The trap of load index

Posted on August 2nd, 2016 in Safety,testing,Tyres by Julian Edgar

This issue we have a story on understanding (and varying) gearing, based primarily on changing tyre diameter. The prompt for the story was the availability of a wide variety of on-line calculators that allow you to very easily correlate road speed with selected gear and engine rpm, and to see how overall gearing changes can be made by changing diff ratio or tyre size.

And there’s nothing at all wrong with those calculators – in fact, it’s easy to spend a few hours trialling all sorts of different combinations of numbers!

However, when looking at making major reductions to tyre rolling diameter, there is a trap that I wasn’t aware of.

And the trap?

Load index!

Load index is the rating given to a tyre that describes the maximum weight that should act through that tyre. The rating is indicated by a number that correlates to a vertical load (in kg or pounds). So for example, a tyre with a load rating of 89 has a maximum load of 580kg per tyre. (And at what tyre pressure does that apply? Again this is an area that most people don’t think about, but that load applies only at a specified inflation pressure – often 36 psi.) Load index tables can easily be found by a web search.

And what governs load index? Most references talk about the strength of the tyre (ie how many layers of steel reinforcement are used, for example) but in fact it also depends to a large extent on the volume of air trapped within the tyre.

And, as you go smaller in rolling diameter, that volume decreases!

Thus, changing gearing by reducing tyre diameter may be difficult if the load index of the smaller diameter tyre has decreased a great amount.

The minimum load index is a legal requirement as stated on the tyre placard. For example, my little Honda Insight, with 165/65 14 tyres, requires a minimum load index of 79 (or 78 in some markets). A load index of 79 means the tyres can cope with 437kg per tyre. That seems really high for this small car – the highest static load the Honda tyres would ever have to deal with is about 330kg – but that’s what the placard says.

If I wished to lower the gearing, changing the wheel size to 13 inch and going with 165/55 tyres (which would give about 10 per cent lower gearing) sounds good – until you realise that the load index of such tyres is only 70, or 335kg. A load index of 335kg is some 23 per cent lower than legal!

In fact, I found it impossible to find a tyre with a legal load index that gave a smaller rolling diameter on the Honda. To go further, I also found it hard to find any cars where these small tyres would be legal, their load indices being so low.

So if you’re thinking of reducing the volume of air inside the tyre (eg a by using a lower profile but keeping the same width, or a combination of smaller wheels and smaller tyres), check the load index of the available tyres first.

It’s honestly not an area I’d ever given much thought to.

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.

A Tyre Trap

Posted on October 9th, 2007 in Driving Emotion,Tyres by Julian Edgar

michelin_pilot_sport.gifIt’s an obvious thing, but not so obvious that I haven’t been caught twice in the last month. When buying a secondhand car, or just the secondhand tyres off a car, check every tyre!

To the latter first. Recently someone in the small town in which I live advertised the tyres from an EF Falcon. ‘Near new’ said the ad. Since at that time I still owned an EF Falcon, I rang up. The seller didn’t know the tyre size but dutifully went away and checked. The size was right, so I went off to inspect the tyres.

Much to my surprise, I found that the tyres – and wheels – were still on the car! A wagon, it had spun and hit a tree. The car was a write-off, but the owner was trying to recoup more than she’d have got from a wrecker. So, the tyres (and wheels) were for sale, as was the engine. Trouble is, she didn’t tell anyone that you’d have to get them off (the wheels) or out (the engine) on your own!

This flummoxed me a little, especially when I pointed out that if I took the wheels and tyres off the car, it’d be awfully hard to move around her steep, grassy backyard – that’s where the car was. So, I suggested, perhaps she should place the car where it would be easy to winch, wheel-less, onto a truck when it was time for the car to finally go. I looked at a couple of the tyres, saw the brand-new tread, and made an offer. She accepted, so I went off for my trolley jack.

It was only when I was taking off the wheels that I noticed that in fact only two tyres were near-new. The other pair was probably half worn. But it was too late to complain… I should have checked the tyres more carefully before making my offer…

And blow me down if the same thing didn’t nearly happen again! This time I was buying a complete car. With the incident of the Falcon tyres still very clear in my mind, I checked with more than usual care the tyres on the car I was buying – a Peugeot 405 SRDT. And, again, there was a surprise in store.

Incredibly, the sizes front and back didn’t match. Now that might not be a shock on a Porsche 911 but it certainly is on a Peugeot 405! The correct standard tyre size for the Pug is 185/65 14. But lo and behold, on the back were 185/70 tyres! I quizzed the owner and he waved away the problem with a sweep of his hand. But why were they different? Oh, he said indifferently, perhaps his son had been responsible for getting those put on the car…

So, as I said, always check all four tyres….

Left foot braking

Posted on September 14th, 2007 in Handling,Opinion,Safety,Suspension,Tyres by Julian Edgar

brake.jpgWhen you were taught to drive I’d wager 10:1 that no-one ever said anything about left-foot braking. The left foot was for the clutch, or in an auto car, for bracing yourself when cornering. (The driving instructor never said anything about that either? Oh well.)

I first started left-foot braking about 15 years ago. After reading a story on RWD handling that described left-foot braking, I decided to have a go. The first thing that I found was that after years of accelerator operation, my right foot had developed a super sensitivity – but my left foot was used to only operating the clutch. Left foot braking therefore resulted in a crick in the neck, until I learnt some sensitivity with that foot as well!

The worth was proved when I found myself pedalling a loan car, one that handled like it was shod with 75 series rubber pumped up to 20 psi.  The auto car had chronic understeer, but – much to my surprise – I found that it could be largely cancelled-out with a dab or two of left foot braking.

Swing axles defended…

Posted on July 24th, 2007 in Opinion,Suspension,Tyres by Julian Edgar

milliken-car.jpgI’d no sooner finished writing A Disappearing Suspension Technology than I came across something that goes a long way to explaining the reason that swing-arm suspension was used by such hugely respected engineers like Porsche and by companies like Mercedes.

The magazine article is on a very interesting car produced by one of the all-time greats in suspension theory. The designer of the car was Bill Milliken and the premise was that by using narrow tyres running a huge amount of negative camber, very good cornering grip would be able to be obtained.

Now there’s a lot more to his car than just that (by clicking on the magnifying glass you can enlarge the article scans enough to print/read them) but the narrow tyres/huge neg camber is a very short summary.

The tyres being used by Auto Union and Mercedes pre-WWII race cars were similar in width to current big motorcycle tyres and so would have been far less susceptible to loss of grip through lifting of the flat tread of the tyre that would otherwise occur through negative camber. In fact, the lateral thrust from the camber achieved by the swing-arms would, as the Milliken car shows, have made a major contribution to cornering grip.

It makes me think that a lightweight car running low pivot point swing-arm (or semi-trailing or leading) suspension and motorcycle tyres could develop a lot of grip while maintaining an ultra lightweight suspension, in turn giving a very low unsprung weight and low total vehicle mass. And the narrow wheels and tyres would also give far lower rotating inertia, improving acceleration and braking still further…