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Millimetres from death

Over the years I’ve attended a fair number of collisions (there is no such thing as an accident in our game as there is always someone to blame for it, so we deal with collisions) and it’s amazing how often life and death is a matter of inches in either direction. For example a security guard who fell asleep driving at night and came off the road: a fence rail punched through the front of the car into the passenger cell, went through his jacket, grazed his side and imbedded itself in the drivers seat! He escaped with no injuries apart from a graze.

On another occasion a motorcyclist brushed down the offside of a car, (almost a head on), causing very serious injuries to his right leg and arm (hospital considered amputation) but a couple of inches to the right and he, and the cars occupants, may have been killed.

Fate obviously has a hand and many things can be out of your control… Many years ago a bus collided with a lamp post, as it was a London bus an unfortunate passenger fell off the rear platform and even more unfortunately the lamp post fell on him, killing him as he laid on the road.

But as motorcyclists we need to think about millimetres, here are some important examples:


New brake pads seem to vary in thickness depending on the manufacture but the average is a little over 4mm.

The UK MOT manual states the following reason to fail:

Brakes Brake linings or pads (other than sintered pads) less than 1.5mm thick at any point (ii) Sintered brake pads less than 1mm thick at any point


Most riders realise their bike’s sole connection to the tarmac is the thin, pliable hoop of rubber encircling the wheel.

The UK MOT manual states the following reason for motorcycles to fail:

The depth of tread is not at least 1mm throughout a continuous circumferential band measuring at least three quarters of the breadth of the tread.

Where the engine capacity is not greater than 50cc, tread depth may be less than 1 mm, if the tread pattern is clearly visible around the entire circumference and across the whole breadth of the tread.

This means you can ride a motorcycle over 50cc with 1mm of tread. 1mm, which is about the thickness of 12 sheets of 80g printer paper, or of a CD!

When you think that tread on a tyre is for displacing water and read below, it seems crazy to have so little tread.

Water displacement

The first most significant feature is a tread. You can have the stickiest compound in the world, but if you have no tread, wet weather performance will be rubbish. As I’m sure you know, tread patterns break up the layer between the rubber and the tarmac so that aquaplaning is discouraged


As a tyre rolls over wet tarmac, a ‘wedge’ of water forms in front of the tyre. If the tyre rises up on this wedge, contact between the tarmac and rubber is broken, and aquaplaning occurs. By breaking up the surface of the tyre such that there are channels for this water wedge to infiltrate, the tyre is less prone to rising up on it. For a similar reason (like-for-like) narrow tyres perform better in very wet conditions because the tyre is less prone to ‘float’.

I was rather hoping for some technical info from more than one manufacturer, comparing the efficiency of water displacement as the tread on a tyre wears, but I have been waiting about seven weeks and it has still not arrived; it would appear that motorcycle tyres are infinitely more complicated than their car equivalents as a consequence of having different compounds in different parts of the tyre, so its not easy to work out… However, needless to say, the more tread you have, the better the water displacement.

When the information is forthcoming I shall pass it on.

Rear Tyre

This image was taken a few weeks ago as the bike was ridden through our county (the mud came from the side of a lay by when stopping), not only is there no tread, the tyre is in danger of blowing out as its internal structure is affected.

PC Graham Pierce

If you have a sensible question about the police, traffic law etc., you can email me at askapoliceman@theridersdigest.co.uk or you can ask it via the comment box below (if you are reading this on the new HTML web site) because you can bet your last shekel that if it’s been bugging you, someone else wants to know too.

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8 thoughts on “Ask a Policeman

  1. I, for the most part, agree that there are few accidents. Occasionally, a tree will drop at exactly the wrong time and that and falling pianos qualify as “accidents” that are unpreventable. In Minnesota, we have hoofed rats (aka “deer”) and they are slightly more brainless than falling trees and way less predictable. I’d still call running into one a “crash” rather than an “accident,” but when one runs into the side of your motorcycle that’s pretty unavoidable.

    Tires, brakes, general maintenance, and riding skills are all critical elements in avoiding crashes or collisions or whatever you like to call preventable incidents. Loud pipes, on the other hand, are a waving flag asking for the general public to take a shot at knocking down an offending asshole.

  2. T.W.Day, I take what you say; in a collision there is almost a reason rather than an accident, even if it’s blaming the hoofed rat.

    In respect to maintenance, here is a link to the U.K MOT, (vehicle testing), regulations for comparison in the USA or general information for us in the U.K
    (MOT Manual for Motorbikes (Detailed Version) UK MOT)

  3. With respect to the photo, whilst in this case it is clear that the tyre is bald as the carcass is showing through, the lack of tread in the centre is not necessarily indicative that the tyre is badly worn, as the text suggests. There are tyres available which have no tread pattern in the middle of the tyre and I suspect this is one of those as the remaining pattern appears to end too cleanly, rather than being worn away. As you say, providing there is tread over three-quarters of the tyre then such a tyre is road legal from new.

    All tyres should have a wear indicator. I’m not sure how the manufacturers do this in the case of the middle portion of a tyre with a ‘bald’ pattern.. I suspect they include small holes in the centre of the tyre (as is done with slicks) so you can poke a depth gauge in and see how much is left. Matey obviously hasn’t done this with this tyre.

  4. Andy,
    as you rightly say with some tyres the tread does not cross the center of the tyre, if you are a rider in all weathers I suspect you would not fit it, but even so depending on the curve of the tyre the footprint may still displace water.
    This is a worse case image, not only for the lack of attention to the riders own, (and others), safety, but for all the wasted rubber around the edges, obviously a rider who is more confident in straight lines!!

  5. From that picture it looks like the owner has just done a massive burnout as there seems to be a fair bit of tread elsewhere on the tyre.

  6. Hi, PC Graham.

    This query relates as much to cars as to bikes, and it’s a product of the overcrowded roads we so enjoy (not!) here in the South East (Kent / Sussex). Picture if you will, a scene from the road to HELL……

    There’s an ~11 mile section of roadworks in the South East corner of the M25 between J5 and J6 (past Clackett Lane Services), where the lanes are narrow and hoards of ‘tens of thousands’ of densely packed vehicles of all types and sizes thunder along hammer-and-tongs at it, 3 lanes each way, stacked ‘waay’ too close to each other at the 50 MPH limit.

    As in: no more than a metre or two from the vehicle alongside in the narrow lanes, and most of ’em a couple of metres behind the vehicle in front of ’em. That therefore means being near-nudged by the guy behind, whose very life appears to depend on getting the guy in front of him out of the way – which is worrying when you’re ‘the guy in front’ on a motorbike! It goes without saying of course, that lane 3 is chokka-full too; they’re all tailgating one another out there at 55 / 60 MPH, chancing-it against the scameras.

    There are a dozen or more pairs and quads of SPECS average-speed cameras keeping a big-brotherly eye on the entire appalling, blood-thirsty, rage-infused, running dogfight, where ‘the devil can take the hindmost’. The scameras at either end of the frantic fandango are facing both ways so as – I guess(?) – to catch the bikers too. As a result of which, everyone has to comply with the aforesaid posted 50 MPH for every metre of those agonising ~11 miles – except the seriously naughty, weaving and speed-heedless bad-boys who’ve either got cloned registrations or nothing to lose anyway.

    The resulting bulging-eyed, blood-pressurised, vein-busting, not-very-well suppressed rage, frustration and aggression to be enjoyed (not!) in that overcrowded 11-mile ‘scene-from-hell’ is something to be scarcely credited, believe me. I’ve experienced it over a number of months now, and it’s not nice!

    Now we all know it’s not a particularly good idea – particularly on a motorbike – to position yourself for any length of time alongside another vehicle on the move. Especially just behind the line of sight of forward observation of the driver of that vehicle. But remember the scenario here: there are thousands of vehicles all doing the same 50ish MPH thundering along in this scene, so it’s impossible to not be alongside someone at any given time.

    So having ‘painted the background canvas’, here’s the thing… The other day, I had a white van man tightly ‘trapped’ in lane one between juggernauts fore and aft of him, alongside myself in lane 2. I was in turn trapped behind a guy doing more-or-less the 50 MPH limit 2 seconds in front of me, and another guy ~2 metres behind my rear numberplate trying to get me out of his face by way of a toxic mix of frustrated willpower and naked intimidation – almost certainly wondering why I wasn’t climbing all over my guy out in front, same as he was all over me. Oh; and a solid line of tightly-packed tailgaters to the offside in lane 3 drifting past at a marginally ticket-risking ~60 MPH in the 50 limit.

    The white van man in lane 1 alongside me got tired of being trapped, and I could see his ‘van-language’ as he started to ‘nudge’ towards lane 2 (and therefore me!) any time he got his nose a metre or 2 in front of me – always baulked by the juggernaut a metre in front of him and by me. No indicators factored into any of this, of course. Conscious though I was of the utter idiot fastened to my rear numberplate and baulked by the 60 MPH queue to my offside, I was therefore effectively bullied into dropping back a little more from the guy out in front of me. I was unwilling to speed up out of the way by getting too close to that guy in front of me – and already at the speed limit, mindful of the vulture scameras beadily watching every MPH for possible monetary pickings.

    Sure enough, said white van man in lane 1 finally cracked, gesticulated and waved his fist, shouted silently at me from a rage contorted face, and swung out directly into my path. Even though I was ‘ready’ for this, the violence of his manoeuvre meant I still had to brake and swerve, narrowly missing the tight-packed lane 3 crew to my offside, and of course very nearly having myself shunted from behind – but fortunately managed to not trigger the almost inevitable pile-up (though only just).

    All incredibly dangerous and stressful, and by now I’m getting more than a little hacked-off with it.

    Any advice, before I get myself pureed and taken-out for simply trying to stay legal? I know you’ll probably deliver the conventional wisdom: “drop back further to let the idiot white van man out”, but remember all the other surrounding hazards, particularly the thick fool jinking around inches behind me…


  7. Dermot,

    I can clearly visualise the scene, thankfully it is a distant memory for me due to my move North and I only have to suffer the increased body count of small winged creatures, general wildlife running into the road and a selection of drivers who have no idea what its like to ride or drive in a city environment.

    In my opinion, (and this has kept me alive for a good long time), there are two ways to ride, defensively or positively, (some would say aggressively).

    Personally I favour the positive approach such as making sure your visible in off side mirrors, dominant road position etc, however…….
    Safety has to be the main concern, but you are in a difficult position to either leave a sensible gap which other drivers see as a sign of weakness and try to take advantage, or be more positive and greatly reduce braking time, either way your safety can be compromised, so are there other options?

    It may be a hassle, but can you use the A25 to miss the problem bit out? or another section of roads? it may take longer, but what price safety.

    If the M25 has to be your preferred route I would increase chances of safety with lots of high vis clothing, brighter running lights on the bike and a bit of extra insurance with a video camera, helmet or bike mounted.

    I know we should not have to accommodate the sort of problem drivers that cause collisions due to impatience or incompetence, but in my opinion its either that, change route or move house or work!!

    Obviously there are other more aggressive methods of riding and I’m sure there are plenty of seasoned despatch riders that may approach it in a different way, but obviously I’m not in a position to suggest or condone more ‘positive’ riding actions.


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