Tyres on 15 inch rims

[swsambler]

I am looking to change my 11 year old CR500 tyres on my 15inch rim Roadsport. The car has 280hp and I do a mixture of Hillclimbing and Road use.

The current 195/45 R15 ( Dia 554mm Section 200mm) in Avon CR500 or ZZS/R seem to not be available anymore. My question is do the 195/50 R15 (Dia 577mm Section 201mm) fit under the cycle wings? Either Avon ZZS or Nankang AR1

[Dave Hardcastle]

I have the Avon ZZS on my current S3, and I also put them on my previous S3. The front are a tight fit, but they do go, at least they have on both of my Caterhams. One thing to watch for is that the larger tyre doesn't rub the indicator wiring away.

[John Vine]

I replaced the 195/45 R15 CR500s on my S3 with 195/50 R15 ZZSs a while ago.  The ZZS is certainly a close fit under the cycle wing but benefits from having a more sloping shoulder than the CR500:

JV

[Kingsley Young]

I changed from CR 500 to ZZS very good tyre  , yes tight under the front mudguard , but it will fit. 

They do take a little longer to warm up though. 

Kingsley 

[swsambler]

Very helpful, many thanks.

[7 wonders of the world]

or have you considered going to 13" .? since you hilllclimb this will offer a greater range of makes widths and compound.. lighter too.

you will idealy need new wingstay though too

[Toughie]

And improves the gearing ratio for acceleration.

Also, Caterham's Technical Director told us at a GTKY7 day that 7s handle better and "feel more grounded" on 13".

[7 wonders of the world]

Fitting 13's doesn't always mean a smaller rolling circumference, certainly if you opted for the 175/55/13 then yes it would improve acceleration but realistically your going to be looking at secton of 205mm plus 

Have a play on the linky below:

https://tiresize.com/comparison/

[swsambler]

I ran 13 inch wheels some 5 years ago, but stupidly sold them as I had 5 years in an OMS single seater. I should have held onto them. Oh! Well.

[swsambler]

Thanks very useful.

[Toughie]

True, 7WOTW.  I concede.

That's a great link. Thank you.

Linked to tyre size, I have found this calculator for gear ratios interesting,

https://www.gearboxman.co.uk/ratio-calculator/

 

[John Vine]

Re #11:

I just tried that calculator.  I entered 15" ZZS sizes and 6-speed ratios. When I entered an ambitious 7000 shift point, I got the following:

Does it simply not work or am I doing something wrong?

JV

[aerobod - near CYYC]

It would need a final drive ratio before calculating anything meaningful, John.

[John Vine]

Quel idiot.  It's just been pointed out to me that I need to enter the final drive ratio.  I'd read "Final" as the topmost gear ratio.

Where's that dunce's cap?

ETA: Thanks also to you, James.

JV

[John Vine]

And it gave the max speed for my R400D as 145mph at 7800.  Theoretically true, but a tad optimistic, methinks.

JV

[aerobod - near CYYC]

Probably going to be drag limited in speed before hitting 145mph, the CR500 195/45-15 had 930 revs per mile to hit 138mph theoretical at 7800RPM, the ZZS 195/50-15 has 888 revs per mile, so the 5% larger diameter would equate to 145mph theoretical.

You can certainly notice the gearing change with the 195/50-15 compared with the 215/530-13 slick I use on the track that has 979 revs per mile topping out at 131mph at 7800RPM, effectively meaning I need to be a gear higher a lot of the time, compared with running the 195/50-15s on the car.

[Blackmamba]

Excuse the simple question but why do you need to change the wing stays if going from 15" to 13" wheels?

Thanks

[7 wonders of the world]

If your running 185/60/13 or less there is a race stay that drops the wing near to the tyre, the normal 13/14" ones are fine for 185/70/13 but look somewhat 'surprised' on the lower profiles.

When using those gear calcs be mindful that some of the softer tyre we use have a reduced tread gauge and therefore an reduced rolling circumference compared to normal tyres of the same size, best to get this date form the manufacturers website, or measure yours with string or chalk