E30 on 17 inch alloys

I hope somebody can answer a couple of questions for me! I've just fitted 17inch Alpina replica's with 205/40 17 tyres and just want to check a couple of things. At the moment the tyre pressures are the same as the standard 205/55 15 previously fitted, i.e factory recommended pressures, is this correct or is an adjustment required to compensate? Also does the change of wheels affect the speedo reading in any way?

Cheers

Steve

Re the tyre pressures.

I've just fitted the same wheel/tyre combo as you, while my son has been running an Astra (I really must get round to giving him a slap for that) on the same size wheel & tyre combo with standard pressures.

I happened to look at his tyres at the weekend and the centre of the tread has worn faster than the outer edges (done about 15k miles) which points to overinflation.  Thinking about it logically, with the same vehicle weight, the bigger the footprint of the tyre, the lower the pressure should be.

I've had a chat with a couple of mates in the racing world and they agree with me.  As a result, I've dropped the pressures on mine by 2psi from standard all round (now at 26 front-27 rear).

It may not sound much, but I know from first hand experience that it will make a hell of a difference.

It's now just a matter of keeping a close eye on them.

Hope this helps Steve.

 

BTW, 325SME, your tyres are ~10mm smaller in diameter than before - but the difference is small enough.

If it was bigger, you may have fractionally better in gear accelaration, higher revs for a given speed and worse fuel economy

Well boys & girls; b318isp really told me didn't he?

Sorry mate, but I found your post to be a little on the harsh side so I think it only fair that I respond in order to ask for some clarification and clear up a couple of points.

1/  If the vehicle weight doesn't change, how can the sidewalls of the tyres carry proportionally more weight?

2/  Wider tyres will give you a bigger contact patch.  It may not be a huge difference but it's a physical fact none the less, otherwise what's the point in fitting them and has the motor racing fraternity got it so wrong for all these years?.   

3/  I believe you may have misread the formula that you quoted.  I think the one you are thinking of is Pressure = Force over Area (at least that's the one shown to me in the SI Units book for Mechanical Engineers).

Using that formula,  where: P = the pressure exerted by the tyre upon the ground at any point within the footprint of the tyre,  F = weight of the vehicle and A = the footprint or contact patch upon the ground.

Assume for this exercise that a tyre has a footprint of 100mm along the longitudinal axis of the vehicle and that footprint areas are perfectly rectangular (I know they are not, but it's easier to show the calculation in this example than an eliptical shape).  Assume also that the vehicle is perfectly balanced, with an even weight distribution of 700lbs at each of the four wheels (total 2800lbs).

With a 195 tyre, the equation would read,  P = 700 � (195 x 100)    

Convert the metric units to imperial and P = 700 � (7.677 x 3.937)

P = 700 � 30.224 sq ins   therefore P = 23.160 lbs/sq in 

With a 205 tyre, the equation would read,  P = 700 � (205 x 100)    

Convert the metric units to imperial and P = 700 � (8.071 x 3.937)

P = 700 � 31.775 sq ins   therefore P = 22.029 lbs/sq in 

The bigger the contact area, the lower the pressure on the ground.  Fact.

4/  Racing tyres are not designed to run at much lower pressures than car tyres.  The pressure is dependant on many variables (type of surface, track temp, air temp, track shape, driving style, atmospheric pressure, moisture content of the nitrogen used to inflate them, etc, etc) and these pressures can be anywhere between 15 and 40psi.

I mentioned racing merely to illustrate that people with a much greater understanding of how tyres work, agreed with my theory!  It was not meant to be taken as a literal comparison. 

I'm not trying to belittle you here mate, because you do have a valid point to make.  Let's keep it friendly, eh?