I was just reading the manual for a C4 and it says that the rear tire sidewall should be 35R and the front 40R. I did not see this and got 35R all the way round. What effect is this going to have?

 

probly just a slightly harsher ride and more likely to get dented rim.

 

Ok so this will have no effect on the mechanics/ axels etc? And i was wondering what the reason is for having 35 on the rear and 40 at the front.

 

by mounting these tire you will "deactivate" the 4wd due to the lack of rotational difference front to rear.

 

oekodriver - deactivate the 4wd! What have you been smoking? Care4era - The 35R and 40R are relative to the width of the tire. Many people will run a lower profile with wider width to avoid clearance problems. Search some of the threads. You will see many people changing from 265/35's on the rear to 275/30 and 285/30.

 

care4era, the lower profile tires "may" give you a smaller tire diameter. I say "may" because it's relative to the tire width. Just check the tire specs, and you shoud be able to figure out the diameter relative to the recommended sizes.

The AWD system has a viscous coupling, and power delivery is adjusted based on rotational differences between front and rear tires. If you don't maintain the recommended tire sizes and change the rotational difference between front and rear tires, the viscous coupling may not function as designed. This will be further compounded if you have PSM and ABS, two other systems that depend on the rotaional differences between the tires and their respective axles. Having said that, whether this theoretical change is significant in the real world or not, I couldn't tell you. I've never challenged the recommendations, and have tried to stay relatively close to the diameters of the recommended tires.

 

What are the exact sizes of your wheels and tires?

 

You see alot of owners running non-standard size tires. I have looked at the relative sidewall heights as follows:

Standard
225/40=90
265/35=92.75

235/40=94
265/40=106

Many recommend 275/30 and 285/30 on the rear

225/40=90
275/30=82.5

235/40=94
275/30=82.5

225/40=90
285/30=85.5

235/40=94
285/30=85.5

With PSM and 4wd the concern is relative size. You can see that There is a wide variation of the delta in the sizes you see in postings here and on Rennlist. I believe these are all milimeters. The stock is only about 3mm, but you can see there are alot of owners running larger deltas.

Anybody out there running 275 or 285's on the rear and the standard 225 or even a 235 on the front? have you had problems?

 

Tire size is only part of the equation. You need to factor in the size of the wheel too, as the tire's overall dimensions will vary slightly.
http://www.1010tires.com/

 

Ok so reccommended is

rear 265/35R 18 I have 265/35R 18
front 225/40R 18 I have front 225/35R 18

So as you can see the only difference is the sidewall on the front tires. Spec says use 40 and I used 35

 

What is the width of your front wheel? 8"? 7.5"? 8.5"?

 

heh, that i cannot tell you. I just know they are 18"

 

@Dharn55

you might want to check your knowledge about the c4 and its viscous coupling.

from design the rear wheel/tire combo diameter should be smaller than front. this achieves the permanent ca 5% torque on the front.

If now suddenly you mount a same size or smaller diameter front wheel/tire combo than on the rear you effectively "disable" the intended effect of the viscous coupling thus 4wd. Of course it will still work in case of massive slip of the rear but not in the way it was designed for.

and btw you should smoke one more.. maybe it helps

@care4era
you should get a set of 225/40 18 for the front

 

I would agree with this too.

 

That means your front tire/wheel rolling diameter is smaller, and therefore rotates faster than the recommended 40 series. Not up to specs, and theoretically boogers up the viscous coupling and ABS, and performance. I'd change to a 40 series.