Old 11-20-2020, 07:44 PM
JLMounce JLMounce is offline
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Originally Posted by 389 View Post
So your telling me you drive a car with the original suspension that positive cambers on compression and you are running modern low profile stiff sidewall tires. Do you have any idea how stupid you look to me and anybody else who knows suspensions right now? The first time you try to panic stop that death trap your driving you will remember this post and what I was talking about.. I guarantee it..
The point you're trying to make is that the softer sidewall of the larger tires found on a 14" or 15" wheel and tire package will react better under straight line braking than the stiffer sidewall of a shorter aspect ratio tire. The assumption is that the taller and softer tire will allow the contact patch to better conform to the road surface, thereby creating more available grip.

It should be noted that fundamentally, the wheel and tire package doesn't change the function of the suspension itself. So when you look at braking performance you do have to look at the camber curve as well. Here is an example of stock 1967 Camaro as recorded by David Pozzi. I think we can all agree he's pretty much an authority on GM short/long arm suspension design.

Although the camber curve is backwards for what we would want to see from a handling and grip standpoint for cornering. the overall curve remains fairly flat, which is more ideal for straight line braking since it will maximize contact patch for whatever tire is attached to the wheel.

It is true that the softer tire will yield more and provide a more square contact patch under heavy braking. However if you look at that curve in stock form, at 2" of compression you're only introducing half a degree of positive camber to the tire. If you make no other change than to dial in half a degree of static negative camber, you effectively flatten your wheel and tire in a panic stop situation.

Even without dialing in some much needed static negative camber, the type of movement these cars see under such a situation is not so much that the modern, smaller aspect ratio tire can't cope with the change. Remember, these are still radials tires which are designed to allow flexing of the tread, independent of the sidewall. Any inherent stiffness gain from the sidewall of the smaller aspect ratio tire does not necessarily correlate to a decrease in contact patch due to the tread's inability to contour to the road's surface. That would be different if you were talking about a bias ply tire.

When you take that situation and combine it with the fact that the modern tire has a larger inherent contact patch than the older tire and has superior rubber compounds, any greater loss of contact patch due to the nature of its construction is made up by the fact that it has a larger contact patch to begin with and typically carries far superior rubber compounds that have much improved levels of grip, comparatively.

In some cases it may actually be a worse situation when increasing spindle height to alter the camber curve. Modernizing the original design in this way may introduce more negative camber in compression than you would have positive camber with the factory suspension. It's a trade-off that is made to get around a corner faster. It's also why positive caster is so beneficial, specifically to these cars, because you get the increasing negative camber gain at larger steering angles without effecting the contact patch under braking.

Anyhow, all that to say that throwing a set of 17" wheels and shorter tires on one of these cars doesn't automatically make it any more of a death trap than the car inherently is to begin with.

One area that I would agree should be looked at is wheel size and weight with factory braking systems. The added weight of larger wheel and the fact that the weight is carried further from the wheel's center increases rotational inertia that decreases acceleration and increases stopping distances.

1969 Pontiac Firebird
Old 11-20-2020, 08:02 PM
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amcmike amcmike is offline
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Originally Posted by JLMounce View Post
One area that I would agree should be looked at is wheel size and weight with factory braking systems. The added weight of larger wheel and the fact that the weight is carried further from the wheel's center increases rotational inertia that decreases acceleration and increases stopping distances.

Excellent point. This is sometimes forgotten. The overall rotating mass and moment of inertia can affect braking and even steering to a degree. But you have to consider also total differences between the original wheel tire combinations. Going from steel to steel will for sure be more (especially if you go wider as well). But aluminum may not differ as much.

"The Mustang's front end is problematic... get yourself a Firebird." - Red Forman
Old 11-21-2020, 10:28 AM
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JUDGE3 JUDGE3 is offline
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I lived the era and for me the only bump up in wheel size that looked far better was taking an early car that came with 14" rims and putting the newer 15" rims on. most of us did that back then and it was very surprising how much an improvement it was. the 14's looked to tiny on them.

If you bump up to funkmaster flex size rims, you have to change everything else with it.

me, I like having all that sidewall and stock everything for a better ride and to me a better look. easier on the car, easier on me!

The One Who Lives Under The Protection Of The Most High Dwells In The Shadow Of The Almighty. Psalm 91

72 Firebird/Trans Am clone/455/Th400/moser 3.73
Old 11-24-2020, 07:47 PM
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DogMeister DogMeister is offline
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I'm running a 461 stroker with a Tremec TKO-600 and a 3.55 posi. 2" lowering springs in the front, back raised maybe an inch. Running 15" Rallye Is on all four, Wheel Vintiques if I recall, 8" in the back, 7" in the front. 255/60s in the back, 215/65s in the front. Looks great, rides great, no clearance issues.

461 Stroker Built by Me -

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