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.

Getting enough clearance

Posted on May 30th, 2014 in Driving Emotion,Safety,Suspension,testing by Julian Edgar

When is enough clearance sufficient?

If you’ve modified a lot on cars, you’ll have come across this question. It might be the clearance between the exhaust the bodywork, clearance of a driveshaft at full suspension bump with a chassis member or subframe, or even clearance between a large turbo and bodywork.

Years ago I read an excellent book written by an automotive suspension engineer working in the 1950s. In it he made the (almost throwaway) line that there’s no need to provide tyre clearance at full suspension bump AND full steering lock – the idea being that this situation almost never occurs, and if contact did in fact occur in that situation, the car would be moving so slowly that it wouldn’t matter much anyway.

These thoughts are intruding because at the moment I am massaging a turbo dump pipe so that it clears a steering tie-rod, with the greatest potential conflict occurring at full suspension droop and with full right-hand steering lock.

At full droop but with the wheels pointing straightahead – no problem. And at full steering lock and with the wheels at normal ride height – again no problem.

It’s just at that particular combination – one that again is very unlikely to ever occur – that I have the issue.

I am concerned because if the car has to undergo full engineering approval, I can just imagine an engineer saying something along the lines that conflict should not be able to occur at ANY combination of lock and suspension movement….

And even if clearance is achieved, how much clearance is enough? If I were ornery enough to throw in maximum engine torque reaction movement at just that moment, perhaps another 10mm of clearance would be needed.

But hold on! How could the engine be developing maximum torque if the suspension is at full droop? After all, in that situation there’s very little – next to none in fact – of the car’s weight on the tyre… so how could it transmit the torque anyway? No torque transmission means no transverse engine rocking!

Hmm, what about if the car has an LSD, and a very stiff front anti-roll bar, and is cornering hard enough (at full lock!) to lift a wheel? Then I suppose one could imagine a situation where something like contact could occur.


Beware black snot

Posted on April 22nd, 2012 in Driving Emotion,Safety,tools by Julian Edgar

If you’ve been sawing, grinding or filing metal, it’s likely that you’ve ended-up with a nose full of it. Not just snot – but black snot.

For years I thought it a just curiosity that resulted from that pursuit.

But now I am rather wary of it.

Recently, after spending a full day cutting and grinding, I started feeling a bit ill. The next day, going back to doing some more cutting and grinding, I wore a light dust mask.

But that night I still had black snot – and a hacking cough.

After a few days of feeling crap, I went to the doctor. I hate going to the doctor, but this one had the advantage of being the most beautiful doctor I’ve ever been to. And what did she say? You’ve got a virus – harden up.

But despite that opinion, I really do wonder if the metal dust that I’d been getting into my lungs didn’t have something to do with it.

Now when cutting and grinding, I wear a half-face respirator that has two double filters, one to catch particulate matter and the other for fumes. The result? No black snot – and filters that after only a few days of work, have changed from white to black.

Better caught in the filter media than in my lungs – or in my snot.

Beware that black snot….

Designing a unique vehicle

Posted on February 4th, 2010 in Aerodynamics,automotive history,Materials,Safety,Suspension,testing by Julian Edgar

Recently I read Thrust, the book by Richard Noble on his life in breaking land speed records, culminating in the development of the ThrustSSC car – the current world land speed record holder. The record was achieved in 1997.

thrust ssc


The book is outstanding on a number of levels, including its honesty and clarity. The section where driver Any Green describes his techniques for steering the car is just amazing, as is the constant battle for funds that occurred every day of the project.

But one small part of the book particularly interested me: the section where the primary designer Ron Ayers describes how he went about designing the car.

The text is reproduced here:

How do you start designing a vehicle that is totally unique? Here are the characteristics of the problem that faced us:

1. By travelling supersonically on land we would be exploring a region where no-one had ventured, where even the problems could only be guessed at, so there were no known solutions.

2. As the aerodynamic forces involved were so enormous, any accident was likely to be fatal.

3. The project would always be underfunded, short of people and time.

4. There would be only one chance. The final car was also the first prototype. The first lines drawn on paper could well be the ones that are made. The very first assumptions and decisions, if incorrect, could put the project on the wrong track and there would be no chance of starting again.

Problem: how do you make those crucial first decisions when so much is uncertain?

First, every decision had to be a robust one. That meant it couldn’t be invalidated by subsequent decisions.

Second, we could only use technology we were very confident with. This militated against using the very latest technology in some cases.

Third, although direct experience of supersonic travel on land did not exist, we consulted widely, with aviation and automobile experts in industry, universities and research establishments. Experience with Thrust2 was invaluable, particularly in pinpoint¬ing practical and environmental problems that might otherwise be overlooked.

Fourth, where possible we left room for adjust¬ment or change, so we could incorporate knowledge acquired subsequently. Nothing was “hard wired”. One reason for using a steel chassis was that it could be modified if necessary.

Fifth, we didn’t try too hard to integrate the systems. If we needed to change one of them, we didn’t want to be forced to change them all.

Sixth, our choice of a twin-engined car made the design massively overpowered. Thus weight was not a critical factor.

The design resulting from such an approach must necessarily be “sub-optimum”. A second attempt, incorporating the lessons learned, would undoubtedly be better. But the design was proved in practice, and there was little about the basic concept that would need to be changed.

The more you read those notes, the more you realise the clarity of thought being employed: it’s also food for thought for anyone building a unique design of anything.

Noble and Green are currently involved with another land speed record car bid – the Bloodhound SSC.

New Car Safety vs Old Car Safety

Posted on April 21st, 2008 in Safety,testing by Julian Edgar

Perhaps everyone has already seen these videos, but I found them very interesting, to say the least.



Automobile Factory of Despair

Posted on April 11th, 2008 in Driving Emotion,Opinion,Safety,Toyota by Julian Edgar

I’ve just read a fascinating and powerful book.

Japan in the Passing Lane was written in the early 1970s by Satoshi Kamata, a Japanese freelance journalist. The English translation was published in 1983. Kamata’s preferred title – Automobile Factory of Despair – is perhaps a better description of the book’s contents.

Kamata joined Toyota as a seasonal worker, one employed for a 6 month period. In the book he describes by means of a daily diary what life in a Toyota plant was then like. The job was harrowing, exploitative, mind-numbing, dangerous – and enormously hard work.

The assembly line moves so fast that Kamata, working in a plant making gearboxes, can barely keep up. That might be understandable when he was first introduced to the job, but it also remains the case months later.

He describes the tasks he has to do at his station:

1. I pick up two knock pins (small pieces of steel shaft) with my left hand from a parts box (where identical parts are stored) in front of the assembly line. I insert them into the upper holes on a gear box and then knock them in with the hammer in my right hand.

2. With my right hand, I take an input shaft out of a tin box coming down the assembly line. I insert it into the center hole of the gear box. Holding the input shaft from the other side with my left hand, I drive it in with the hammer. (Sometimes it doesn’t go in easily.)

3. With my left hand, I screw a synchronizing ring to an end into which an input shaft has been driven.

4. I turn the gear box around.

Driving Fast

Posted on December 11th, 2007 in Driving Emotion,Opinion,Power,Safety by Julian Edgar

derestricted.jpgYears ago – say getting on for 15 or 20 years ago – people used to ask why I had a performance car.

“There’s no where you can drive fast,” they’d say, “so why bother?”

I’d enigmatically respond with something like: “Oh, there are still plenty of places left to drive fast.”

And, in those days, there were.

The Northern Territory had no open road speed limit, and while the other Australian states and territories had 110 km/h limits, the philosophy of enforcement was then completely different.

There were no speed cameras – all radars were hand-held and, a little later, mobile in-car. In most states, radar detectors were completely legal, and all police communications were unscrambled voice. Trucks didn’t have speed limiters and on the open road typically sat well over the speed limit. CB radios were constantly used by trucks to communicate the presence of police cars (“double bubbles”) and police motorcycles (“Evel Knievels”).

In my Commodore VL Turbo I ran a radar detector, CB radio and police scanning radio. And they weren’t there for looks.

My BMW 3.0si ran to an indicated 220 km/h, my Commodore Turbo to 210 km/h, my Liberty RS to 220 km/h (which seems slow but that’s what I remember), my Daihatsu Handi turbo to 180 km/h and my R32 GTR to 260 km/h. And none of these were figures I got from just reading a book…

Any tight, windy road was a challenge there to be taken: the chances of being caught were tiny. In addition, the speed limit for the stretch of road was seldom set on the basis of the corners, so it was common for a 100 km/h limit to be in place on a road that included corners with advisories down to 30 km/h.

In those days turn-in understeer at 150 km/h was a real consideration; lightness in the steering at over 200 km/h was a right pain in the butt, and anything less than 130 on the open road and you must have had Grandma on board. I remember I boiled the auto trans fluid in the Commodore when going for a top speed run – the car was slipping its clutch-packs and the fluid got so hot it came out of the breather onto the exhaust. A guy I know used to sit on 180 km/h on the open road, ear plugs firmly in place.

Without any doubt the roads today are much safer – I’m sure the enforcement of speed limits and low/zero blood alcohols have resulted in less fatalities and injuries.

But now there really aren’t any places to drive fast. These days, they literally put you in jail if you drive fast, and take away your car if you have a few quick traffic light races. I am not saying that’s bad; what I am saying is that the road use of a performance car is now so limited that I wonder at their purpose.

I live at the top of a steep and windy country road. There’s about 15 kilometres of it – and I know it far better than the back of my hand. I’ve at times driven it extremely quickly, but any time I have done so I’ve been risking my license – the speed limit is 60 km/h. At sixty I can go around every corner without slowing.

Apart from flicking through an urban roundabout quickly (so what…), there is nowhere – literally nowhere – that I can drive fast. And that’s living smack-bang in the middle of what many call the best drivers’ roads for hundreds of kilometres.

And is it any different for other people? I was in a workshop the other day and the proprietor told me how the Falcon XR6 Turbo out the front had 450kW at the wheels. Or was it 550? – I don’t know, I wasn’t really listening. The prop went on to say that it was a really hard car to dyno because of wheelspin. Apparently, on the road it wheelspins up to 4th.

Now, honestly, apart from dyno bragging rights, what is the point of having that much power in a road car? As I have implied, once upon a time it would have been really useful – 100 to 200 km/h in just a handful of seconds. But now, spinning wheels will cause a police booking, a quick traffic light race ditto, and exercising anything like the top-end potential would immediately result in jail time.

Wouldn’t it make a lot more sense for people to have cars so low in power that you can have fun at what can only be described as slow speeds? Or instead of spending money modifying an already powerful road car to make it even more powerful, invest in kart, budget open-wheeler or dedicated drag car?

Cuts and abrasions…

Posted on October 4th, 2007 in Opinion,Safety by Julian Edgar

dettol.jpgThose of us who are not full-time workers on cars, but instead are enthusiasts who don’t get their hands dirty on a daily basis, are most likely to benefit from this product.

So what is it? It’s an antiseptic liquid that you dispense from a pump-pack and then wipe over your hands. The stuff dries quickly and doesn’t leave a sticky residue.

I find it particularly effective when I cut my hands (looking now I can count six minor cuts, three of which are open) as very often heavy duty hand cleaners have the unfortunate side-effect of infecting cuts.

So you’re working on brakes or an engine or bodywork and, at the time you inflict a minor cut on yourself, your hands are filthy. If the cut or abrasion is of little consequence, you leave it and then later clean your hands with a heavy duty soap or dedicated hand cleaner.

And if that’s all you do, it’s likely the open wounds will get infected.

But if after cleaning your hands, you give them a wipe-over with Dettol Instant Hand Sanitizer, the later infection is avoided.

There may well be other products that work in the same way – it’s just that I haven’t come across them. For minor cuts and abrasions it’s much better than the antiseptic cream you might apply to a more serious wound – you can quickly and easily spread the sanitizer over both hands by just squeezing some of the liquid onto a palm and then rubbing your hands together. It doesn’t need to be wiped off – it then evaporates.

The product costs $5 or $6 and it’s available at supermarkets.  Note that it’s highly inflammable, so use it on your hands only after you’ve left the workshop…

(And no the sample wasn’t a freebie – I bought it just like anyone else.)

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.

When the throttle jams…

Posted on August 30th, 2007 in Opinion,Safety by Julian Edgar

skidmarks.jpgHaving the throttle jam fully open is a pretty exciting idea. And an even more exciting reality.

I’ve had the throttle jam fully open twice in my driving career.  Once it was in city traffic, in second gear.  I’d just had the throttle body-to-plenum rubber hose off, and when I’d replaced it I hadn’t orientated the hose clamps the right way around.  The result was that when the throttle lever moved past the clamp screw it couldn’t get back: instant sustained Wide Open Throttle. 

After thinking “Shit!” I turned off the ignition key and then swapped lanes to the curb.