#21  
Old 11-20-2020, 07:44 PM
JLMounce JLMounce is online now
Ultimate Warrior
 
Join Date: Sep 2005
Location: Greeley, Colorado
Posts: 3,679
Send a message via AIM to JLMounce
Default

Quote:
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.

__________________
-Jason
1969 Pontiac Firebird
  #22  
Old 11-20-2020, 08:02 PM
amcmike's Avatar
amcmike amcmike is offline
Ultimate Warrior
 
Join Date: Oct 2004
Posts: 1,733
Default

Quote:
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
  #23  
Old 11-21-2020, 10:28 AM
JUDGE3 JUDGE3 is offline
Ultimate Warrior
 
Join Date: Sep 2011
Posts: 1,122
Default

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!

  #24  
Old 11-24-2020, 07:47 PM
DogMeister's Avatar
DogMeister DogMeister is offline
Senior Member
 
Join Date: Nov 2013
Location: Jasper, GA
Posts: 180
Default

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 -
  #25  
Old 12-04-2020, 01:48 AM
jimib's Avatar
jimib jimib is offline
Chief Ponti-yacker
 
Join Date: Dec 2003
Location: Boca Raton, FL
Posts: 681
Default

Quote:
Originally Posted by gtospieg View Post
Here are pics of my 67 with 255/45/17 on 17x8 wheels with 5.5 backspace on all 4 corners.
gtospieg: Your car looks fantastic with the 17's and I like the slight rake.

__________________
1967 GTO, hard top, 400 Block, Butler Performance Build, EFI Holley Sniper, Tremec 5-Speed, Moser 373 Rear, 4 Wheel Disc Brakes
  #26  
Old 12-04-2020, 02:37 AM
Scarebird's Avatar
Scarebird Scarebird is offline
Ultimate Warrior
 
Join Date: Nov 2002
Location: ABQ, USA
Posts: 4,998
Talking

Quote:
Originally Posted by JLMounce View Post
Yeah these cars are real soft when compared to modern cars. Not too many people in the market for cars these days desire a floaty disconnected feel from their vehicle.
I sprung for a set of 17 x 8 US Mag Bandit U109's shod with Vogue Signature V's in 235-55-R17. The rims are lighter than I thought they would be.




  #27  
Old 12-04-2020, 08:57 AM
gtospieg gtospieg is offline
Ultimate Warrior
 
Join Date: Apr 2012
Posts: 1,225
Default

Thank you jimib....not inexpensive

  #28  
Old 12-04-2020, 01:18 PM
JLMounce JLMounce is online now
Ultimate Warrior
 
Join Date: Sep 2005
Location: Greeley, Colorado
Posts: 3,679
Send a message via AIM to JLMounce
Default

Quote:
Originally Posted by Scarebird View Post
I sprung for a set of 17 x 8 US Mag Bandit U109's shod with Vogue Signature V's in 235-55-R17. The rims are lighter than I thought they would be.



Cast aluminum wheels aren't too bad on weight in 17" sizes when compared to a 15" steel wheel. Usually within a couple pounds on either side, depending on the wheels being used. Something like an AR salt flat is going to be heavier than a Torqthrust.

At the same weight though, there's still a bit more rotational mass since the weight tends to be carried further from center. It's a performance trade-off.

__________________
-Jason
1969 Pontiac Firebird
  #29  
Old 12-04-2020, 03:36 PM
71HOT/A 71HOT/A is offline
Senior Chief
 
Join Date: Aug 2007
Posts: 279
Default

Don't just look at wheel weight, look at wheel/tire weight. Some tires are heavier than others. The tire's mass is even farther from center than the wheel.

Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -4. The time now is 04:53 PM.

 

About Us

The PY Online Forums is the largest online gathering of Pontiac enthusiasts anywhere in the world. Founded in 1991, it was also the first online forum for people to gather and talk about their Pontiacs. Since then, it has become the mecca of Pontiac technical data and knowledge that no other place can surpass.

 




Copyright © 2017