Tyre sizes ??

[random hero]

Ive got some boxster carrera rims i was going to use as track wheels there 8.5 rear with 255/40/17 's, the fronts are 7" with 205/50/17's but these just look odd as the profile seems too high. My 17" cups i had ran a 225/45/17 will this size fit the 7" rim or will it be over tyred ???

 

[sawood12]

Standard size up front is 205/55/16 so you need to make sure the overall dia remains the same. By my calcs 205/50's mean you end up with an overall wheel dia that is only 4.9mm larger than the standard size which is as close as you can get. This is the tyre size I have up front.

 

[Diver944]

Scotts right - 205/50's are the correct size for a 7" front but I have seen 225/45's on one too which is the sameish overall diameter

 

[berg944]

205/50's are the correct size for a 7" front but I have seen 225/45's on one too which is the sameish overall diameter

I need to change my 225/50,s as they rub the nearside arch roll under heavy braking.

I always thought that it was a simple calculation ..... 50 - 45 = 5mm RAD. = 10mm DIA...... Is this incorrect?

 

[sawood12]

The calc I use is ([rim size in inches]x25.4) + 2x([tyre width in mm]*([aspect ratio]/100))

By this calc 205/55/16 is 631.9mm dia whereas 205/50/17 is 636.8mm. As Paul says you can also get a 225 tyre on a 7J rim and 225/45/17 would be 634.3mm dia which is even closer to std dia.

 

[mik_ok]

Berg - the "50" etc is an aspect ratio as opposed to a mm measurement.

205/50 ~ sidewall height is 50% of the width (205mm) = 100mm
225/45 ~ sidewall height is 45% of the width (225mm) = 101.25mm

I toyed with the idea of 17" Cups whilst we had our S2 (several times!).

I much prefered the idea of 225/45/17 front as opposed to 205/50/17 front. I haven't done a back to back comparison, but the latter sounds too narrow to me (when compared to the 255/40 rears you'll be running) for a car with 50/50 weight distribution. The 968 ran 225 fronts and 255 rears, which kinda supports it as being "correct".

225/45/17 will fit on a 7" rim. Toyo list it as an acceptable rim width for that tyre size (although 7.5 is ideal). When I had my Leon I ran 225/50/16's on a 7x16 without any problems at all.

 

[sawood12]

Don't forget the rear tyre has to transmit the power onto the ground as well as provide grip in the corners therefore it makes sense that the rear tyre is wider than the fronts. Also Porsche probably intended the fronts to be slightly narrower than ideal in order to induce a bit of understeer on the limit to warn the driver that the car is just about to bite back!! The Boxster is 50/50 distribution and has 255's at the rear and 205's up front. However Mik is right, the car can cope with more rubber up front. I'll be going for 225's up front next time.

 

[random hero]

ORIGINAL: sawood12

Standard size up front is 205/55/16 so you need to make sure the overall dia remains the same. By my calcs 205/50's mean you end up with an overall wheel dia that is only 4.9mm larger than the standard size which is as close as you can get. This is the tyre size I have up front.

What size rims are you running on the front ??

 

[sawood12]

7".

 

[berg944]

Berg - the "50" etc is an aspect ratio as opposed to a mm measurement.

Thanks, that helps me work out the size I need.

I still don't know if I should fit-

16 x 215/55

or

16 x 205/55

205's will give me the same ride height as the 225/50's I have now but how will they affect handling and braking?

I have Toyo 245/45 on the rear and want to fit Toyo on the front, I am told that they are one of the widest fitting tyres.

Mike[:'(]

 

[mik_ok]

Berg - what are your wheels?

I ran 205/55/16 on the front S2. These were on std D90s (ET55), but with a 7mm shim pushing them out (making them an "effective" ET48. They were still miles from the front arches.

Turbo's ran 225/50/16 on the front (and the 245/45/16 rears you mention on the rear). Their 7.5" front rims are half an inch wider than the S2 fronts but more heavily inset (ET65), so the outer face of the wheel/tyre has the same relationship with the wheel arch. Hence you could easily stick 7mm shims on the front of a Turbo running 225/50/16 and not have any fouling problems.

 

[berg944]

My wheels are from a 993.

The front are 16' with 7' rim and 55 offset.

 

[berg944]

As you can see, not much room...and thats the side that doesn't rub!

No spacers or shims

I have medium big black callipers but do not know what the hubs are from.

Mike[:'(]

 

[sawood12]

ORIGINAL: berg944

Berg - the "50" etc is an aspect ratio as opposed to a mm measurement.

Thanks, that helps me work out the size I need.

I still don't know if I should fit-

16 x 215/55

or

16 x 205/55

205's will give me the same ride height as the 225/50's I have now but how will they affect handling and braking?

I have Toyo 245/45 on the rear and want to fit Toyo on the front, I am told that they are one of the widest fitting tyres.

Mike[:'(]

215/55/16's are about 11mm too large a dia. You'll get away with it with them being on the front (i.e. it won't affect your speedometer readings) however you need to be within a certain % of the original wheel size or your ABS can screw up. Unfortunately 215/50/16 is about 10.5mm too small so it seems 215 wheels are not a good match on 16" rims.

 

[berg944]

No ABS to think about but they don't make T1R's in 215/50/16

215/55 will only lift the front by 6mm but that might rub somewhere else!

205/50 will be the same height but how will a tyre that small affect handling and braking?

I don't need to loose 10mm off the outside of the tyre to clear the arch so could move it out a little bit with a shim.

Mike[:'(]

 

[sawood12]

I would guess that braking would not be that affected greatly. There is a trade off at some point but if you have a narrower tyre then the weight under braking is spread over a smaller area hence more pressure bearing down on the tyre per square inch therefore increasing the adhesion of the tyre. A wider tyre spreads the load and reduces the adhesion of the tyre. But there will be a trade off at some point because it stands to reason if you keep on making the tyre narrower and narrower then grip will ultimately suffer. I'm not sure where that kink point would be though.

In terms of handling you are really talking about the understeer/oversteer balance of the car. I know my car with 205's up front can be a bit understeery when pushing mid corner. With a wider tyre up front you'll be changing that balance and understeer will occur at higher speeds, however as the tyre gets wider there will be a point where the back end will loose grip first and you get oversteer. It all depends upon what feel you are trying to achive.

 

[berg944]

When I first got the car the rear tyres were an unknown make and would step out often. Since fitting Toyo's of the same size the rear is solid but the front will push out turning hard at low speeds.

I am hoping that smaller Toyo's will be better than the larger Wan-Kings fitted at the moment, I will just have to try it!

I think a narrower tyre might help tramlining as my Lux on 195's doesn't do it.

Mike[:'(]