Don’t crash on the gas!

The ‘Winter Riding Season’ usually kicks off with a series of spills at junctions, where riders usually report that “the back end just came round on me” or something similar.

The accident is particularly likely where riders are turning right out of a side road – the distance you travel encourages trying to power through the turn, plus the fact you’re crossing two lanes of traffic (so generally fewer gaps on a busy roads) leads to riders who are in a hurry.

In this case, it was coming off a roundabout late at night in spring on a wet surface. The back end came round and the rider went down on the low side. It seems it’s a roundabout the rider uses all the time so why did it happen?

First of all, let’s get the usual excuses out the way.

Cold Tyres. Do motorcycle tyres need to be warmed up? There are all sorts of stories going round about how you need to warm up tyres before they’ll offer proper grip, some riders are even resorting to using tyre warmers in the garage before they go out for a ride!

Is this likely? If street tyres needed warming up before use to the point where they offered dangerously low levels of grip when cold, the riders all over the place would be crashing and able to point the finger directly at the tyre. Where would be the first place they would be running to?

It wouldn’t be the tyre warmer shop, it would be the compensation courts, particularly in the USA! Think about it. Even road legal trackday tyres have to produce a reasonable level of grip on the road from cold; it’ll just be a lot less than they would on a sun-warmed race track!

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[EDIT] 23 May – I’m going to change my opinion on this somewhat.

At the recent Rider Performance day at Castle Combe where I was giving one of the presentations, I got to listen to a tyre expert who’d come along from Avon Tyres. The most thought provoking observation he made was that in the conditions (13c, and wet) the tyres that would work best on the track would be the high silica sports touring tyres, NOT the supersports tyres.

Yes, he said on the TRACK, not just the road.

He said the sports touring tyres would offer just as much outright grip as the softer tyres under the wet conditions, and furthermore wouldn’t need warming up but would work from cold. The supersports tyres would need to be worked hard to get them up to temperature, and worked hard to keep them there!

The obvious conclusion is that for anything but dry, warm roads on a sunny mid-summer day, you’re better off on a sports touring tyre!

I still think a rider with reasonable feel and understanding of how the bike is performing under them could ride round the issue, and tyre warmers are clearly a fantasy, but if you’re a bit ham-fisted it does throw some light on the annual rash of cold-weather crashes! [/EDIT]

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Diesel. Fuel spills do happen. But generally they’re thin streaks or drips, and you might slide as you hit it, but you’re soon off onto clean tarmac and the tyre grips again, after a twitch and a wobble.

Very occasionally there is a BIG spill you can’t avoid – I have had to negotiate a couple over the years, including one that caused three other riders ahead of me to crash. But a spill that size you can see and even smell. Police accident analyses do not point to diesel and petrol spills as a major cause of crashes in the UK.

So what does cause slides that the bike can’t recover from? Simple; it’s lean angle and throttle! At the same time. To excess.

I have lost count of how many times I’ve explained this on the bike forums I contribute to, yet people still read magazine articles which bang on about feeding the power on as you start to pick the bike up out of the turn. What they discover is that if you feed in TOO much power, you end up picking the bike up out of the ditch instead!

The short answer. Get the bike upright THEN feed the power in. That way if you exceed the available grip, you get upright wheelspin – no big deal.

The long answer. Why is this “feed the power in” technique a poor one? It’s basically down to road surfaces. We’re not on a track. For starters roads don’t grip like a track, but in the wet they get much worse. A GOOD CONDITION wet road (shellgrip excepted) will deliver around 15% less grip than it will in the dry, a worn surface considerably less.

The tyre can deliver a certain amount of grip, depending on
a) how grippy the tyre is
b) how grippy the surface is

Let’s look at the first part – tyre grip. If you’re leaning over, you’re asking the tyre to grip to keep it turning. If you’re adding throttle, you’re asking the rear tyre to deliver grip to drive the bike forward.

Now, let’s factor in what happens as you feed in the throttle and the bike speeds up whilst you’re leant over in the middle of a corner – you need increase your lean to keep the bike on the same line. So you’re now asking the tyre for MORE grip to keep the bike turning at the same time as you’re asking for MORE grip to keep it driving! This split of grip between acceleration and cornering is sometimes known as the “Traction Pie”.

It’s not rocket science to see that by twisting the throttle mid-corner it’s easy to try to carve off a bigger slice of the Traction Pie than is available – the grip the tyre was delivering quite happily a moment earlier suddenly isn’t there any more. In essence, it’s the reverse of the problem of applying brakes mid-turn, and riders are quite aware of the danger of that.

The second issue is road surface – it’s half the deal – if the surface can’t deliver its half of the grip, then the tyre won’t stick to it if you try to bite off a big chunk of the traction pie!

Just like a soft compound tyre will allow more throttle and lean angle than a hard compound tyre, so a high friction road surface like ‘Shellgrip’ will offer more than worn out and slick surface; you can brake and lean almost as hard in the wet as you can in the dry on Shellgrip.

The difference is that surfaces aren’t consistant and constantly change. You don’t change tyres half way through a bend! (Though come to think of it, someone will no doubt mention dual compound tyres, but note these offer more grip at bigger lean angles, not less!)

In particular, high grip surfaces are often laid where cars are turning or braking, but these surfaces are rarely extended far out of the junctions or corners, and motorcycles with their wider lines can run off the Shellgrip onto the ordinary tarmac whilst still leant over. Consider the potential consequences!

OK, having explained at some length why bikes lose traction mid-turn, why do riders continue to do it? Two reasons:

• it’s a track technique – it’ll get you round the lap faster (at the risk of crashing out) so it’s “sexy”.
• it’s in the advanced police riding manual “Motorcycle Roadcraft” – which was written when police were trying to squeeze the highest speeds possible out of 50hp Triumph Saints and BMW R75s.

Neither is a good justification for using the technique when even newly qualified riders are buying 100hp 600s. I’ve wheelspun a CG125 in the wet, so it’s not difficult with a 100hp bike.

So why can’t you shut the throttle and get it back?

You need to understand how tyres deliver traction – unfortunately the sums don’t work the same for losing and regaining traction. It’s just the way tyres work beyond the normal friction rules (which incidentally is why you can lean over more than 45 degrees without falling off) – once you break traction you have to reduce what you’re asking the tyre to do in terms of the traction pie beyond what it was offering when it lost traction.

It’s a bit complicated to explain without resorting to graphs and equations, but trust me that it’s much difficult to regain grip lost during a slide than it was to to lose it in the first place.

If you’re on the throttle, once a tyre has broken traction, you’ll find it difficult to keep it from wheelspinning. And if it’s wheelspinning, it’s delivering next to zero grip. In a straight line, you just shut the throttle and the tyre stops spinning and you regain traction. No big deal

But if it spins up at the same time as you’re leaning, then the tyre slides out sideways at an angle to the front wheel, because of the design of a motorcycle as two wheels with a hinge in the middle.

Now you have a big problem. If you try to reduce the throttle imput to regain grip, the bike is sideways on. When the tyre grips, the next stop is usually an aerial exit from the machine in the form of a highside. If you were watching British Superbikes from Oulton Park on Easter Monday, you’d have seen some excellent examples of bikes losing traction, getting sideways and being unable to regain it, with the riders being thrown off – one of them even crashed behind the pace car!

Alternatively you try to steer the bike upright to take away some of the lean angle. At least, that’s what riding manuals will say; “steer into the slide”. There’s a clear disadvantage there because there’s a reason you’re leaning over – to get round a bend. Steering into the slide takes you off the road or worse into oncoming traffic.

In reality, once the back end has gone mid-turn, it all happens far too fast for the average rider (me included) and if you stay on at this point it’s divine intervention. Those not so blessed will go down lowside if the tyre continues to slide, or highside if the tyre gets broadside enough for the contact patch to “grow” and grip again.

And incidentally, this is why it was very unlikely to be diesel that caused the crash – as soon as he got back on the non-diesel part of the road, the level of grip would have gone right up and the bike would have snapped back into line. As that didn’t happen and the bike continued to slide out and down, it’s far more likely his combination of throttle and lean angle just outrode the wet road surface! Police accident investigations don’t find diesel actually causes many bike crashes – but it’s an easy excuse for the rider!

So, how to avoid?? Two suggestions.

The first one is to use “Point and Squirt” cornering. Rather than try to use big lean angles, sweeping lines and feeding the power on whilst turning, use the Point and Squirt. Slow down upright, get mid-turn speeds down, square turns off by turning tight, get the bike upright again and THEN put the power on.

Second, look at the tyres. Many people fit soft compound tyres that are really designed for trackdays in the belief they’ll give more grip on the road. They do allow bigger lean angles than touring tyres.

Or rather, they do on a CONSISTENT surface.

Just like any tyre, if the surface can’t deliver its half of the grip, then the tyre won’t stick to it – but now because of the confidence offered by the soft compound tyre, the rider is leaning the bike further, feeding the power in harder…

…all the conditions that make recoving a slide more difficult if not downright impossible.

Touring or Sport-touring tyres don’t deliver the outright dry grip (though I’ve got my knee down without too much bother on the old Mk 111 Avon Roadrunners in the past) but they let go more predictably in the wet – you usually get a series of warning “slips” before they slide.

Final point. A slower, squarer turn gets you upright again sooner. And because you’re upright, you can get on the power harder and make a quicker getaway than the rider who’s still leaning getting round the corner!

Sorted!