Tyres - yes...but why ?

[Stoff]

I have a 1.6k Roadsport SV with 205/45/R16 and Avon ZZ3's.

 

After the Cheshire Area meet a couple of months ago I have been looking into tyres. The general concensus appears to be to go for R13 shod with Avon CR500/Toyo888/ what ever you can afford.

 

I can understand why changing the rubber affects grip, how pregressive it is etc. But why are R13's "better" than R16's. Is it simple tyre availability, weight, something to do with how a tyre wall works or none/all of the above?

 

Currently I'm doing mainly (dry) road use but am booking onto track days. I'm trying to understand a bit about what's going on.

 

Thanks in advance.

 

Chris

[rgrigsby]

Tyre availability is certainly one factor but I think the main reason is the tyre and wheel weight.

 

16" wheels are generally far heavier than 13" wheels which when the rest of the car is so light makes a huge difference. (Someone else will come along shortly and give you the technical explanation with the correct words such as unsprung weight etc 😬)

 

Rob

[mav]

Rotational mass and unsprung weight.

 

As Rob says there is a bigger range of suitable performance tyres in 13".

 

You'll also find the steering a fair bit lighter and possibly more responsive.

 

Budget for adjustable platforms if you've not already got them as a move to 13's could easily cause a big reduction in ride height leading to sump / road interface.

 

I run my SV on 13's on track and 16's on the road.

[Pendennis]

Basically comes down to cost, availability, weight, rolling circumference (gearing) whilst not affecting suspension and geometry. Look at the racing scene, whilst cost doesn't really come into it, all the others do

[Gridgway]

The size of the sidewall makes a big difference to the handling. The difference between 13" and 16" rims is huge. I hated running 16" rims on my first caterham. Replacing them with 13s was fantastic.

 

GRaham

[Mavic82]

Personally I prefer the 16" wheels and low profile tyres. The 15" wheel with CR500's are similar in profile to. To much flex and give with 13" wheels - all a matter of personal choice

 

For tyre choice Toyo now doa super sticky tyre in the correct size.

[Gridgway]

Interesting, you are the first person I've come across that prefers the 16s on a caterham! Of course there will no be a deluge of others!

[Anton]

Chris - I have a 1.8K SV 03 plate

I changed the original Avons to Toyo R888 205 45 ZR 16 in 3G soft/medium compound which improved the roadholding in the dry and made a huge difference in the wet.

 

I tried 13 inch for a short while, but whilst there was an improvement, I found I hit to many things with the sump or gearbox, so went back to the 16s and fitted the Toyos. I don't track it, but drive fairly briskly and would recommend them for fast road use. They don't wear out that fast, either.

 

Try putting it into the search function - you'll find lots of advice!

 

 

[rgrigsby]

A long time ago I weighed a 6.5" SLR 5 spoke with a CR500 on and it was around 11kg so that's 22lb. Quite a difference.

[Eugene]

Tyre choices can be very personal...

 

I have 13" rims with 70 profile 021Rs, and find them perfect.

The tyre wall compliance make a real difference to handling...

 

But, you also need all other suspension components to work in partnership 😬

[Stoff]

Thanks for the replies.

 

I hadn't even thought about messing up the speedo or grounding the sump even more than I have already (due to a speed humps and a few dirt tracks) !!!

 

I understand that to a degree its down to personnal preferances and driving style. it was more the physics and tyre mechanics I was ineterested in. So far it sounds that weight and the rotational mass is a key factor. I wondered if the flex in the wall somehow helped.

 

Interesting to see that the choice between 16" and 13" is not as cut and dried as I had assumed. I like the idea of one set for touring and another for track use. If only finanaces would allow !!

 

Thanks for the feedback and views.

[MacP]

There are a number of factors which mean that 205/45R16s are unlikely to be optimal:

 

1. Geometry - the suspension on 7s was originally designed for narrower tyres and putting wider tyres on tends to result in more tramlining, bump steer and heavier steering (particularly if you have a 21..). My 1.6k Seven came with 185/60R14s which are a more natural size. Even the current R400 makes do with 195/45R15.

 

2. Weight - based onToyo T1-Rs (which are relatively light) a 205/45R16 weighs 19 pounds whereas the 185/55R15 is 16 pounds - 3 pounds a wheel. Both tyres have about the same diameter. A lot of this is down to the load index which tends to be higher on 205s.

 

3. Weight bis - the 16 inch HPC wheels are about the heaviest put on a 7. Up to 2 pounds heavier than the 15 inch anthracites (if you believe official weights). 13 inch wheels are lighter still.

 

4. Choice of rubber - best choice is in 13 or 15 inch.

 

Everyone normally says you should go for 13 inch wheels because this minimises the weight but you would need adjustable ride height (or be stuck with 185/70R13 A021Rs) If you go for CR500s you would go from around 589 mm diameter to 554 mm - 35 mm - that's half some people's ground clearance!

 

But if you can live with the ride height CR500s on 13s is probably the way to go for ultimate handling (might also want to check what it would look like on an SV).

 

In favour of 15 inch is tyre choice - CR500, Toyo R888, Yoko A048 but you would also have the option of Toyo R1Rs which have got some pretty favourable reviews on here as well as AD08s which no-one seems to have tried yet. The main tyre option you lose is the A021R. With the exception of the CR500 most of these come in 55 profile which would preserve your ride height but you could go lower.

 

For road use there are loads of normal 195/50R15 tyres which are very cheap - £36 a corner for T1-Rs - and these are likely to be better on the road in the rain or in low temperatures than the out and out track day tyres - T1-Rs ought to be similar to your current ZZ3s - maybe a bit better.

 

Ultimately to settle whether 13s make a big difference someone would need to take exactly the same car and run it with Caterham anthracites and CR500s first on 13 inches and then on 15 and see what happens to the lap times. For tyre bores like me that would be a great low flying article!

[keybaud]

MacP,

 

I love you and want to have your children (alternatively, just accept my thanks for your tyre summary).

[Nifty]

A long time ago I weighed a 6.5" SLR 5 spoke with a CR500 on and it was around 11kg so that's 22lb. Quite a difference.

Or more precisely .. about 24 lbs *tongue*

[irrotational]

16's are too bling 😬

[oldbutnotslow]

Its a very interesting subject and would take someone far more qualified than me to start to go into the technicalities.

 

However.

As has been said beware your ride height. Again you can reset most Billies by moving the spring platform on the non adjustable dampers. It depends on where the groves are set. Its not a big job to have additional groves put in by an engineering company if you need to. Ive had this done for customers and it cost just a few quid.

 

Do remember that the rear dampers are upright and the fronts are at an angle so an increase of 20mm on the rear would need to be increased on the front to compensate.

 

You also need to ensure that you maintain a rake of about 15mm front to aft. You dont want it looking like a speed boat and handling like one either!

 

By their very nature 45 section tyres have stiffer side walls. Therefore they tend to be harder to get temperature in them. Greater temperature in very simple terms = better grip up to the point where the rubber starts to become plastic ie more fluid.

 

Again the slip angle can be worse on a lower height tyre ie it will let go that little bit sooner. It can also create issues with under steer/over steer.

 

The side wall flexing is an important part of the suspension which is why tyre pressures are so critical. If you think of it at 16psi a 2lb difference is 12.5%. If you run a tin top at 36 psi +12.5% would equate to 40.5psi. Now you probably wouldn't think of running this much more but rarely do we start a run by checking and adjusting tyre pressures for the ambient temperature etc.

 

Because of the greater stiffness there can be a tendency for the inside / outside edge of the contact patch to become unloaded. We run negative camber in an effort to maintain the tyre as vertical to the road surface in a corner as possible. Changing the tyre profile can also affect the suspension in terms of the anti roll bar stiffness required.

 

If you look at F1 the tyres they are all 13" but all have massive side wall height ie full or even 110%. The tyre accounts for almost all of the suspension movement in their case.

 

You have a greater risk of making the wheel rim go out of true if you hit pot holes with low profile tyres. Aesthetically bigger rims and lower profile tyres may look better as they fill out the wheel arches but you will find that you get a far more relaxed ride on the road running a higher profile tyre.

 

In no way does spirited driving on the road equate to driving on track. So in the end its all a compromise and it will depend on how you use your car. So you make a judgement, pay your money and if you are happy with the performance dont worry too much further....... just go and enjoy yourself.

 

 

 

 

 

[Eugene]

Just as a reminder, you should NOT simple adjust spring platforms to alter ride height...

 

Spring platform settings should be for handling only.

 

The 'nominal' setting is to have the lower wishbones parallel to the ground, with the rears set to approx 20mm higher than the front (as measured from the chassis).

 

For some old but interesting info see the following:

 

Suspenstion and the round bits

 

Sticking to the black stuff

[Tony Whitley]

Quoting MacP: 

If you go for CR500s you would go from around 589 mm diameter to 554 mm - 35 mm - that's half some people's ground clearance!

but ground clearance will only reduced by the radius - 17.5 mm.

 

But what do I know - changing to CR500s without raising the suspension cost me a tour of Scotland and a new sump

[Mavic82]

I think Comapring to F1 is a bad example. Look at touring cars or GT cars, the profile is much lower.

 

Ultimately you should let any peice of the car do the job it was intended to do, so let the suspension sort out the suspension and the tyres do the job they are intended to do. Don't mix and match. Suspension is designed to run in a certain configuration so grossly altering the ride heights in the neutral position will bugger things up. But perhaps most importantly you must get the right set up for the right combination of parts. A car with big wheels and low profile tyres will need different spring rates and geometry to a car on 13" rims and 60 or 70 profile tyres. Ultimately the aim is to keep as much of the tyre in contact with the road at all times.

 

Guy

[Old Caterhamian]

To help with the difference between tyre/wheel sizes this is a good calculator.

Link

 

Andrew F

 

[sjmmarsh]

http://www.sjmmarsh.f2s.com/side.JPG

 

Edited by - sjmmarsh on 15 Nov 2011 14:01:28

[Fil M]

My SV roadsport has 16 inch wheels... currently booted with Yokohama AO21R's. At this time she is purely a road car and her current speed and performance is ideal within road constraints for progressive driving. Ideally one would have a road 7 and a track day 7 and no compromise to worry about. Ultimately a CSR would be my ideal as it would do both jobs to a level that would suit me and the skill level I would aspire to. I decided to leave as is .... for now 😬

 

Fil

[sjmmarsh]

Fil

 

The Purple SV above is on A032s, but if you want a CSR - try this one:

 

http://www.sjmmarsh.f2s.com/CSR.JPG

[MacP]

@keybaud thanks for the love 😬.

 

Tony Whitley's response to my post makes the ground clearance point much more eloquently .

 

My 21 started off on 205/45R16s, for the last three years was on 195/60R14s and is now on 185/195/55R15 on Anthracites.

 

The car was very briefly on 13 inches and CR500s because I sold my wheels and Chris at the 7 workshop needed something to wheel it around on. Apparently it looked like a roller skate.

 

 

[Graham Perry]

I currently have 13 and 15 inch wheels for my Caterham and have in the past also run 14 and 16 inch.

 

What I like about the 13's is more choice of road legal motorsport rubber available and that on 13's I find I get much more warning that the back is going to let go. With 15's and in particular the low profile 16's I once had, it can just snap away from you.

 

The other thing you have to consider is that you will likely find the softest rubber compounds at 13 inch in road legal sizes. The tyre manufacturer has to assume as part of the European Certification process that the bigger the wheel then potentially the heavier the car when producing a road legal tyre. That usually means that the softer compounds in the larger wheels are not generally as soft as they will be in 13 inch sizes for fear of overheating when being used on cars that are more than double the weight of the seven. An engineer from Goodyear motorsport particulalrly emphasised that point to me in a conversation a few years ago.

 

Edited by - Graham Perry on 15 Nov 2011 14:40:55