Hi guys

I recently went from HRE wheel offset 35 to 40 on the rear. This has brought the rear wheels back in line with the rear wheel arches (before they were sticking out 5mm each side).

Driving the car in the damp the other day I noticed that the rear end seem to be much friskier coming out of left/right road junctions. PSM was flashing several times which is rare for me.

So my question is - do I need a new geo to reset rear toe and camber after switching to different wheel offsets?

Obviously the rear wheel footprints are 10mm less than before but surely that shouldn't make a huge difference?

 

Road conditions are probably more of a factor than just changing the ET on the wheels. Check your alignment and tire pressures.

 

Do you still have your tire-pull virginity?

 

I haven't noticed any "pull" and I can think of a left and a right junction where I came out of and the back end got frisky.

I am going to take the car down to my local tyre shop today to get an alignment check. If its different to what was accurately set just before the new wheels then I will go back to my suspension tuner and get the geo adjusted.

Ambient temperatures when testing were around 12 deg C and this happened both when the tyres were cold (37 psi on the rears) and warm (40 psi on the rear).

One thing that has changed is the tyres have air in them now instead of Nitrogen. So I definitely want to get that fixed.

 

Considering "air" is 78% nitrogen, I doubt you will find any difference there.

Probably road surface temps or dampness and/or tire temps played the biggest part. I don't know that you can base tire tread temp on tire air pressure. Would really need a pyrometer to do that accurately.

 

I'm no scientist, but from my understanding, tires inflated with nitrogen have repeatedly shown a more consistent air pressure vs. those running regular air. Small difference, but with a high performance car it might be more noticeable.

 

One thing my suspension tuner suggested was that my rear toe geometry settings were set with a full tank of fuel (adjusted) and when running 60kg lighter (low fuel) my toe settings won't be as great and the grip will decrease.

So it was probably that, the wet/cold roads and my tyres not having warmed up much at that point.

 

Nitrogen apparently is 'dryer' than regular air ie. holds less water vapour content. The water content is what has a dramatic affect on pressures - hence why nitorgen filled tyres have a smaller pressure range when cold<=>hot.

 

In this case, "dramatic" is in the eyes of the beholder. In normal sized tires, there is about .4 (four tenths) psi difference with compressed air compared to 100% nitrogen when the gas is heated from ambient to 200 degrees fahrenheit.
If the compressed air is processed through a typical dryer as is found in most commercial compressed air systems, the difference is almost immeasurable.
You are well informed about expanding gasses. ALL gases, nitrogen, oxygen or whatever, expand at the same rate when heated. It's science. It is only the partial pressure of the water vapor that is a variable and it is VERY TINY.
Due to its molecular size, Nitrogen will also leak THROUGH the tire at a slower rate than oxygen, but the rate is about 2-3 psi per YEAR.

There is another ugly secret about commercial "Pure Nitrogen" that is not well known. It is usually only about 95-96% Pure. Compressed air is already 78% nitrogen.
You can probably tell that I am a real cynic about nitrogen in tires. It is mostly hype and often very expensive.
Compressed nitrogen is commonly used in aerospace and other commercial applications to reduce corrosion in aluminum and magnesium alloys. Both magnesium and 7075 aluminum are highly susceptible to corrosion and are commonly found in these applications.

 

^ thanks for the clarifications!

Putting N2 in my tyres cost me a whopping £6.80 at my tyre dealer at the bottom of the road. As a result my TPMS says (dependant on time of year):

Cold tyres (air) = 29-31 psi front | 34-36 psi rear
Warm tyres (air) = 34-36 psi front | 40-42 psi rear

Cold tyres (N2) = 31 psi front | 34 psi rear
Warm tyres (N2) = 36 psi front | 40 psi rear

I like having my tyres dead on the recommended 34/40 PSI front/rear and with N2 there is certainly less range.