the problem is now resolved. Here's what I did.

Full Delrin bushings all the way around. It turns out my alignment shop was able to get it to the specs per the manual.

All wheel hubs replaced. When disassembling for the bushings, I noticed the rear drivers hub was not smooth in it's rotation. That accounted for an annoying noise I had at speeds. The others were smooth, but I figured at 20 years old, they all needed to be replaced.

Once the above steps were done, despite going back to where the cam bolts were before, the alignment was absolute garbage at this point. It was barely drivable, but I managed to get into the highest rated shop in Phoenix (they have extensive experience with Ferrari, Lamborghini, Bentley, lowered, bagged, and even having done several GTM's in the past.). The guys said my alignment was as far as 4 degrees off in the front, which was visually noticeable, but they said that's common when moving away from the rubber bushings and going to solid ones. They were able to get it to spec according to the manual. Once the first alignment was done, they had me drive around and get it settled in, then return to the rack for a fine tuning. Once the fine tuning was done, they did a weight compensated alignment with me in the car. Finally, they made sure the steering wheel was dead straight without any preload and settled it in. Even when I bought the car from Laurence 8 years ago, the steering wheel wasn't straight even then. I know it's a minor thing, but it's so nice to have it correct now.

Results are amazing. The car runs as straight as an arrow now, no more loose feeling at high speeds. I haven't tested 100+ yet, but it was even noticeable at highway speeds, but no longer. It's very effortless to drive now. I guess I just got so used to it being the way it was over the past 8 years that I just accepted it. Well, it's like a whole new animal.

Again, thank you to everyone for the recommendations and feedback.

[Dave 53]

Quick update. Thank you everyone on the feedback. It appears the problem is now resolved. Here's what I did.

Full Delrin bushings all the way around. It turns out my alignment shop was able to get it to the specs per the manual.

All wheel hubs replaced. When disassembling for the bushings, I noticed the rear drivers hub was not smooth in it's rotation. That accounted for an annoying noise I had at speeds. The others were smooth, but I figured at 20 years old, they all needed to be replaced.

Once the above steps were done, despite going back to where the cam bolts were before, the alignment was absolute garbage at this point. It was barely drivable, but I managed to get into the highest rated shop in Phoenix (they have extensive experience with Ferrari, Lamborghini, Bentley, lowered, bagged, and even having done several GTM's in the past.). The guys said my alignment was as far as 4 degrees off in the front, which was visually noticeable, but they said that's common when moving away from the rubber bushings and going to solid ones. They were able to get it to spec according to the manual. Once the first alignment was done, they had me drive around and get it settled in, then return to the rack for a fine tuning. Once the fine tuning was done, they did a weight compensated alignment with me in the car. Finally, they made sure the steering wheel was dead straight without any preload and settled it in. Even when I bought the car from Laurence 8 years ago, the steering wheel wasn't straight even then. I know it's a minor thing, but it's so nice to have it correct now.

Results are amazing. The car runs as straight as an arrow now, no more loose feeling at high speeds. I haven't tested 100+ yet, but it was even noticeable at highway speeds, but no longer. It's very effortless to drive now. I guess I just got so used to it being the way it was over the past 8 years that I just accepted it. Well, it's like a whole new animal.

Again, thank you to everyone for the recommendations and feedback.

After getting my final engine tune on my 818S with its new found power, my car was dancing around under hard acceleration. This was about the same time you started this thread. Replies to this thread and my post on the 818 forum inspired me to get a proper "race car" suspension tune. Ditto... I guess I was use to how it drove before, but now it's like a whole new animal! (and I got my steering wheel centered too) It's Factory Five RACING and these cars have a race car suspension that needs a race car shop to set up properly. Thanks to both the GTM and 818 members for pointing me in the right direction.