C-Class Tyres??? Which Size??

[a6jus]

Hi
I have a C-Class W203 C320 Avantgarde
It has 205/55/16 tyres on all round, I have seen some C-Classes which have 225/50/16 tyres fitted at the rear??
Any reason for this, could I replace mine with 225s instead of 205 and any benefits??

 

[marnix]

With 225/50/16 tyres, you'll get better traction in dry weather condition but worse in the wet. The fatter the tyre, the more water to displace for road holding
As the tyre profile is lower, the ride will be harder and you will feel any bump in the road a bit more.
You will not be able to swap the tyres around to even out the wear.
The only benefit is probably visual, as it will look a bit more sporty with fatter tyres at the back

 

[a6jus]

Not much of a benefit then? But Merc and BMW do seem to like to have wider back tyres so may be some benefit somewhere to the driver??

 

[jberks]

No benefit really. Its just fashion. Gives it a more sporty stance.
I suspect the reason they started to make the rears wider is that
a) it looks good from the back
b) with space savers they can have them different sizes now
c) Wider tyres on the front tend to tramline and make the steering heavier.

On the downside, they are a bit noisier, have greater drag on fuel economy and will cost more.
Not really worth the effort.

 

[a6jus]

Shall stick to my 205/55/16 tyres I think and save myself £60 per pair!

 

[Rory]

With 225/50/16 tyres, you'll get better traction in dry weather condition but worse in the wet. The fatter the tyre, the more water to displace for road holding

I used to think this, but apparently it's incorrect - the footprint area on the road remains the same (assuming the pressure is the same).

As the tyre profile is lower, the ride will be harder and you will feel any bump in the road a bit more.

Yep - I have the staggered set up on my C270 Avantgarde and the ride is terrible on anything other than billiard table surfaces. It came with Bridgestones (noted for stiff sidewalls) and it felt like it was running on iron rings. The Michelin Primacy's on now are better,

You will not be able to swap the tyres around to even out the wear.

That's a real pain - especially as tyres on Merc's tend to wear in the centres on the back, and the edges on the front. I can't even out the wear. I also can't take advantage of Costco and Michelin's offers as you have to replace all 4 tyres.

The only benefit is probably visual, as it will look a bit more sporty with fatter tyres at the back

Yep. People do comment on it. The 225 rears look huge on the C Class.

 

[a6jus]

That has really made my mind up!!

 

[angus falconer]

I suspect the reason they started to make the rears wider is that
a) it looks good from the back
b) with space savers they can have them different sizes now
c) Wider tyres on the front tend to tramline and make the steering heavier.

....I think it is also related to sustained high speed loading in the rear axle - so, say, for a C class AMG with four people and luggage hrtling down the autobahn Merc decided to up the rears to cope with the loading. So I read somewhere.

Looks good but the rears are harsher than the fronts. You don't need 'em.

 

[tom7035]

I used to think this, but apparently it's incorrect - the footprint area on the road remains the same (assuming the pressure is the same).

By definition, 225 tyres MUST have a larger 'footprint' than 205's?

 

[marnix]

Of course wider tyres have a bigger foot print.
Otherwise there should only need to be one tyre available for all the cars.
The wider the tyre, the bigger the foot print and the greater the traction.

 

[jberks]

Foot print is trickier than you'd imagine. I don't actually know the exact facts, but playing devils advocate...

Assuming the same inflation pressure, tyres 'squish' down at the road surface, spreading the contact patch. How much this spread occurs will depend on sidewall strength and the psi pressure required to transfer the weight of the car to the road surface (not the tyre pressure). It's the old story that you're better off being trod on by an elephant than by a woman in Stilettos as, even though there is more pressure, the psi pressure under the elephant's foot is far lower, having more 'square inch' to spread the pressure over.

Assuming a similar sidewall strength between a 225 and a 245 (based on the same wheel size, the sidewall should be identical in size and construction), the tyre will therefore have the same deformation characteristics.
So, assuming it needs 10cm2 of contact surface to transfer the weight, that 10cm2 will be achieved with less F/R deformation on a 245 than on a 225, but the contact patch remains the same at 10cm2.

There are clearly lots of caveats to this principle including tyre diameter, construction, side wall strength, suspension geometry, tyre air pressure etc but in principle he may be right ....

 

[Rory]

Of course wider tyres have a bigger foot print.
Otherwise there should only need to be one tyre available for all the cars.
The wider the tyre, the bigger the foot print and the greater the traction.

Someone who seemed to understand these things explained to me that, of course the footprint is wider, but (assuming the pressures are the same) then the front to back measurement of the footprint will be smaller.

You can calculate the weight of a car if you know the area of the footprint (x4) and the tyre pressure. So, if you assume the weight of the car and the tyre pressures are constant, then fitting wider tyres must maintain the same size footprint.

 

[angus falconer]

Someone who seemed to understand these things explained to me that, of course the footprint is wider, but (assuming the pressures are the same) then the front to back measurement of the footprint will be smaller.

You can calculate the weight of a car if you know the area of the footprint (x4) and the tyre pressure. So, if you assume the weight of the car and the tyre pressures are constant, then fitting wider tyres must maintain the same size footprint.

I have heard (or read) the same thing

 

[tom7035]

Wider tyres therefore are entirely pointless with this argument.

 

[jberks]

There may be some benefit under high G conditions. effectively the weight will increase with increased G and there will be a subsequent increase is load and hence contact area. As the wider tyre is using less F/R spread than a thinner one, there is more to go at as the load increases.
Also, wider tyres tend to sit wider on the car, giving the car a wider track and hence a slightly lower centre of gravity.
However, in realistic situations the difference from 225 to 245 will be bugger all and you'd never tell!

 

[Rory]

Wider tyres therefore are entirely pointless with this argument.

Is it true though?

I'm trying to get my head around whether the tyre would last longer, the wider it is?

 

[jberks]

I'm trying to get my head around whether the tyre would last longer, the wider it is?

Basically no - it won't last any longer. The overall differences will be fractions of percentage points so 2 hastilly taken roundabouts will make more difference to overall tyre life than the width. The same is true of tyre pressure - the greater the pressure the lower the subsequent contact area and potentially the longer the life of the tyre (but with less grip in the dry and possibly more grip in the wet).

 

[mstar]

not sure if this is the case but with higher pressures as mentioned will ever so slightly improve fuel consumption? as greater forward motion as the circumference more rounder, then say less pressurised tyres? however if theory is correct air has weight so increasing the air may cancel out any benefits of ever-so-slightly increase in fuel consumption? and lack of grip during standing starts (i know ever so slight not worth mentioning but multiply by that in a year and its significant) require more right foot pressure to move/grip the car to move.

I think personally the weight issue may not be significant but love to hear if someone got a explanation for this?

 

[jberks]

not sure if this is the case but with higher pressures as mentioned will ever so slightly improve fuel consumption? as greater forward motion as the circumference more rounder, then say less pressurised tyres? however if theory is correct air has weight so increasing the air may cancel out any benefits of ever-so-slightly increase in fuel consumption? and lack of grip during standing starts (i know ever so slight not worth mentioning but multiply by that in a year and its significant) require more right foot pressure to move/grip the car to move.

I think personally the weight issue may not be significant but love to hear if someone got a explanation for this?

Well, higher pressure should reduce the contact patch (slightly) so the rolling resistance, both from friction of the contact patch and also the energy required to deform the tyre (try pedalling a bike with half flat tyres). The weight of the extra air is probably considerably less than the sweet wrapper in the ash tray - so we can discount that.
Traction reduction would occur, so you'd have to be more gentle with the throttle with with the subsequent improvement in consumption.

 

[big x]

Of course wider tyres have a bigger foot print.
Otherwise there should only need to be one tyre available for all the cars.
The wider the tyre, the bigger the foot print and the greater the traction.

No, the footprint is the same size but a different shape ! The only way to increase it is to go up a wheel size.In practice the wider tyre on the same wheel size does handle extreme loads better because the lower profile tyre distorts less at extremis.The cars power,weight and the desired handling profile determine which size is fitted. This link explains in DETAIL everything about tyres.

http://www.chris-longhurst.com/carbibles/tyre_bible.html

adam