Two differnt tyre types

[paulmchurd]

Hey,

So in for a service and the mechanic says I have

205/40/17 tyres on my rear wheels and the 195/40/17 on my front wheels.

He said this would be a mot failure. Is there a risk driving about on two different types of tyre? Ive heard the 205 are cheaper wheels and must be the reason why they have been fitted.

Thanks
Paul

 

[maggi200]

I think he's wrong, unless the rules changed in that respect. If it were the other way around I would be concerned but as it is, I'd find another mechanic. It's a perfectly acceptable way to run the tyres, As long as they're the same across the axle

 

[paulmchurd]

OK thank you. When talking to him I did feel like he was just trying to get some more money out of me. I told him Id leave it any way.

Why would the previous owner put larger tyres on the rear is it for better traction? Could different sizes damage the car?

Thanks
Paul

 

[maggi200]

I ran it like that for a while. You want your best on the rear, on the road understeer is preferable to snap/lift off oversteer.

You also want value for money so when the fronts need rotating it makes sense to stick the old rears on the front to get the life out of them, and then the new tyres, which happen to be wider and newer, on the back.

 

[singlespeed]

Reason for rejection. 1a. One tyre is of a different nominal size or aspect ratio to any other on the same axle.

Front or back is OK. Mixed left/right isnt, hence space savers aren't allowed to pass

 

[paulmchurd]

You've cleared things up for me

Thanks for the advice really appreciate it.

Paul

 

[paulmchurd]

Ok so it's actually the other way round the 205 are on the front and are 195 are at the back.
The rear tyres are older.
Is there a major concern about this?
Thanks
Paul

 

[maggi200]

Swap em round. As I said above, you don't want the back end overtaking you if you can help it.

 

[wrighty94]

Aslong as the tyres don't rub and they are the same size on the same axel it's a pass! Bmw and mercedes come with different size tyres front and rear

 

[waitey]

Aslong as the tyres don't rub and they are the same size on the same axel it's a pass! Bmw and mercedes come with different size tyres front and rear

Yes but the wider of the two is always on the REAR.

 

[oscar]

Aslong as the tyres don't rub and they are the same size on the same axel it's a pass! Bmw and mercedes come with different size tyres front and rear

Yes but the wider of the two is always on the REAR.

that's not true whatsoever. The basic rule of thumb is that tire size should be roughly proportional to tire loading, on a front wheel drive car wider rear wheels will increase understeer as the rear axle has no turn therefore on cornering they drag : increasing the contact surface of the rears on a FWD car, in essence putting wider tyres on will increase understeer by creating more drag on turns. understanding the ackerman angle of toe out on turns is where this comes in to play, so with the biggest proportion of the cars weight over the front axle thus providing you with the most grip over the driven (not trailing) wheels, incorporating wider front tyres onto the front wheels of a FWD car will make your entrance into turns much better promoting to the side of understeer for FWD racing applications assuming that both front and rears are the same compound.

larger rear tyres on a RWD car is a little more complicated as this balance depends on the engine position. F1 car with larger rear tyres with the engine directly over the driven axle.

larger tyres over the driven axle promotes traction.

 

[waitey]

Oscar last time I checked there is no FWD BMW as of yet?

 

[oscar]

that's true man, it wasn't to debunk you though. just that some people don't understand the concept of traction! traction is basically weight over driven wheels in it's purest form and it varies like i said. mercedes have the A class and that's FWD, certainly not a car that needs mentioned though as it's a travesty to mercedes and most of the smaller engine models in the mercedes bmw range don't come with staggered wheels. move upto the higher powered cars and you'll find wider rear tyres for sure.

if anyone was planning on increasing lap times in their FWD respective cars, you'd want wider fronts to promote oversteer for a better and faster turn in to corners and straights as the driving axle has more contact area and can put the power down more wheras on a RWD car wider read tyres will improve straight line speed but suffer on cornering as the drag over the rear axle will make cornering worse. for daily driving though on a FWD car, it's much better to promote understeer definitely safer. remember seeing those 3534535435435 bhp honda civics on the drag strip with normal rear road tyres and humungous wide drag tyres on the front? this is adrian newey zones of chat now haha

 

[singlespeed]

Aslong as the tyres don't rub and they are the same size on the same axel it's a pass! Bmw and mercedes come with different size tyres front and rear

Yes but the wider of the two is always on the REAR.

that's not true whatsoever. The basic rule of thumb is that tire size should be roughly proportional to tire loading, on a front wheel drive car wider rear wheels will increase understeer as the rear axle has no turn therefore on cornering they drag : increasing the contact surface of the rears on a FWD car, in essence putting wider tyres on will increase understeer by creating more drag on turns. understanding the ackerman angle of toe out on turns is where this comes in to play, so with the biggest proportion of the cars weight over the front axle thus providing you with the most grip over the driven (not trailing) wheels, incorporating wider front tyres onto the front wheels of a FWD car will make your entrance into turns much better promoting to the side of understeer for FWD racing applications assuming that both front and rears are the same compound.

larger rear tyres on a RWD car is a little more complicated as this balance depends on the engine position. F1 car with larger rear tyres with the engine directly over the driven axle.

larger tyres over the driven axle promotes traction.

Thats discounting the change in ride height, which will affect the rake angle of the chassis and the change in the front/rear weight balance from raising the front or rear. This will also increase or decrease the castor angle of the front suspension, affecting the feedback and self centering force on the steering.

Also, the roll center will alter. Raising the chassis with larger wheels on the front will raise the roll center, increasing the tendency for body roll. Whereas the rear would have less difference

 

[oscar]

exactly neil, but the basics of it are true but basically what you just said is soo true, you know when people say ''when i lowered my car, it just turned into a go kart!'' ^^^^^ the reason for this is everything that neil said! the best thing you can do to a car is tune the chassis before you consider putting power through it, start with figuring out how you want it to drive and then power should be reserved for straight roads and corner exits and entry's. i couldn't imagine two different sized tyres on the same axle in a renaultsport. hopefully the OP paul has got a better understanding as to why this is dangerous and isn't left too baffled

 

[singlespeed]

Personally, if the 205s are the newer ones, I'd have them on the rear and wear out the 195s as fronts then replace them with 205s as well.

The compromise being slightly less castor angle and therefore less weight to the steering.

 

[singlespeed]

Coming back from leeds the time before last made for some weird handling.

195/50/15 winters on the rear and 205/40/17 normal tyres on the front to save the winters getting destroyed on the rollers. I put the full set of 17s on this time.

 

[wrighty94]

I'm talking about passing a MOT not pert romance and traction just thought id state that! As I do work in a garage where we do MOTs and have come across this many a time

 

[paulmchurd]

Thanks for all the Info. Think ill have to re read over it to fully understand some of what your talking about. Problem is I don't have a friend who can swap my wheels round for 0 cost. I guess the last owner put the smaller wheels on to do some power sliding around the corners. haha

 

[wrighty94]

Take it to garage and say I'll give you a drink to swap them round