Speedline Corse Challenge Rims - Tyre sizes

I have a set of Challenge Rims on my 348 spider and I have the need to change the tyres. The previous owner fitted them with 295 35 18 on the rears and 235 40 18 on the fronts.

I know that these are a popular choice of wheel on many a 348 and 355, but I was curious what size/make tyres others have been running on these wheels?

Thanks

Stig

 

Tire math:

225/40ZR18s go with 265/40ZR18s (355 original) <- and yes I know this is for a 348

(225*0.40*2)/25.4+18 = 25.08 rolling radius 225/25.4 = 8.85"
(265*0.40*2)/25.4+18 = 26.34 rolling raidus 264/25.4 = 10.43"
265/225 = 1.177

(255*0.35*2)/25.4+18 = 25.02 :: 255/25.4 = 10.0"
255*1.177 = 300 -> so your choice is between 295 and 305.
(295*0.35*2)/25.4+18 = 26.13 :: 295/25.4 = 11.6"
(305*0.35*2)/25.4+18 = 26.40 :: 305/25.4 = 12.0"

So;
255/35ZR18s go with 305/35ZR18s
245/40ZR18 are too tall
295/35ZR18 are too short

At this point anyone with access to a calculator can determine the required tire sizes.

There are a number of combinations that work on the rims, however, the tires comming off the car are tall at the front and short at the back. This SHOULD have caused an oversteer issue (improper rake on the chassis) unless the nose was lowered (or rear raised) to deal with the insufficient rake. Since we don't know, and it is likely you don't know, you will want to find this out--and should consider putting the car back at its specified ride height.

Another sublty is that lower aspect ratio tires will want to run slightly less camber front and rear in the ratio of:

(WSM spec camber)*(original footprint)/(new footprint)

So with 2.0 degrees of negative camber in the back, and switchinig from 265/40 tires to 305/35 tires one would want:

2.0*265/295 = 1.8 degrees of negative camber.
0.5*225/255 = 0.44 degrees "

This puts the same amount if inward thrust from the tires onto the chassis with the new tires as the old tires did.

 

please can you post some pics of car with the 295s on the rear

I am running 275s on 360 modena wheels on my 348 and was thinking to go up to 295s on the rear.

 

the best idea would be to send those challenge wheels over to me, and choose some nice new shiny ones for yourself

 

Top info, mate, but will take while for me to digest. I guess it would be pretty key for me to know the actual widths of the wheels I guess, but I cannot see it written on the rims anywhere....

Have not been experiencing oversteer though. In fact, on the track I get a little understeer if anything, but in all fairness this is more likely down to poor technique and lack of talent than any mismatched tyre heights. It was the former owner of the wheels that had this tyre combo and he had very trick suspension on his 348 (Andy HLS's car if you are aware of it), and I can pretty much be certain that the chap I bought the actual car off did not dick around with the suspension.

 

You got two chances, my friend; fat and slim!

 

Vroom, vroom, baby!

BTW, this is with 15mm spacers on the rear and no spacers on the front.
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oh brother
that is super HOT
i love 348 spiders

 

The only rear view I can find (car fishtailing after alarming oversteer (see my earlier comment regarding lack of talenet) coming out of Gambon at Topgear Test Track)...
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thats the hottest car in the universe

 

Why, thank you, kind sir. I take it you dig the yellow/black vibe?

 

looks fantastic

 

Challenge wheels are 18X8 F and 18X10 R.

 

The tire size that will maintain your speedometer accuracy is 225/40/18 front and 285/35/18 rear. Giving you respective revolutions of 25.1 and 25.9.

Your stock rear tires meant to work with the speedometer are 255/45/17 with a 26.

That gets you the closest and that's what I have as well.

Kai

 

Love it!

 

The standard tyre size for a 348 is 215 50 17 and 255 45 17 which gives a metric diameter of 647 (front) and 661 (rear).

My current tyres are 235 40 18 (diameter 645) and 295 35 18 (diameter 661) so it would seem that the sizes on the car at the moment are of an appropriate diameter (for the design of the suspension). Don't know how width affects things. Perhaps one of you fine gentlemen could enlighten me...

If a 355 has 225 40 18 (diameter 637) and 265 40 18 (diameter 669) then that would probably throw my speedometer out a bit, but enough to make a real difference?

Doing the math, I suppose my limit on the rears would be 255 40 18 (d 658) and the fronts 215 45 18 (d 654) but it is probably getting marginal on width as Rob points out that the wheels are 10" and 8" wide.

Good god, my head is spinning with this, but it has been a massive help guys, it really has.

What would be the advantage/disadvantages of going to a narrower tyre. I guess that would get hotter on track, but how would it affect the feel of the car on the road?

 

Hi Stig,

Long time since you have been over here?

Stick with the sizes you have now, they are the nearest to the original 17" sizes!

Here is a tyre size calculator to help (thanks to Mouser57), saves the head hurting!!

Regards,
Phil.

http://www.trailtiredistributors.com/TireCalculator.aspx

 

Don't let the speedo variance get you down on tire selection...here is an easy cure

http://www.healtech-electronics.com/

Always remember there is a hugh difference between the "how bad it looks bling thing" and setting the car up for performance...bigger is not always better.

 

That's was the part about ::

245/215 = 1.139
versus
295/235 = 1.255
whereas you want:
295/1.139 = 258.9 fronts to go with 295 rears
or
235*1.139 = 267.6 rears to go wth 235 fronts

 

Doesn't the 348 speedo read high from the factory? The slightly taller tires may be dead on.

 

I kinda meant with regard to how the car would operate in a qualitative way, rather that a quantitative way.

How would the car "feel" on narrower tyres?

 

Hi Phil long time since I have been either here or there to be honest, and a bloody long time since I saw your good self. Maybe I'll make it up to Silverstone for the FOC day next year.

BTW, I broke my head with the maths (not a chore, because I actually like maths (don't tell anyone as they may think that I am weird)), but did actually find a calculator to corroborate my wildly inaccurate manual computation, but ta for the link anyway.

I could stick to the sizes I have, I just did not want to rob myself of the opportunity to explore the advantages of a narrower tyre width (if indeed there were an advantage), hence my stimulation of this discussion.

 

A narrower tire should heat up quicker so if you're doing short stints (Auto-X) or just some spirited street driving that may be an advantage in colder situations. They will also weigh less which will help all other aspects of performance aside from outright grip.


Rob

 

If you get the footprints matched, the oversteer/understeer relationship will be unchanged.

Wider tires will always have a heavier feel at the steering wheel.

Anyway, if you get close to those equations, the tires will be close enough to balance with air pressure.