its mostly down to tyre compounds which are different for different parts of the tyre so harder comps in the middle and softer towards the wall. aero is a bit more difficult on a bike because of the lean thing, you need the aero on the straight but if it was the same on the corner the bike would fall over, i suspect  as the bike is leant over the aero is also leaning so doesn’t have the same effect. Now for the experts to come along!

Friction between road and tyre resisting centripetal force.  Newton’s equal and opposite reaction.

The angle is determined by how much friction is available.  That depends on the road surface, what the tyre is made of, and how much of it is in contact with the road.  I guess a racing tyre will be made of a soft rubber rounded to squidge more as the bike tilts.  Better grip than a road tyre in exchange for rapid wear.

Obviously limits to how far a bike can tilt, and motor cyclists are extremely vulnerable to oil, water, ice, leaves, gravel, metal covers, or smooth asphalt repairs.  Or expecting a road tyre to allow Guy Martin style cornering on the way back from the pub.

I’ve avoided the maths because I will certainly get it wrong.  Look up centripetal force, or perhaps someone good at sums will explain!

Dave

Could there be anything gained by the fact that the sidewall of the tyre will be trying to “climb up” the road surface as the angle becomes more acute? Or more accurately “across the road surface in the direction of the top of the bike”, dragging it into the bend.

Rob

Not quite on the subject but older members may remember the Dunlop triangular race tyres. The great Mike Hailwood, when he returned to the IoM in the late 70’s commented that, due to the great improvement in tyre compound and construction over the 12+ years he was out of the sport, he had to rethink his braking points.

In two words counter steering.

Reducing things to classic point and line simplified dynamic analysis its clear that a motorcycle cannot run round an arc, ie go round corners, as this requires a centripetal force to be applied trough the centre of gravity towards the centre of the arc.

Being a single track vehicle a motorcycle is unstable side to side so any centripetal force will cause it to fall over!

Obviously motorcycles go round corners quite happily so there must be some trick that lets the bike think its going in a straight line whilst the rest of the world sees it going round a corner. Sticking with point and line dynamics this condition is met if the bikes centre of gravity lies on the outside of cone whose base is the radius of the corner being followed by the tyre contact patch.

Fairly obviously there will be some centrifugal force acting through the C of G to help hold the bike up. Effectively levering against the tyre contact patch. On its own this is clearly insufficient to stop it falling over. Gyroscopic effects from the spinning wheels help but that sort of thing is far too painful to discuss in polite company.

Extending the point and line model to include the length of the machine, so now there are two extra lines through the centre of the tyre contact patches, counter steering means that the front wheel is pointing a little outside the turn. The bike actually moves slightly crabwise relative to the nominal radius of the turn. The front wheel contact patch moving around a slightly larger radius and the rear wheel a slightly smaller one.

By the time these forces have finished chasing each other around the bike the net result is that some of the grip from the tyres is deflected from stopping the bike sliding outwards to jacking it up against falling over.

Hence the ability to reach spectacular angles of lean so long as the tyre grip holds out.

Given a modern big. heavy bike shod with radial tyres it’s quite easy to verify this analysis by simply playing with the counter steer force to modulate the angle of lean in corner. The funny front end Yamaha GTS with its swinging arm front suspension is the purest example of this as, correctly ridden, it is directed solely by counter steer. A revelation when it appeared in 1994 and something of an acquired taste.

Clive

On 6 October 2024 at 21:13 Clive Foster Said:

Robert

Not sure whether to thank you or not for finding that report. Splendid example of bureaucratic buckpassing attempting to prove that if sufficient rules are followed perfect safety is possible. Things have clearly gotten far worse since I separated from MoD 20 years ago. These people need to get a life.

…
Clive

Maybe Clive, but the report is an actual real-world example of what happens when there’s a bad accident.  No buckpassing, rather a determined attempt to identify causes, including a positive check that everyone involved did their job properly.  Plus suggested improvements.

All the organisations involved will have had to produce similar.  The MoD report is particularly detailed because the rider died whilst representing the RAF – an employee paid to be there!

I’ve been involved a few times supporting H&S enquiries.  My experience is that those with any degree of responsibility become extremely stressed, even if they did everything right.  Costs a fortune and goes on forever too.

In the past people were less likely to be called to account or held responsible for their mistakes.  Not so today. Clive’s experience of what the MoD have to do in these cases is out-of-date.

Things tightened up a lot during my career, and since I retired.  Root cause is avoidable accidents caused by people whose shortcomings extend the “Duty of Care” after every stupid incident.

We might not care for this level of scrutiny after accidents, and maybe it has gone too far, but I’m afraid it can’t be ignored.   Like it or not believing folk shouldn’t be held responsible is no protection when things go badly wrong.   As is, expect probing questions if you had a role, and worse if your answers are found unsatisfactory.

In Robert’s report, no-one was at fault.  Nonetheless the MoD identified a bunch of minor improvements, all more work for someone, and of the type Clive hates!  But I hope it’s obvious, as an example, why the MoD should be concerned that their line-management system thoughtlessly gave a man permission to ride even though he had very recently reported sick.   Fortunately, being ill didn’t cause the accident: if it had, the employer would be fined and required to pay compensation.  This stuff can’t be ignored.

Keep in it all proportion though. Model Engineering is safer than many hobbies, and certainly not in the same league as racing motorbikes!   Nor would hobbyists be considered employees.  But clubs can’t dodge their duty of care to visitors or passers by.  It’s why they take out insurance!

Managing H&S risk isn’t difficult unless something unusual is afoot.  The main problem is it requires some thought and then documenting the results is tedious. No-one likes paperwork!

Dave

PS, meant to add those MotoGP rear tyres are 200mm wide, almost 8″, so bascially double and more than that of the classic era. Fronts are typically 120mm or almost 5 inches, vs 3.00 inches in the classic era. So more, and better, rubber on the road. Made possible by modern kevlar etc cords in the tyre “carcass” that allows them to be large but stiff with low profiles.

(As an aside,  MotoGP bikes do have aerodynamic components, just like F1, to aid adhesion but their affect would be more in a straight line rather than a turn.)

( This is not a ‘physics’ description. Physics tells us that centrifugal forces as fictitious or apparent forces). It is worth considering what happens when a motorcycle does not lean in a turn. If the motorcycle is prevented from leaning into a turn then the forces will act on the upright motorcycle causing it to lean away from the turn and ‘flip over’. We can see this happening with a sidecar. The motorcycle is prevented from leaning into the turn and the ‘centrifugal force’ acts on the motorcycle to cause it to lean away from the turn, often causing the inside sidecar wheel to lift.

By leaning, the force is partly directed through the frame of the motorcycle to the road. This can be felt when riding as the force is sufficient to compress the suspension of the motorcycle. As the speed increases the force does too, so that greater angles of lean can be achieved the faster you ride. MotoGP racers can manage these angles much better than us mere mortals.

Obviously, the lean angle is dependent on the adhesion between the tyre and road. And that adhesion is affected by  the materials concerned but also by the size of the contact patch between the road and the tyre. Modern tyres are designed to always maximise that contact even at extreme angles of lean. Older tyre designs never achieved anywhere near as much contact.

I have a modern, BMW, motorcycle and an older (1950s) , Norton, motorcycle. The difference in adhesion is unbelievable. No doubt some of this is down to the respective frames and suspensions but a much greater part is down to the modern tyre design and material which places a much larger contact patch on the road and uses material in the tyre which gives much greater adhesion.

I can only think that if these forces did not act then a large part of the enjoyment of motorcycling would disappear.

MotoGP bikes do have aerodynamic components, just like F1, to aid adhesion but their affect would be more in a straight line rather than a turn

Not these days – what used to called “fairings” are now refered to as “aerobodies” & the lower sections are designed to increase downforce when the bike is leaned over in a turn . KTM are sponsored by Red Bull & apparently use the F1 teams aerodynamics people though currently Ducati’s package seems to be working the best overall, with maybe Aprilia’s cornering aerodynamics currently the best.

Tyres are very different to road bike versions, offering extreme grip levels but only within a very narrow range of operating parameters (temperature and pressure) over a very short life, typically wearing off around 1kg of active compound over a maximum of the race distance (100-130 km). But during qualifying a tyre will be “done” with as little as 4 laps – an “out lap” on new tyres to scrub in & get the temperature up to optimum (can’t be too slow or the tyre won’t be worked enough to buld the required temps & too fast will probably result in an “off” due to lack of grip) then an “on it” lap to get sighted in & up to speed, then 2 (possibly 3 ) qualifying pace laps. After that, in for new tyres – but not too often, as the tyre allocation for a race weekend is only 10 fronts & 12 rears.

Same as fuel (which is also limited – 22 litres to do the out lap, forming-up lap & the race) tyres have to be managed during the race to ensure there is rubber left at the end – go too fast too soon & there will be no grip at the end.

Nigel B.

If I remember rightly, the old Dunlop racing triangular tyres had an angle between the sides of 57 degrees. They had a reputation of being rather dicy when starting the lean from upright as the contact was very narrow until full lean was achieved. Modern tyres are a world away from triangulars, as is the horsepower now. Mike Hailwood’s 500 Honda had 105hp and he admitted it was a challenge to ride.

I don’t buy the contact patch claims either. Simply put this can be approximated by dividing axle load with tire pressure, I’m sure that the stiffness of the rubber complicates this somewhat but not THAT much.

But did we know about counter steer, in practice rather than academically, in those days?

I don’t recall noticing it as a thing prior to the arrival of bias belted and radial construction tyres with their softer sidewalls. Certainly my Norton Commander wasn’t noticeably countersteery (sic) with the original Yamaha XJ900 front end and old style tyres.

I learned about countersteering from one of the American bike mags (Cycle or Cycle World probably) in the early ’80s. First time I tried it on my GS425E I very nearly ended up in a hedge, having put in way too much input. Over a number of years I worked on the technique to the point that I just do it without thinking about it now.

I had an XJ900F in the mid-90s and I just rode it in the same manner as anything else – which included countersteering by default. Don’t recall any handling issues with it – actually don’t recall much about it at all, as it was a pretty bland “UJM” and it wasn’t around long. Lovely paint job, though – probably one of the best looking bikes I have owned. I do recall thinking that it was a better bike overall than the BMW K100RT I had run (equally briefly) a couple of years before – smoother, more economical & equally long-distance comfortable. The XJ900 was the point that I realised that it wasn’t any of the inline fours I had owned (all briefly) that was the problem for me, it was the realisation that I just don’t like inline fours. Haven’t had one since – twins or singles (4T and 2T) since the Yamaha deprated early ’95. I did complete the Stella Alpina on in though, so it can’t have been all bad.

Only bike I recall riding that really didn’t respond to countersteering was a ZZR600 that my wife ran for a few years. I truly hated the handling on that & it was very reluctant to turn in without an appreciable shift in body position – which isn’t me. I guess my “stlye” (such as it is) is much more Hailwood than Rossi in that regard !

Nigel B.