PFF - Pennock's Fiero Forum, PDA Edition

Upsizing Tires and Wheels (Page 1/1)

Dennis LaGrua

AUG 10, 04:44 PM

Many of us have upsized our tires and wheels for a more modern look. I now run 225 x 45 x 17 rear and 205 x 45 x 17 front Goodyear Eagle tires. IIRC they are on 8" in Sport wheels.
Changing wheels and tires did change things a bit. The car required an alignment and the 17" tires firmed up the ride to the point where I had to scrap the KYB shocks and put Monroe's on the front. Now the ride is better. Tires are inflated to 34 psi all around but what concerns me is the weight balance of the Fiero. The weight distribution is something like 60% rear and 40% front. Probably 70/30 with my 3800SC engine swap. Since there is less weight on the front of the Fiero its seems that tire pressure in the front should be be less but this has not yet been tried. Any tire experts here know if running lower front tire pressures under these circumstances is recommended?

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Darrelk

AUG 10, 06:12 PM

First, nope, I am not a pro. I have been building lighter weight vw kit cars, trucks, and Fiero based kits for the last 40 years or so. Some of the most difficult kits I have done have been in that magical 1400 to 2200 pound weight range in both rear and mid engine configurations. VW guys are fanatics about working out tire pressures because the wheel/tire combos that end up on kits and dune buggies are so radically different from the OEM wheel/tire combos that you might as well just start with a blank piece of paper to figure things out. Add in more unsprung weight from big brake kits and more suspension components like Kafer trusses, mid mounts, sway bars and such and the cars are just a mess to tune for good handling/ride characteristics. My last vw based kit car shared a lot of similarities with a car like yours however it was just much lighter. It was a rear engine supercharged 13 b rotary at about 320 h.p. and an all up wet weight in the mid 1800 pound range. This is an article that is worth reading and especially check out the Chalk Method B. on here....
http://www.thedirtreport.co...ce-and-non-oe-tires/
And yes, I know a lot of that is for racing but I think the basic chalk method at least gets you close to a good starting psi. The idea is to at least get that first good, complete, chalk print and slowly lower pressure and road test. On that car I mentioned it ended up being between 18 and 22 psi depending on what wheels I had on the car. On my 17 lb. front and 19 lb. rear wheels we found that 22 psi cold on the front worked pretty well, good ride, still able to run at well over 130. The backs ended up being right at 32 psi. When I would go to a lighter weight (14 lb. all around ) I would go down to 18 psi front and about 30 rear.
And I probably don't have to tell you this.....something I am sure you have picked up with 17 inch wheels is extra unsprung weight which must be managed by the susupension. I might be completely wrong about this but that extra weight must be managed with the lesser sidewall of the newer short sidewalls soooooo.....you might find that taking tire pressures down at all might be problematic...... Nothing written in rock, just sayin'........

Spadesluck

AUG 10, 08:48 PM

What is it you are after? I know from my drag racing days I would fill up the fronts plenty and let out of the rears so that the car would squat more. This was also done with ET Streets. What tires are on the car, which was already mentioned, would play into the PSI though process as well.

Dennis LaGrua

AUG 10, 10:10 PM

quote

Originally posted by Spadesluck:

What is it you are after? I know from my drag racing days I would fill up the fronts plenty and let out of the rears so that the car would squat more. This was also done with ET Streets. What tires are on the car, which was already mentioned, would play into the PSI though process as well.

The objective is to balance the car relative to weight distribution and tire pressure. On certain cars varying front and rear air pressures was common while on other s the tire pressure was the same all around . I believe that this was done to optimize handling and to keep tread wear equal.. Porsche, VW and the Chevy Corvair used unequal tire pressures.

Tony Kania

AUG 11, 02:10 PM

108* here yesterday. Temperature is important.

I keep records of my tires. I am on my third set for this Fiero, but have data for over 23 years from different vehicles that I have owned. I keep a small pad next to my tire gauge.

On this Fiero, I generally keep 4 pounds less in the front. KYB with Eibach all around. With the temperatures here, I have 30 lbs in the front and 34 in the back. I will put up to 44lbs in the back depending on temperatures.

Pressure also depends on how I am driving that day. Some days I want to blast through the mountains. I will take flack for this, but I like to slide and drift. I will lower pressure in the front, and max out the back.

All relative to my mood I guess.

Dennis LaGrua

AUG 12, 10:24 AM

quote

Originally posted by Tony Kania:

108* here yesterday. Temperature is important.

I keep records of my tires. I am on my third set for this Fiero, but have data for over 23 years from different vehicles that I have owned. I keep a small pad next to my tire gauge.

On this Fiero, I generally keep 4 pounds less in the front. KYB with Eibach all around. With the temperatures here, I have 30 lbs in the front and 34 in the back. I will put up to 44lbs in the back depending on temperatures.

Pressure also depends on how I am driving that day. Some days I want to blast through the mountains. I will take flack for this, but I like to slide and drift. I will lower pressure in the front, and max out the back.

All relative to my mood I guess.

When it comes to tire pressures, I thought that I was the only one that cut it fine. If you think about it, racers have always adjusted tire pressure to suit the condition to which they are used. Now after reading the comments here, articles online and considering my engine swap that offset the weigh distribution a bit, I have decided that lower tire pressures in the front is the way to go. Since I use 17" tires I will also measure the sidewall height at the lower pressures. It would guess that 34 lbs rear and 30 lbs front may work well for ride optimization.

2.5

AUG 14, 04:15 PM

quote

Originally posted by Tony Kania:

On this Fiero, I generally keep 4 pounds less in the front. KYB with Eibach all around. With the temperatures here, I have 30 lbs in the front and 34 in the back. I will put up to 44lbs in the back depending on temperatures.

I run about 30 front and 35-40 back with my 17s.

Dennis LaGrua

AUG 14, 06:18 PM

quote

Originally posted by 2.5:

I run about 30 front and 35-40 back with my 17s.

Since Tony and yourself are running them about that way and it was my guess that it would work, I have decided to go with 34-35 rear and 30 front. It makes sense especially for my Fiero where I have added a bit of weight with my powertrain swap.

theogre

AUG 14, 09:50 PM

Stop guessing weight.
Go to scales and weigh the car when no-one is in a hurry and ask the operator. Total, front and rear. (Many trunk scales weigh 3 sections, front, rear and trailer.)
Trunk stops and others have public scales. Examples: https://catscale.com/cat-scale-locator/ http://www.publicscaleslocator.com/locations.html
Other private scales may allow you to weigh a car but many only have one pad to weigh whatever and can have problems weighing non center loads. IOW many can't read front/rear only on the scale correctly.
Some "Race" shops have or know who has portable scales that maybe easier to weigh each axle. (Police often uses these too but most won't allow just to weight a car. They use them for overweight/overloaded vehicle stops. You may here a cop calling for scales on a scanner... this is what and why. many agencies only have 1 or 2 sets because of cost, training, etc.)

front/back weight % change does not = extra weight, if any, of new engine or trans because center of mass is in front of rear axle not On the axle or Worse Behind the axle. 3800 cast iron block and heads are close to same for Fiero V6. If has Aluminum block and/or heads likely less then Fiero V6 then add SC close to same.
if has 4T6x that is heavier TH125 but not a lot heavier.
If Battery is moved to front then can equalized added weight in the back. I don't love this for several reason but moving ~32lbs battery can help here. In most setup that ~32lbs Directly On the front axle. In winter many Fiero owners put 40-60lbs of sand etc in the front... bag(s) is on to just behind front axle depending just how loaded in there.

Tire pressure is not always the same but start w/ OE pressure = 30psi for Fiero. OE pressure is often same for many tires on the car not just stock tires.
Note: Never go over cold inflation limit "printed" on sidewall. The limit allows added pressure when the tires are hot and not wreck them. More confusing, Two tires of different models/brands may be same size but have different cold inflation limits. Always check the sidewall limits.

Yes Adding more tire pressure = adding total spring rate to the car means you will often get a rough ride.
NASCAR and others adjust inflation just for this reason but you tires are too small to guess how much change in rate for ±1psi inflation. Don't think you are getting same results as NASCAR tires.

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