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Tire Air Pressure GTV 115

3K views 14 replies 6 participants last post by  Del 
#1 · (Edited)
I'm wondering if there is a fairly linear grip difference between front and rear as air pressure in the tires are changed say from 24/26 F/R to 26/28 F/R to 28/30 F/R to 30/32 F/R? I would imagine there would be a small change as the tire pressures go up. The reason for the question is that optimum tire pressure for tires wear I'm sure is different from optimum tire pressure for grip. And then there is tire model/size, and suspension set up. Also, what about using a 4 psi differential, e.g., 26/30 F/R, etc.? Too many variables. The tire size I'm using is 185/70x14.
 
#2 ·
I run Pirellis that size on a '72 Super 1.3 (115) .. and it seems happy at 30/30 on the road. I'd start there and experiment with 2 psi increments. Each tire manufacture and compound will give you different feels so depending on the tire and the age of them I would expect no two cars to feel the same. Personally, I think anything lower than 30 on the fronts creates a understeer (snowplowing) that will accelerate wear. A ggod place to check that is a clear on ramp onto the interstate.
 
#3 ·
What I have been working on is oversteer. This is after have an alignment done by a professional who is very familiar with Alfas and other similar sports cars. Upon turn-in to a relatively sharp corner at 50 mph, the car exhibits significant oversteer. Several suggestions have been to lower front tire pressure by at least two psi relative the the rear pressures. I think I'll try 28/30 F/R next. --Thanks
 
#6 ·
Upon turn-in to a relatively sharp corner at 50 mph, the car exhibits significant oversteer.
You may have already gone over the rear suspension, but this is a classic symptom of worn out rear trunnion bushings, or missing spacers between the bushings and the trunnion.
 
#5 · (Edited)
Tire pressures

As Ed suggested, the Front tires carry more weight, do more work (steering) and need to have more grip.
Lower pressures in the rear give a more stable turning experience with less tendency to step out/oversteer.

Rule of thumb setting up tire pressures is factory air pressure in rear, 4 lbs more in front.
 
#7 · (Edited)
I was always taught that for front engine rear wheel drive cars, you wanted a higher pressure in the rear, not the front, as higher pressures in the front increase oversteer. Certainty did in any front engine rear wheel drive Alfa I've driven through the decades.

Front wheel drive cars on the other hand, such as the 164s I drive now usually call for higher pressures in the front to reduce the larger amount of understeer that front engine/front wheel drive cars have, the weight distribution being more weight up front, and the tires doing more work in cornering.

The 115 model Alfas, having a closer 50/50 weight distribution, will usually have a smaller difference between front and rear pressures, but the handling characteristics will still hold. I usually used the same pressures front/rear in those, as with our Milano, as Pete Lovely had suggested, never more in the front.

The Tire Rack web site, for instance, notes the following handling changes:

------------------decrease understeer----- decrease oversteer
Front Tire Pressure:----- Higher------------------- Lower
Rear Tire Pressure:------ Lower------------------- Higher

The above becomes more evident at higher speeds. Usually, manufacturers design in a tad of understeer for normal highway driving, as it is a safer way to drive, fewer high speed spinouts.

Racing setups are a whole different kettle of fish so to speak.
 
#12 ·
Air pressures Front and rear

Try this:

Rather than read, set your air pressures on the rear to factory setting for those size tires. Then set the front tires 4 PSI higher. Drive the car and note the difference in handling and responce.

Lower rear tire pressure keeps the contact patch more stable and gripping. Less contact loss on irregular surfaces.
Lower front pressures allow more tire roll and contact patch loss inducing understeer. This is preferable to corporate engineers, as most drivers are very poor at controling oversteer.

It really concerns me when people argue a point because they "READ" something contrary, and haven`t tested the concept.

I bet most amature racers are still running higher air pressures than needed, because if their reading material.
 
#13 · (Edited)
That seems to be the crux of the conversation, that of mixing racing setups with ordinary driving on public highways, where potential higher speed spinouts, such as I've witnessed, can endanger the driver, passengers, and nearby drivers.

Yes, one can make the car seem more spritely in low or medium speed cornering by increasing oversteer, but at higher speeds this can lead to terminal spinout oversteer. This is regarded by engineers as not desirable for a real reason, it being potentially dangerous for the driver, his passengers, and those drivers around them for normal driving on highways. It's just the dynamics of the vehicle.

I did mention that racing setups can be quite different, and for some racing situations and track designs, increased oversteer can be desirable, but not for ordinary driving on higher speed highways.

The Stig most are not, lol.

I've been driving Alfas for 5 decades here around the country and abroad, and have done testing with various models, including a little track time with Alfa test drivers, and this was their conclusion for the average Alfa driver, even those with some skill. I know I would rather have a little more understeer in order to avoid excessive high speed oversteer, seeing as how I may drive at 80-100 mph on some freeways, 80 mph (at least) being legal on some highways) in the West, on vacation trips to Texas or Yellowstone NP. Think about that deer or elk jumping out in front of you at speed!

Having said that, however, go ahead, trying different tire pressures front and rear at various speeds on a winding road, and on the highway, trying a little gentle swerving at different higher speeds. Experiment. Draw your own conclusions based on the type of driving you will primarily do. Just understand the engineering basis for what you find.
 
#14 ·
I did mention that racing setups can be quite different, and for some racing situations and track designs, increased oversteer can be desirable, but not for ordinary driving on higher speed highways.

The Stig most are not, lol.

I've been driving Alfas for 5 decades here around the country and abroad, and have done testing with various models, including a little track time with Alfa test drivers, and this was their conclusion for the average Alfa driver, even those with some skill. I know I would rather have a little more understeer in order to avoid excessive high speed oversteer, seeing as how I may drive at 80-100 mph on some freeways, 80 mph (at least) being legal on some highways) in the West, on vacation trips to Texas or Yellowstone NP. Think about that deer or elk jumping out in front of you at speed!

Having said that, however, go ahead, trying different tire pressures front and rear at various speeds on a winding road, and on the highway, trying a little gentle swerving at different higher speeds. Experiment. Draw your own conclusions based on the type of driving you will primarily do. Just understand the engineering basis for what you find.
Really good advice... track conditions usually give you better control of your environment than road conditions and balance of the car on the road can be more easily upset by outside influences ( deer ..elk..idiots). I've swapped ends with Porsches regardless of tire pressure but never an Alfa and don't want to start. Driving at 85% v. 110% on public roads is probably the best cure for most oversteer in an Alfa. Also, it is amazing how much better ride you will have in a straight line if the pressures are right. That might not be optimum for performance but all around it improves the experience of the drive.
 
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