GT5 and wide front rim geometry

There has been discussion here about the steering jiggling with 10" wide front rims creating issues.

The thought was that the wider rim changes the scrub radius and changes the front geometry beyond the original design considerations.

The simplest solution seemingly is installing a EPS system since it is not possible to correct the situation otherwise.

Yes, that is somewhat normal for GT5 / GT5S cars with increased scrub radius of the wider wheels and any play in the steering  seems to amplify it. I spent a long time dialing out as much play in the steering as possible, the column slip joint can be one major culprit. I also modified my upper a-arms to provide 7 degrees of caster, which helps but makes steering at parking speeds heavier. Some have counteracted that with an EPAS power steering unit.

One of the most notable improvements I made was when I switched tire brand, but choices for OEM 15" wheels are very limited.  Tire pressure of 24 psi seems a bit on the low side, I run 32 psi in my front tires, period documentation suggests 35 psi.

While I agree that wider rims and tires increase wander, I have never had the sensation of the steering wheel being ripped from my hands. Something is wrong. I have offset bushi8ngs and mild reduction in width on the upper outside hub carrier to maximize caster. Nothing radical. Frankly, I didn't notice a difference in tracking from before.

Check your steering rack bushing for slop. Additionally, check your suspension bushings and wheel bearings by jacking the front  tire off the ground, grabbing the wheel at 9 and 3 o'clock and trying to rock it w/o the steering wheel moving.

You either have slop in the suspension/steering or a bad tire.

If someone with a widebody car lives near you, swap tires with them to test.

I had a set of Pirellis that absolutely wandered everywhere, switching to Michelins , albeit with shorter sidewalls, offered much improvement. To be fair, probably a new set of Pirellis would have as well.

You need to verify that you have toe in. 3/16" total is a good number.

If you have toe out, then the steering can pull radically to one side or the other suddenly.

I run a 1/16th to 1/8" toe out on my race cars and my Panteras. My rational for the Pantera is to assist with initial turn in and reduce understeer. I did this when I tracked my Panteras as well. I do not buy into the notion that it generates more directional instability than toe in.

JT

Thanks for the replies. Tracking is slight toe in, I don't remember the numbers. I'm pretty sure there's no mechanical slop anywhere. Front tyres are new Michelins.

On a smooth road, there's no problem. Over here, the roads can be tight and twisty, and I do experience wander, especially if I go over the crown of the road. If I hit a bump, or something like a bevelled curb (slowly) then the steering tries to wrench itself out my hands.

Front end alignment in reality is experimental from car to car and aimed at resolving certain perceived issues that the driver wants to improve upon.

Only you can determine if a setting is optimum to solving an issue that you have with the cars reactions.

I'm not a novice or newcomer to any of this but neither do I claim to be the expert either.  It is conventional thinking that toe in improves directional stability but in the case of Panteras where positive caster has been increased from the original design perimeters, the results of radical changes from that although predictable, may have somewhat unexpected consequences to them.

For sure tire choices have changed since 1970 considerations, so exactly the combination to improve the handling characteristics, not just change them are going to be somewhat of an experiment under the best of conditions.

Using an EPS system is going to stabilize the steering to a great extent and probably at least enough to where sudden steering changes will be dampened acceptably.

Only time will tell to what level any of the cars need to be altered to.

"Occam's Razor" suggests that the simplest solution may be the best but it is apparent that everyone has different perspectives.

Dang2407: that sounds like "bump steer" to me. Where is the steering rack mount on your car? Does it use the factory steering rack mounting shims?

What are the camber settings?

On my original post I was going to ask if this is a converted early car (perhaps the reference to 2407) as Doug states the later L cars had shims to lower the rack and dial out some bump steer.

Also, are these 15" rims? What Michelin tire are you using?

It is a later car #9557. There are spacers approx 1/4" above the steering rack and from below the bracket fits snugly, on the other side of the rack.

Camber on the front is 0.75° and toe 1mm on each side. Tyres are TB5R on 15" rims.

The TB5R's are essentially a semi slick road race tire, I'm not surprised they track more than a street tire.

The GT5 manual that came with the car (photocopied) states 37 psi front and 41 psi rear...wow, is this for real? It's going to help tremendously with my fuel economy if I go with that....

I would ask what others with GT5's are running alignment wise.

I presume that you mean -.75° camber? That means the top of the tire leans in. It IS possible that the nature of that tire is going to NEED much more just to track straight?

Previous personal experience tells me it could be it needs -1.5 to 2°? With a soft compound like you have, that means it is going to wear out in a couple of thousand miles, if even that much.

I would think that you could use more toe in also. It might reduce the dartiness of the steering wheel but you may need to play with settings to get this reasonable.

I would START with 1/8"-3/16" total toe in and test the results. Going with toe out settings is really race car specific. For instance NASCAR "stockers" are set to turn left for 500 miles, and the steering alignments for that are specific and can use toe out on one side of the car.

If you don't want to get killed in this thing, you had better know exactly WTF, Whisky Tango Foxtrot, you are doing with that?

Race cars are typically worked on extensively to reduce the bump steer. I don't know of many, or any that claim they can completely eliminate it.

Not every racer or team is forthcoming on tech info like this. Many consider that information proprietary. There are quite a few You tube videos with in car cams that clearly show some cars are really quite a handful on the track with often violent steering wheel changes.

You really can't presume that there is no bump steer existing in the car now. You need to test it. That isn't simple to do. The wider the tire tread the more effect even small amounts will have on the steering.

As others have stated, many have modified the front upper a-arms to gain additional + caster.

Stock caster on "US Ford era" cars is about +1.5°. Some cars now are running as much as +7°. That is a possible solution, as some have suggested, but that will tend to make the steering effort noticeably heavier.

US sold Panteras were sold as US street cars with national speed limits of 55mph. Ford didn't want to be accused of building race cars for the street and didn't want to hear people complain about steering effort with a car that had no power steering so +1.5° was what the corporate policy was on the Pantera.

Those are very sticky compound tires. They may need specific adjustments not necessary on other less capable street tires? You may need to bring in NASA to consult?

A quick search suggests 28 psi front and 32 psi rear for the TB5's, as a Pantera is reverse to a front engine car, I'd go up to 32 psi front.

Recommended alignment specs for wide body (10" front, 13" rear wheels)

Front: camber 0 degrees, castor max possible, 1/16" toe in. Rear: camber 0 degrees, toe 0 degrees. For track change camber to negative 0.75 to 1 degree front and rear.

@joules posted:

A quick search suggests 28 psi front and 32 psi rear for the TB5's, as a Pantera is reverse to a front engine car, I'd go up to 32 psi front.

Recommended alignment specs for wide body (10" front, 13" rear wheels)

Front: camber 0 degrees, castor max possible, 1/16" toe in. Rear: camber 0 degrees, toe 0 degrees. For track change camber to negative 0.75 to 1 degree front and rear.

Wow. With settings like that, you need to ensure that the car is ballasted preciously and sitting exactly level during alignment.

For one thing, toe in IS YOUR FRIEND. Zero in the rear on a mid engine car is "just a bit 'hairy' ". I'm very nervous just thinking about driving it.

The big block Cobras were like that. They are front, mid-engined cars and initially were a problem to keep the rear ends from coming around. That was fixed with rear toe-in. Toe-in keeps the rear from steering.

This is race team stuff. A week at the "track" setting up a car. I'm the team now. Let's just say I'm enjoying the retirement from that?  So now this is all above my pay grade fortunately.

I don't know? Try it. Maybe "Mikey likes it?". Good  luck

I had a similar experience with my 72 pre-L when one of the "new" wheel bearings started to fail.  I had less than 500 miles on a new set of bearings, so it can happen. 

John

#3590

Thanks again everyone for contributing. Panteradoug especially for taking the time to explain stuff. Yes I do have negative 0.75°  front camber. Joules thanks for the setup info. 

I'm thinking the scrub radius is the real culprit and the additional positive caster will reduce the impact from side hits as "attack" angle diminishes. Also, I'll raise the tyre pressures.

Modern cars and race cars have wheel offset "inboard" so that the scrub radius is reduced. The fix with Pantera would be to bring the steering pivot outwards relative to the front wheel, but that's not going to happen for me....

It would be good if some other GT5 or Grpr4 owners could chime in to confirm...

@dang2407 posted:

The steering wheel almost gets ripped from my hands when one of the front wheels hits a bump.

I am thinking it is a bump steer problem.  As Julian mentioned, the factory added spacers to lower the rack and flatten the angle of the tie rods.  Are the shims in place on your car?  There are postings and articles about how to measure and fix bump steer on Panteras.  I remember reading about making a bump steer gauge.

Regarding caster, the primary benefit of additional positive caster is that the car returns to center better.  The down side it that the car is harder to turn at slow speeds.

This appears to be a good thread applicable to your situation…

https://pantera.infopop.cc/top...nment-and-bump-steer

Stevebuchanan, yes shims are above the rack, approx 6mm thickness. The car is already an upper body workout at low speeds....

There is one very significant factor in all of this regarding bump steer that no one else has mentioned.

That is 1) you can't eliminate all of the bump steer, just reduce it.

2) the stiffness of the tires sidewall is a major factor in what you feel through the steering wheel.

The most dramatic change would be going from the bias tires to a radial tire.

Even so, when you change the profile from the original Michelin's with a 78 profile down to probably what a wide body car would be with 10" wide front wheels, a 35 or 40 profile, even though those are radials, you are stiffening the wall of the tire AGAINST flexing. That is the point of the profile change.

That is going to negate the road shock absorption the tire provides and you will feel more of the shock through the steering wheel of the bump steer.

The Michelin tire that you are using is essentially a race tire legal for street use. That is even STIFFER in the side walls providing even less side wall flex and giving "greater road feel".

The TIRE selection you made is LIKELY the largest contributing factor to the change you are feeling in the steering.

So really this is one of the tradeoffs that you will need to endure.

There just isn't going to be a perfect solution but I go back to my original suggestion, particularly with the wide body cars. You can DAMPEN the shock in the steering wheel by installing an EPS system.

To me, that is the simplest solution and the most effective. Occam's Razor.

That's one beautiful GT5S, you should probably give up on all this steering nonsense and sell it to me!

There are numerous GT5S owners driving around with no modifications to their cars and not complaining of steering issues. I reached out to some of the Brits, and it seems recommended settings are updated from those of the factory to 0.5-degree negative camber front and rear with 1/8" toe in front and 0.1-degree toe in on the rear.

If it were me, I'd spend the money on a set of classic Pirelli Cinturato P7 tires in 285/40-15 and 345/35-15 and ditch the TB5R's

Joules, where have you seen a photo? If it's the detomaso registry, could you please post the photos here, or at least pm me? I can't get acces to the registry...

Panteradoug, surely the sidewall height is the same - if the width has increased the profile % reduces to keep the tyre diameter the same? I agree the construction will be different with the TB5s.... I recently acquired a set of Roin wheels - the man maths was to avoid damaging the expensive originals - so it would be an idea to get the pirellis for them...

PS,  it depends on the road surface - if people are driving on smooth, wide, flat roads, they won't have any steering issues...

I would suspect that of the few options thrown out…  toe-in adjustment & alignment, tire change, offset bushings, suspension modifications for camber & caster…

EPS installation is the MOST complicated choice, and if the problem isn’t corrected (or minimized) before EPS installation, it’s more of a band-aid than a root cause corrective action….

But that’s just someone looking at it from my arm-chair perspective…. They were able to significantly improve the steering behavior in the thread I linked above, and I know & trust Johnny Woods!

Rocky

PS. There are a few threads on EPS installation, as well, and I have spoken to two owners who have done it…  there’s a lot of “fettling” involved.

And when I say “fettling”, I mean pulling the EPS assembly in and out at least 5-8 times, while grinding off pieces of the structure around the steering column.  This means laying on your back in the cockpit, and fitting, marking, grinding and fitting, then doing the wiring.

Not a bad job if someone else is doing it, and your not paying for it!

Geez-ee!

I haven’t had any discussions with wide-body owner’s post-EPS install, either…

Maybe Doug is a wide body owner with EPS, and I’m overestimating the scope of the task?

The Roin wheels are going to be a bit heavier, so that will also impact steering feel. If you have the 26/61-15 TB5's then that's equivalent to a 285/40, which is OEM spec.

I'll PM you on the photos from the Registry....

No, I just have the solutions that are looking for problems to solve.

Nice car.

I'm not seeing this as a problem. It is a characteristic that is looking for a modification.

These cars are a lot like very attractive women. Initially the package is often extremely enveloping. Eventually, if you are lucky enough to get close, then you start to discover the quirks you didn't know were there?

Maybe evens some "fettling"? Gee-se!