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Michael Dalke's 1967 Firebird

2020 March
of the Month

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360Rocket

a wide track question of sorts

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So I recently bought a wheel and tire package just to change the look and performance of my GTO by going to a 17" aluminum wheel with a 255/45/17 tire and I expected a ride difference but not as big of a disparity as I am experiencing. The ride is stiffer, I expected that. There is more road feel I expected that. I had 15x7" R2's with a 245/60/15 almost identical in size. The difference I didn't expect was the car didn't track and it seemed more effort was required to keep her in a straight line she seemed to wander all over the road. Just out of curiosity when I returned home I measured from outer edge of tire to outer edge of tire both back and front and the front was 73" wide and the rear was 70". My old set up had all the same wheels with all the same backspace as the new wheels are but I didn't experience the "tracking" issue before......any ideas? Thanks in advance.

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What tires are you using? I’ve had good and bad luck with different radial tires. Tread patterns can have some effect. Have you considered a front end alignment? 

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Frosty I just had an alignment put on it before the tires and wheels went on These are Khumo Ecsta tires 255/45/17. I'm going to swap them for NITTOS since these tires are old. Gonna go with a taller and wider tire in the back as well and then drop the rear back down with a set of 2" lowering springs. I just recently got back on FACEBOOK thanks to work and I have been killing it on Pontiac Parts in the MARKETPLACE.

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Posted (edited)

More than likely the difference in the way car  tracks is directly related to considerably stiffer sidewalls of the low profile tires..

Even though the overall tire diameter is really close (the 17s being slightly taller) the actual sidewalls are narrower..and a lot stiffer.Due to the inside diameter being larger for the 17 inch rims and don't allow the same amount of side to side flexibility between the road surface through the tires to the rims...So any imperfections in the road that would normally be absorbed with the 15 inch wheels and the taller sidewalls in the tires are being transferred more directly through the suspension and ultimately the steering wheel..With the lower profile tires...Any loose or worn suspension parts that would have gone unnoticed will show up with the wider tires and/or stiffer sidewalls also with the contact patch of the tires being considerably larger it is picking up more of the road surface imperfections....

I have low profile tires on a couple of my cars..I can run over a cigarette butt and tell if it has a filter on it or not..

It Would be a good idea to thoroughly inspect the front suspension for any loose or worn parts and do an alignment....with special attention paid to the tow in/out...To help compensate for the wider tires...

Edited by TWO LANE BLACK TOP
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3 hours ago, 360Rocket said:

Frosty I just had an alignment put on it before the tires and wheels went on These are Khumo Ecsta tires 255/45/17. I'm going to swap them for NITTOS since these tires are old. Gonna go with a taller and wider tire in the back as well and then drop the rear back down with a set of 2" lowering springs. I just recently got back on FACEBOOK thanks to work and I have been killing it on Pontiac Parts in the MARKETPLACE.

360, I think you have several things going on! So I would like to help you step through it before you spend more money to get something less than you could actually have. 

First of all if you kept offsets the same as they were before as you state than something is wrong! The original track width for the car was 61” front track, 60” for the rear track. That 1” difference should still be the same! The only things that would change it are offset, different tire size or toe in! So if tire size is the same front and rear and the same as before, remeasure the front track again, but measure the front of the front tires and then the back of the front tires! If that number is more than a 1/8 to 1/4 inch that is part of the problem.

Also by going to a different size rim, 17” vs 15”, this can change the camber drastically, which in turn changes the caster. Also a shorter sidewall tire actually requires more air pressure than a larger sidewall tire of comparable size. This may sound wrong, but it’s not! Think of it this way, old style bike tires use less air pressure than an English bike! The English bike uses almost twice as much! So in the new 17” tires you should be running at least 36 psi in the rear tires and 37 psi in the front and that should be done with the tire is cold, not driven yet, at 70 degrees! Add or detract 1 psi for every 10 degrees of temperature above or below 70 degrees.

Next tire compounds make a huge difference in ride, both handling and stiffness. Everybody has their own preference, so that’s a personal choice. You might try to find out what the compounds are that are in the Khumo or Nittos, but as a rule neither one of them use the friendliest of compounds.

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Wow I love this detailed response and your help. My width measurements were from Outer sidewalk to outer sidewalk. We’re you measuring from the bare hubs? Also I believe that the rear 12 bolt axle is from a 67 GTO and I know I went to swap axles from a 68 Chevelle 12 bolt the Chevelle axles were longer by over an inch each. I’ll check air pressures as well. Do you know the widest tire size I can go with in the rear by chance? I’ll get you the backspacing when I get back home. Thanks for your assistance. 

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2 hours ago, 360Rocket said:

Wow I love this detailed response and your help. My width measurements were from Outer sidewalk to outer sidewalk. We’re you measuring from the bare hubs? Also I believe that the rear 12 bolt axle is from a 67 GTO and I know I went to swap axles from a 68 Chevelle 12 bolt the Chevelle axles were longer by over an inch each. I’ll check air pressures as well. Do you know the widest tire size I can go with in the rear by chance? I’ll get you the backspacing when I get back home. Thanks for your assistance. 

Well when I measure for tire alignment I usually use a a good wide stiff tape measure. I go to the back and front tires, because with FWD’s you align both front and rear, I look to see what and where the limiting height restriction is for a straight across measurement is. I measure that height and than measure both sets of tires at that height, at the back and the front of the tire. I use the hook part of the tape measure and get a hold on the last tread edge, closest to the outside edge, but not the outside edge of the tire, but the last tread edge. I measure from that edge to the mating edge at the other tire, but overall it doesn’t really matter as long as you have a dependable location to attach to that can be matched up on the other side & front and back as well as front and rear tires, all the same location. This not only gives you toe in, but if you know your front and back track difference between the front track and rear track, which in your case should be 1”. Then you’ll know you’re where you should be. If you have changed the front or rear track and want to check it then you need to make a setup that butts to the outside of each tire at again the highest point you can measure across from one tire to the other. Make that measurement and that’s the track width.

Also meant to mention that increasing the air pressure in a shorter sidewall tire, obviously within reason, will give a better ride.

Also, if you have a two foot level, have the wheels pointed straight, place the level vertically against each front tire, note where the bubble is and let me know. That disparity can really impact handling.

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