Expensive tyres?

Posted on February 5th, 2008 in Driving Emotion,Handling,Tyres by Julian Edgar

kh18.jpgI am starting to wonder how much people should spend on tyres.

Years ago, when I owned a Subaru Liberty RS, I bought a set of sticky track tyres of the type that were only just road legal. They gripped phenomenally well, even in the wet. Given the minimal tread depth, the latter was a real surprise to me.

And at other times I have also bought other very expensive tyres, largely being guided by brand name and word of mouth.

But now I am not sure that on cars of less than stratospheric performance, it’s worth spending a lot of money on tyres. Instead, I am starting to think that if there are problems with handling, the money should be spent on the suspension instead.

To put this another way, upgrading tyres without doing anything else is a band-aid approach; tweaking the suspension (even with things as simple as anti-roll bar or damper changes) will give a similar improvement – and one that won’t wear out as fast as the tyres!

The other point to keep in mind when considering costly tyres is the law of diminishing returns: a tyre twice as expensive is not twice as good.

A car that is well balanced – it can be driver-tweaked into neutral, oversteer or understeer attitudes – will be fast and enjoyable, even on relatively slippery tyres. One that grips and grips and grips – and then lets go in an almighty rush – might be initially fast, but the enjoyment will soon be tempered by sweaty palms!

If the car is obscure and off-the-shelf up-grade suspension parts are not available for it, selecting better quality tyres is still a good way of getting an improvement. This is what I did with my Prius – I fitted a rear anti-roll bar and since further suspension steps would have been very expensive, upgraded the tyres by going to a better compound with a lower profile and wider carcass.

But if the car already handles well, or can be easily upgraded in suspension, I now chose to use lower cost tyres. The EF Falcon, fully upgraded in suspension as an AutoSpeed project car, stayed with the oddly-named budget Maxxis tyres throughout the time I had it. And, point-to-point on a twisty road, that was a very fast car.

My current Peugeot 405 SRDT, a quite sweet handling car in standard form, recently got new tyres.

And, after thinking the above thoughts, I selected standard size Kumho KH-18 tyres. At an incredible AUD$85 each fitted and balanced, you’d think that the result would be disastrous.

But so far I have found them to be very quiet, progressive and predictable, and to have quite competent grip…

11 Responses to 'Expensive tyres?'

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  1. Tom Westmacott said,

    on February 5th, 2008 at 9:45 am

    Another interesting article, however I can’t wholly agree. A good set of tyres doesn’t just improve dry-road cornering. They also give better traction, better braking, and better wet grip. Traction is particularly important on front engine, rear-drive cars where the driven wheels are lightly loaded. Braking is obviously an area where any improvement is welcome – if you’re going to be going faster around the corner, you’d do well to be able to stop better when you get round it and find a tractor in the road. And the difference between good and mediocre tyres is exaggerated in the wet, so an expensive tyre is less adversely affected by bad weather.
    Another thing is that generally, a good tyre will improve any car, so it’s a straightforward thing to do, whereas with suspension, different cars benefit from different tweaks, and improvements in one area can make things worse in another. Magazine tests are a good source of tyre recommendations, certainly some of them are almost worthy of Autospeed in their scientific approach.
    Finally, at least here in the UK, if you start fitting dampers or anti-roll bars, you have to tell your insurance company, who will charge you extra. But you can fit sticky tyres without any extra charge.

  2. Darren Roles said,

    on February 5th, 2008 at 1:24 pm

    I have to agree, I fitted low profile name brand tyres on wider aftermarket rims and lowered the suspension.
    With out replacement of the dampers/anti-roll bars/bushes or anything else I wasn’t happy with how it handled; I reckon it was worse than standard.
    So I put the standard springs, rims and tyres back on and just increased the tyre pressure to 40psi. Best thing I ever did – sensational value for money.

  3. Peter Tawadros said,

    on February 5th, 2008 at 1:38 pm

    Available tractive force (outright grip) is a function of the friction co-efficient (both static and dynamic) of the road-tyre interface, and the area of the interface.

    One of the main aims of a suspension system is to keep as large an area of your tyres on the road as possible, and keep it planted there as long as possible. So there are basically two ways of increasing outright grip. The first is to increase the contact patch with the road, which can be done by lowering your tyre pressure or revising your suspension so it works to keep more of the tyre that you have on the road at any given time.

    The second is to increase the co-efficient of friction at the road-tyre interface. Simple, stickier tyres.

    However, thats where the simple bit ends, because cars aren’t about outright grip on the road. The way a car handles is also dictated by its transient response.

    Humans like a linear response curve, and unfortunately for us, rubber is far from linear. This is especially so when tyres under-inflated (like for instance in a bid to get more contact on the road). This is where you have a point Julian, because if the suspension setup is not doing a good job of keeping the contact patch relatively constant in size, then the handling of the car will feel something like an amplification of the tyre’s non-linear response curve – predictable up to a point then too quick to catch when it changes. This is why it can often be a far better idea to revise your car’s suspension if you just can’t predict what the car is going to do when you turn the wheel. Also, it might be worth checking your tyre’s inflation pressures.**

    If on the other hand, you know exactly what it’s going to do but whatever it is will involve a lot of squealing (on the car’s part) and groaning (on yours), then maybe you need some stickier tyres.

    **It’s worth noting that on the road the vast majority of motorists I’ve met report better handling when they increase tyre pressures over and above the manufacturer’s recommendations (based on anecdotal evidence). This is despite the actual loss of outright grip due to a smaller contact patch. I would conjecture that this is because as the tyre is more inflated it behaves more linearly (with much less sidewall deflection and a higher spring co-efficient in each direction), and so in normal driving (not at the limits of adhesion) the average driver is better able to predict what the car is going to do.

  4. Rick Miles said,

    on February 5th, 2008 at 3:08 pm

    I tend to agree with Julian on this one…

    Here’s my views on road tyres:-

    1) I’ve been spoilt by ‘R’ spec tyres on my race car, so in comparison, all road tyres, regardless of cost, are very ordinary…

    (not the fault of road tyres… try driving ‘R’ specs on the street and you will very quickly realise how much comprimise is in a street tyre to make them comfortable and quiet! Not to mention durable…)

    2) Due to wear/age/improvements in technology, the next new set of tyres will almost always feel much better than the set you just replaced.

    3) It’s almost impossible to make an informed decision on brand/model as once they’re fitted to your car, you’ve bought them! Magazine articles can help, but what works on the test car may not work on your car. How a tyre performs after some kilometers/time is also unknown…

    4) Some tyres seem to ‘go-off’ after time. One set I had went hard, and I dumped them with over 50% tread depth as they were VERY low on grip and extremely noisy. How can you possibly avoid buy a set of tyres like this?

    In conclusion:- Go for the best deal you can get on a set of ‘middle of the range’, well known brand tyres….

  5. Monty said,

    on February 5th, 2008 at 6:12 pm

    When considering the cost/performance tradeoff with car tyres it is probably also necessary to consider the operating cost. Michelin claim impressive reductions in rolling resistance on some of their tyres. This reduced friction can reduce fuel consumption and therefore running costs. Yet another difficult-to-quantify factor to include in the equation!!

  6. ldv dan said,

    on February 6th, 2008 at 4:38 am

    i suppose the best way to assess optimum tyre pressure would be to measure tyre temp of the middle & outer edge of the tread surface with a heat sensing camera or a laser thermomitors. ive never had the equipment to try it. Dan

  7. Blair said,

    on February 6th, 2008 at 7:56 am

    Quote: I have found them to be very quiet, progressive and predictable, and to have quite competent grip…
    I am sorry but you have it completely backwards Tyres are all about grip, not just lateral but most important is under brakes and especially in the wet. Anything else is a bonus, and by all means that effects the buying process, but the benefit of a quite tyre is pretty quickly lost if it takes even 50 cm more to stop from 60 kph and that 50 cm is the difference between another car or a child.
    Like one of the other posters I also race and I spend an obscene amount of money on tyres that do not last long, but grip all the way to a lap time, so any road tyre is poor in comparison, but you can still make the best choice possible within the constraints you have, usually financial. Improving your suspension will make the most of the tyre you have, now doubt, but your ultimate limit/potential is dictated by the tyre, nothing else
    I do agree brands are irrelevant – Michelin/Pirelli/Continental, best tyres in the world may be, but they also make cheap and nasty goods aimed at people who don’t care. A better “lesser” brand is usually better than cheap “premium” brand

  8. Ben said,

    on February 9th, 2008 at 2:31 am

    While in NZ I switched the 195/55 tyres to 205/55 on my ’97 Mazda Astina, and comletely screwed the handling. The car became far more cumbersome and slow-witted. Changing back to the original 195 size restored the handling, so clearly the wider tyres were too wide for the rims allowing excess lateral sidewall flex to occur. In the UK I’m using Toyo Proxes T1S which I’ve found to be excellent and sensibly priced at around £60 each (say AUS$130) from the internet.

  9. Darren Roles said,

    on February 22nd, 2008 at 8:26 am

    The technology in an average car tyre has come a long way. The class rules for racing early 70’s vintage HQ Holdens dictated ‘standard’ cars; engines, single barrel carb in the (I think) red 202ci motor etc.
    They were allowed to change brake pad material but had keep the standard size calipers & rotors. Control tyres were the Bridgestone RE88 scrubbed down to 3mm tread depth with standard size rims.
    The last time they raced during Bathurst weekend I remember hearing one of the commentators saying the winning lap times were faster than when Moffat won in a V8 GTHO. Sure there are other factors but the tyres are what’s keeping them on the road.
    As I approach 40 I’m slowly starting to realise how much bang for my buck is a particular modification going to give me rather than “I gotta have it”. If that question is answered do I want to be more uncomfortable than when I started?
    Standard size tyres look more and more appealing to me then I just look at the air pressures instead.

  10. Robert said,

    on March 3rd, 2008 at 10:06 am

    PeterT, increasing the contact patch won’t necessarily increase grip. This only works for low-traction surfaces. Grip is also about the weight on the tyre.

  11. Ben said,

    on March 3rd, 2008 at 8:58 pm

    Robert: It almost certainly does increase grip. One of the things that tyre companies don’t normally release, but continously test, is the relationship between weight applied to a given piece of rubber and the amount of sideways force that can be held by that piece of rubber.

    Weight is on the horizontal axis, and sideways force (we’ll call it thrust, for ease of use) on the vertical. The line starts at zero on both axis’, but thrust rises faster than weight initially, but starts to fall back soon after. An example is that a given compound might be able to make (per square centimeter) 150kg of thrust at 100kg weight, 250kg thrust at 200kg weigh, 300kg thrust at 300kg weight, and 340kg of thrust at 400kg weight.

    The idea of increasing the contact patch is to get the weight (per square centimeter) lower, so the thrust (again, per square centimeter) is higher. You then end up in a situation where you might have had 400kg on one square centimeter (and so be capable of 340kg of thrust), but you now have that same 400kg across two square centimeters. The total thrust is now 500kg.

    Obviously those numbers are wrong, but that is how tyres work. I think that graph, when labelled differently, is also known as a G-curve, or traction curve.