Changing from 15 inch wheels to 13's.

[Andy135]

I have a new to me Sigma Superlight on the way, which is currently on 15 inch 10 spoke anthracite wheels and CR500's (2007 vintage if I remember correctly, which seem too old to be safe). I've read the threads on here and PH regarding 13's vs 15's but what's not clear to me is whether for Sigma cars 6 or 8 inch rears are preferred. Any suggestions or opinions either way?

Next question would be on rubber. What are the current preferred tyres for a car that will be 80/20 road/track?

[Jonathan Kay]

On width: how much power will that be? There's a crossover point that's often suggested.

Thanks

Jonathan

[Andy135]

It's a fixed cam 150.

[ptr_drvr]

I haven't any other experience, but I run 13 x 8" rears on a Sigma 140 with ZZRs. Never lacking in grip, that's for sure.

[David_Stamp]

I run 13" Apollo staggered (6" ZZS on front + 8" ZZS on rear) on a Sigma Roadsport 150 SV.

Others have said that 6" front and 6" rear are 'adequate', but I didn't get my Caterham to be 'adequate'...

[Check if you need to change your front cycle wings because of the change in wheel diameter from 15" down to 13". I have seen others say that this might be a problem. I moved from 14" wheels to 13" wheels which avoided this issue]

[StevehS3]

I have had both. Handling: I think I prefer 6" front and rear. Looks: I prefer wider rears particularly as I like a slightly higher ride height and they fill the arches better.

[DerekH]

I'm also running 13" Apollo's with 6" fronts and 8" on the rear on a 270S sat on ZZS's. 8" and 215's on the rear just look better.

Recommend you get it flat floored and the ride height adjusted to suit the 13" wheels. Ride height on the front drops significantly going from 15 -13

Best change I've made to my car!

DerekH

[7 wonders of the world]

185/205 Toyo R888R on 6"/8" will offer a light more by way of ground clearance, excellent tyre too.

[Golf Juliet Tango]

2007 vintage if I remember correctly, which seem too old to be safe

Far too old, replace as soon as you can. 

[Andy135]

#9: I quite agree.

As for the 6's vs 8's debate, I'd prefer predictable, readable handling, and from what I've read 6's on the rears are better for this than 8's. Does this bear out for those of you who run 6's? I love the look of the wider 8's but would prioritize handling if there's a difference to be had.

 

[aerobod - near CYYC]

I run 15x6.5 wheels with 195/50-15 tyres all around for general touring purposes and 13x6 with 185/60-13 front and 13x8 with 235/45-13 rear tyres for blat or track use.

There is definitely more tail-out action with the narrower rear tyres, the wider ones obviously give better grip out of tight corners, but shift the tail-out action to a higher speed. The handling is just as good and predictable with either set.

The narrower set win out in the wet due to higher road pressure leading to less wheel spin on anything wetter than just a damp surface.

[Andy135]

#11, thank you - that is just the sort of insight I was hoping for.

[Phil G]

I assume the speedo will require recalibration if the rolling radius changes.

[aerobod - near CYYC]

Hi Phil,

It does need recalibrating if you want the most accurate reading, but with the electronic Caerbont speedos is easy to do. I used to swap backwards and forwards, but now just calibrate for the larger diameter wheels (576mm for the 195/50-15 tyres, 542mm for the 235/45-13 tyres), leaving the 6% difference when on track or blat on the 13" wheels, as I swap wheels at least a dozen times each driving season.

Swapping the suspension settings for the different tyre sizes (mainly due to slight rake change with the 13" staggered wheels in my case) is more of a challenge, but I keep mine optimised on the 13" wheels and accept a bit of a compromise on the 15" wheels.

[Phil G]

#14 Yep, just pointing out to thread originator. 

[Jonathan Kay]

"I assume the speedo will require recalibration if the rolling radius changes."

Yes, and the ease of doing that depends on the type of speedometer.

Jonathan

 

[John Vine]

#14:  For info on how to recalibrate a Caerbont programmable speedo, see Part C of this Guide.

JV