I don't understand tyres...

[whitenemesis]

It's more usual to have 7.5" fronts and 8.5" rears with those tyre sizes. Had them on my S203

 

[Naraic]

Still is common, a friend of mine has an arch roller, very good bit of kit, the "euro look" boys do it.

The theory on wider rears is to put more rubber on he road therefore increasing grip, aesthetically it looks better as the rear arch is filled.

For example I have 8x17" ET35 wheels all round on my W203 but have 225/45 front and 245/40 rear tyres which are correct for the car and maintain the rolling radius of the wheel / tyre combination and keep the speedo reading what it should.

Ermm...wrong.

Wider tyres do not fill the arches better than narrow tyres...unless you get it wrong...in which case the CC and other systems will not operate as they should.

In fact, contrary to what you think, your rear tyres are actually (diameter wise) smaller than your fronts by 6mm!! So having the same size all round (225/45) would fill the rear arches better.

 

[Carabosse]

The only way of filling your wheel arches better is by lowering the car.

 

[television]

Then lower the car and back to a harder ride

 

[Carabosse]

Then lower the car and back to a harder ride

Not necessarily. Choice of springs, shocks, rims and tyres all come into the equation.

My last two cars were both lowered. On one, the ride improved significantly and on the other the ride quality was unchanged (probably because I went from 16s to 18s as well as lowering).

 

[whitenemesis]

Lowering improved the ride in my S203, it became much less harsh. It all hinges on the correct selection of springs and compatible shock absorbers.

 

[hotrodder]

The theory on wider rears is to put more rubber on he road therefore increasing grip

There's no theory about it, it's simple (pre GCSE) maths and wider tyres DO NOT put more rubber on the road

Size of a tyres contact patch (rubber on the road) is a function of the downward force on the tyre and the air pressure in it. As most of us still do tyre pressure in psi we'll use lbs for weight to keep unit conversions out of it and pretend weight distribution is perfect so each tyre is supporting 1/4 of the cars weight.

Say each tyre has 800lb of weight on it. If it's pumped up to 10psi each square inch of tyre area can support 10lbs so the contact patch will be 80 square inches hence a typical car tyre looking flat @10psi. At 32 psi the contact patch will be 800/32 = 25 square inches

Very roughly* if the tyre is 5" wide then (@32psi) the contact patch will be around 5" wide and 5" long. If the tyre is 10" wide then the contact patch will be 10" wide and 2.5" long.
* Obviously (with the exception of hateful run flats) tyres are flexible so the contact patch isn't exactly rectangular at rest and changes shape when cornering etc.

The narrow tyre with it's longer contact patch means the tyre has to flex more than the wide tyre. This flexing creates heat and since more of the tyres circumference is in contact with the road cooling is hindered i.e. the tyre will overheat more easily/quickly than a wide one. Doesn't help 'turn in' feel either which brings us to slip angle. This is where things start getting complicated hence me settling for the gross generalisations i posted back on page 1... wider tyre can generate more grip cornering at the limit but will 'let go' with less warning than the squidgy narrow tyre. It WILL produce less wet grip, be noisier, reduce fuel economy, increase unsprung weight, be more prone to tramlining etc etc. On the plus side it'll 'fill the arches better innit' :lol:

 

[rlw]

I don't know if this was meant to be arrogant and condescending but that's how it came across.

I accept how this and any other car is specified. What I am trying to do is understand the theory/geometry/physics or whatever behind the principle, i.e. the why. Please excuse me for trying to broaden my education and understanding of how and why cars are put together differently!

Neither arrogant nor condescending. For an extreme example of why the car is like it is, look at an F1 car. They all have wider rear tyres. Helps with traction and ultimate grip. Doesn't help in standing water though.

It seems to me, perhaps wrongly, that someone looking to buy a reasonably high performance saloon car ought to be a bit more clued up. Sorry if that offends.

 

[Gkinghorn]

To be fair to the OP he is asking a reasonable set of questions. There is a fair amount of info here that I and others have found interesting.

Not everyone can know everything about wheels/tyres.

Remember the only stupid question is the one you don't ask!



sent from my Galaxy Note 8 via Tapatalk

 

[JBell]

Ermm...wrong.

Wider tyres do not fill the arches better than narrow tyres...unless you get it wrong...in which case the CC and other systems will not operate as they should.

In fact, contrary to what you think, your rear tyres are actually (diameter wise) smaller than your fronts by 6mm!! So having the same size all round (225/45) would fill the rear arches better.

Ermm...right

Width ways they do, having had both sizes on the rear of the car the 245 look much better and do not loose traction as easily as the 225 (before you say they were different tyres they were all Pirelli P Zero MO Spec)

 

[JBell]

It's more usual to have 7.5" fronts and 8.5" rears with those tyre sizes. Had them on my S203

The wheels I have are only available in 8x17"

 

[Wirral_guy]

Neither arrogant nor condescending. For an extreme example of why the car is like it is, look at an F1 car. They all have wider rear tyres. Helps with traction and ultimate grip. Doesn't help in standing water though.

It seems to me, perhaps wrongly, that someone looking to buy a reasonably high performance saloon car ought to be a bit more clued up. Sorry if that offends.

THERE is no such thing as a stupid question, only a person too stupid to ask a question when they need too.

The OP was unsure therefore asking a question, how else are you supposed to gain knowledge? How did you get so clued up? Asking questions by any chance?

I know plenty about tyres but I have learnt something from several posts on here (hotrodder's posts for one).

Some of the forum responses of late really are bordering on offensive at times - it is supposed to be a place to gain\share knowledge not for displays of arrogance and bad manners.

Dave

 

[Salforal]

Neither arrogant nor condescending. For an extreme example of why the car is like it is, look at an F1 car. They all have wider rear tyres. Helps with traction and ultimate grip. Doesn't help in standing water though.

It seems to me, perhaps wrongly, that someone looking to buy a reasonably high performance saloon car ought to be a bit more clued up. Sorry if that offends.

No offence taken but I would disagree with your thinking that anyone buying such a car "ought to be a bit more clued up" without knowing exactly what you mean. And I'm still not sure you understand where I'm coming from with this thread - apologies again if that too is not the case. I am not questioning the design process or mechanics or physics because I think it's wrong but out of curiosity.

In the same way, for example, I would accept a physiotherapist telling me I need to carry out certain rehabilitation exercises following an injury without question because he or she is the expert. However while I wouldn't need to know why those exercises are most suitable I may choose to ask how they will benefit me for my own understanding.

Here I have chosen to ask why the tyres are different sizes out of interest, no more.

I'm quite happy and prepared to have a discussion with you away from this thread to save boring everyone else, if you wish. I'm not looking to fall out with anyone or cause offence or upset.

 

[JBell]

Here I have chosen to ask why the tyres are different sizes out of interest, no more.

Because that is what MB do and it looks nice

 

[JBell]

This one looks nice
http://mercedescarfinder.co.uk/detail.html?car_id=170581

 

[Naraic]

Ermm...right

Width ways they do, having had both sizes on the rear of the car the 245 look much better and do not loose traction as easily as the 225 (before you say they were different tyres they were all Pirelli P Zero MO Spec)

Well of course width wise...they are wider after all. I suspect when most people talk about filling the arches they usually mean as viewed from the side of the car...not the rear. Sorry I assumed wrongly.

 

[rlw]

I'm quite happy and prepared to have a discussion with you away from this thread to save boring everyone else, if you wish. I'm not looking to fall out with anyone or cause offence or upset.

No need and neither am I. End of story.

 

[muzam321]

This thread has been very informative actually. My wife curbed my rear alloy the other day and caused some damage to the tyre. When I looked into replacing it, that was the only time I realised that I had different tyre sizes front/back.
Call me ignorant but when I came to buying my car, it wasnt something I had ever come across so I never even paid any attention.
From what I have read here though, would I be right in saying if I were to buy a totally new set of alloys then there would be no problems having the same size on all corners?
I am not planning on changing the alloys just because of the whole wider rears debate. I had planned to do this anyway and based on the info I have read here, it seems that I will hardly notice any difference and my future tyres will be cheaper.
Thanks in advance

 

[EmilysDad]

.....
Thanks in advance

Be sure to tell your insurer that you've changed the spec of your car

 

[npuk]

Yes you can change for alloys the same width all round. The fitment chart on alloywheelsdirect lists the correct size, ET and tyre size for your car. They also sell genuine Mercedes alloys. Even if you dont buy from them it is a usefull site to get the part numbers for the wheels you want. http://www.alloywheelsdirect.net/

If you have a decent insurer and you go for genuine Mercedes wheels they shouldnt have an issue. Admiral didnt when I changed from the factory fitted 17" Menkib to genuine 17" AMG.