Motorcycle Throttle Control – a Cautionary Tale

Firstly many thanks for Alan Bussell for allowing us to share this with you.

Allan is a very experienced biker and has done as much advanced training as he could pack in from the usual suspects on the roads and track, and was booked to come on one of our track training days for road riders on 30th June, after winning a competition in Motorcycle Monthly.

Sadly he’s just had an off and very luckily escaped with just a few bruises, but the bike is probably a write off. Could have been far worse – note a lack of trees and lamp posts. We’ve known that ‘loss of control’ is a major factor for some time in big bike injuries (>500cc) from the accident statistics.

Allan:-

‘This shows how easy it is to come off.  Gentle acceleration as I was moving out to overtake an elderly couple.  They’d seen me and had kindly pulled over for me to go (no need to because the road was empty). When overtaking I try to show the same respect (if not more) than what was shown to me so I go past without having the engine screaming.
Road2
The conditions were obviously damp so gently does it but when you hit the slippery tar then any power going to the back wheel just spins up the back wheel and before you know it I’m sideways on motocross style then it bites when it hits the grippy stuff.  This is the result’. 

 

Side View

The problem with modern bikes is even tourers have very powerful engines and very light flywheels, so when the rear wheel loses traction, if the throttle has been opened wider than it needs to be, then the revs rise almost instantly. If the tyre does then find a grippy piece of tarmac it’s a complete lottery what happens next, depends on how far sideways you have got and in which direction, before some grip is found.

This is the classic ‘high-side’ if you are banked, but even a seemingly flat straight road can have enough camber to force the bike sideways. White lines are better than they were but can still be treacherous, and a badly maintained road where all the aggregate has worn off just leaving the tar can be like ice, particularly when wet.

It is worse in a higher gear as you get so little response from opening the throttle wider, so may not realise how far it is open, which is why you should always consider which gear to be in (and change down before taking).

Traction control should have kept Alan upright, but the advice is not only to open the throttle slowly and smoothly, but also never get it further open than you need, rolling it open steadily as the revs increase, ‘dragging’ if you like, the tacho needle around the dial.

Modern injection systems are ‘ride by wire’ with no direct connection between the twist grip and the throttle butterflies (which restrict the flow of incoming air and fuel – or a second set of butterflies controlled by the computer on older bikes). No matter how far you twist the throttle, at lower revs the butterflies won’t open much as the engine can’t digest any more fuel efficiently – until the revs rise and the engine management system then opens them more and more as the revs build until it reaches a point set by the twist grip, when the butterflies stay partially open dependent on how far you’ve turned the throttle.

So if the throttle is fully open at lower revs, if the rear wheel loses grip the engine will instantly hit the rev limiter – peak revs and peak power produced as the tyre tries to grip again. When the rider instinctively reacts and shuts the throttle the bike will likely be sideways when the tyre grips again, which often then throws the rider off.

So the advice is to use your gearbox and consider changing down before overtaking which means the bike is more responsive, twist the throttle gradually as the revs increase, so if you do lose traction then the engine revs will only rise so far, giving you a good chance of saving it.

Motorcycle Track Clothing

One piece leathers probably give riders the best protection, although good quality textile clothing should be good enough. Two piece suits, whether textile or leather, are better if either the top and bottom can be zipped together, or if the jacket has a robust crotch strap.

We don’t accept kevlar jeans, as we don’t know how good they will be.

Both textile and leather suits should have CE armour in elbows and knees as a minimum. Shoulders, back and hip protection should also be better. There are also chest protectors available. Check your jacket has a CE marked back protector and not a piece of thin foam. Consider fitting a CE protector in the pocket, or using a separate back protector.

The European standard is EN1621-2 (Level 1 for regular use and Level 2 for sport/track).

Some leather suits are made from very thin leather, could be sheepskin. The quality of the stitching is also vital. Leather is not automatically better.

Here is some interesting general information and advice from Byson Leathers http://www.bysonleather.co.uk/BYSON%20LEATHER%20AboutLeathers.htm

 

 

Braking from High Speed – Components

We’ve had some response regarding brake pads and discs, and whether these effect braking distances in any way, so here is a quick review of braking components.

Brake Pads

‘Softer’ pads with higher coefficients of friction require less lever effort for the same braking effect, but wear out quicker and can fade. Different construction of and materials used in pads can also effect the ‘feel’ during braking which is vital particularly on a track, and being able to sit the bike in it’s nose with one finger is great.

Make sure you buy a well known brand from a reputable source. I bought some ‘Armstrong Racing’ pads from Newark Auto Jumble some years ago, then went straight at the Esses at Donnington on a trackday. Very lucky to find the problem there, as I was off to the IOM on bike the following week. They were probably OK for normal road use, and of course may be OK now, but had I found the problem going down to the Creg on Mad Sunday…….

Brake Lines

Braided hoses, if not already fitted, can make a big difference as they don’t expand as much when pressurised, which makes the brakes feel sharper and uses marginally less lever movement. These do need to be properly made as they operate under high pressure. Most modern hoses are made to measure and swaged rather than you having to assemble your own fittings. Buying a bike fitted already with after market bolted hoses is always a risk that they have been assembled and torqued up properly. If in doubt change them. Again buy a major brand from a recognised supplier, and make sure they are certified correctly. See for example:-

http://www.helperformance.com/certification/

Master cylinders and calipers

The first bike we had which could stop with one finger, was an Aprilia Mille ‘Sound of Thunder’ bike, with the early Brembo racing radial calipers and master cylinder, which are now commonplace on top spec OE bikes. Absolutely amazing at the time. You can retrofit better master cylinders relatively easy (changing calipers usually mean changing forks, or some complicated bracketry for older bikes), but you need to be very careful to use the correct bore and displacement master cylinder, and adjust it correctly.

From experience, everything seems great tested statically, but if the master cylinder piston is not returning far enough, then the fluid cannot return to the reservoir , heats up and locks the front brake. Had this happen to a rider on a race bike with us a couple of years ago , as well as personally on an old TZ350 trying out a smaller bore master cylinder.

Brake discs

Also be very careful where you buy these safety critical parts. Here is a CB500 disc off a race bike where it has cracked and failed at Mallory – the rider was badly hurt.

IMG01910-20110716-1632

The same problem was found on a second bike the same day. These are non-floating cast iron, not the modern steel discs. However, steel discs can warp pushing the pads back, which you suddenly  discover when it takes two pumps to get the brake to work. The old thin fixed steel disc are very prone to this, so you need to make sure that pads are equally spaced and that the disc is not being bent when the brake is applied (another old TZ problem), which is why ‘floating discs’ were invented.

Some floating discs seem to wear the ‘spools’ very quickly, one side went recently on my GXR1000K7. You can here them ‘tinkling’  when you wheel the bike (some Italian bikes used to do this from new), but we cannot find what the service limits should be. You also sometimes get a ‘clonk’ initially when braking. If in doubt change them.

Modern sintered pads can wear out the discs quite quickly, so always check for bad scoring and minimum thickness (usually stamped on the disc).

Again buy OE or recognised brands from a reputable source.

Brake Fluid

Needs to be changed every two years on road bikes, more often for race bikes. Dot 4 is usually recommended, and there is a Dot 4 Racing with a higher boiling point for track bikes. The problem is you don’t know you have a problem until the fluid overheats and boils, as it absorbs water over time, and the brakes simply fail. There is also a new DOT 5.1 which has a longer life, not to be confused with DOT 5 which is silicone based and must not be mixed with any other brake fluid. DOT 5 doesn’t absorb water so has a long life, has a high boiling point, doesn’t act as a paint stripper, is better at very low temperatures, but is supposed to be more compressible making brakes spongy, and is not suitable for anti-lock systems.

Apparently some Harleys have DOT 5 fluid.

Always refer to your bikes handbook, only buy small bottles of fluid to top up and keep the caps on. For racing brake fluid, refer to the manufacturers advice for usage and frequency of changes.

 

What finally limits braking distances on a bike, is that braking forces much over 1 g have the back wheel in air, as the centre of gravity is so high, so braking harder flips the bike. This usually happens quite slowly, so riders have time to simply ease off the brakes.

 

 

Braking from High Speed – Reaction times

We came across this piece of research from the Transport Research Laboratory which appears to have gone under the radar, which we present on our road training courses. Our previous paper on braking a motorcycle (or car) from high speed used the TRL 0.7 second standard reaction time to hazards, but this research shows how much this can vary on circumstance.

There are two factors as to whether you can avoid a collision or minimise the impact speed:-

a. How quickly you can react

b. How quickly you can stop (which we covered in the earlier post) or swerve.

These figures (see below) were reaction times using a simulator in different scenarios.

1. Cars and pedestrians emerging.

Average reaction time at 0.85 is actually a bit greater than that currently used in Highway Code stopping distances.

2. A vehicle braking ahead.

This at an average 1.3 seconds is nearly twice the figure used in Highway Code stopping distances, and the ‘two second rule’ leaves you with a 0.7 second gap in which to brake (or 70 feet at 70 mph). It is also clear that most driver and riders amazingly don’t expect the vehicle ahead to brake suddenly, as from observation they ride/drive far too close and would often simply hit the vehicle in front without even beginning to brake.

3. A vehicle ahead veering off the road and knocking down an overhead motorway gantry.

This I think we can all agree is ‘unexpected’, but reaction time is surprisingly only slightly longer than a vehicle braking in front.

4. A stationary vehicle in front the centre of the lane

This is the real concern as clearly riders/drivers see a vehicle ahead and don’t realise for some while that it is stationary, taking on average nearly 4 seconds to brake or 5 seconds to swerve. A massive 400-500 feet to react at 70 mph. This can also apply in any situation where you do not immediately recognise a hazardous situation, which is why ‘hazard perception’ is such an issue and part of any licencing test, as is planning ahead.

This is known by the emergency services which is why you will see emergency vehicles now always parked across the road or diagonally, to indicate they are stationary.

Also included is the effects of fatigue on reaction times, which is very noticeable, adding more than a second in all circumstances. So if you’re tired a 2 seconds separation distance from a vehicle ahead is nowhere near enough.

So what are the conclusions?

1. Always leave a good gap ahead – 2 seconds is clearly not enough – particularly at high speed.

2. Pay attention!

3. Don’t drive when tired. Stop and get a coffee – a large cup from Costa (other providers are available) not only kept me wide awake riding home the other day, but for half the night.

Again most braking actually occurs in the last few feet, so the earlier you react has a disproportionate effect on  the lowering the speed of any impact or avoiding one (which is better).

Mike Abbott, British Superbike School 25.11.15

Braking a motorcycle with a Pillion Passenger

This article is in response to an interesting question on Twitter from BikeSafe Glos, to an earlier blog on tyre grip, as to whether the extra tyre grip from the weight of a pillion would reduce stopping distances?

The short simple theoretical answer is it makes no difference. The physics is as follows:-

The kinetic energy of bike and rider(s) = m times v squared / divided by 2, where m = Mass (Weight) and v = Speed.

The braking force = umgd, where u = coefficient of friction between tyre and road, m = Mass, g = gravity, and d = distance.

When braking, the two equations are equal, so ‘Mass’ cancels out as it’s a multiplier for both sides. So (in theory) weight on its own makes no difference to stopping distances.

The additional grip via the passenger’s weight is balanced out by the increased momentum of the additional rider. This does however seem counter intuitive from experience, so there may be other factors.

The rider often has to cope with the extra weight of the pillion passenger pressing on him or her, which has to be held back via the rider’s arms, legs and crutch against the tank (usually) which can be ‘eye watering’ – for blokes anyway.

What limits stopping distances is also the position of the centre of gravity of bike and rider(s), as modern bikes can flip over forwards like a pedal cycle. This limits braking to about 1 g – where cars can do much better (Formula One cars can stop at >5 g with downforce) with a far lower centre of gravity.

However, having a pillion shifts the centre of gravity backwards potentially allowing a faster rate of de-acceleration without flipping. Sportsbike pillions sit higher, so the effect is less than with other types of motorcycle.

img457

Pillion braking 2

Illustrations from Motorcycle Dynamics by Vittorre Cossalter .

I’ve taken out the effects of wind drag on braking (and no we’re not on commission).

The red line shows the effect of the shift in the centre of gravity rearwards and slightly upwards, as the included angle decreases, meaning that de-acceleration can be greater without flipping. When the angle is 45 degrees the maximum de-acceleration would be 1 g, whereas tyre grip can result in up to >1.3 g, so the location of the centre of gravity is key.

So in theory having a pillion would reduce stopping distances, everything else being equal, but is the theory actually achievable? Front suspension can bottom out with the extra weight transferred forward, the bike becomes less stable as trail reduces further, so it is advisable to change the suspension settings as recommended by the manufacturer for a pillion and luggage if necessary. Some bikes are too likely sprung, like the early AprIlia RSV’S, even for solo riding, so aftermarket suspension mods might help, but always go to a reputable company.

As an aside, is also probably worth pointing out that accident first responders will go looking for a pillion if they find a crashed bike had its rear footrests down, particularly sportsbike crashes in rural locations.

Motorcycle Tyres and Grip – some myths dispelled

There are a few myths surrounding bike tyres and grip, and for sure the tyre companies are not going to give anything away.

The first is that the bigger the contact patch area then the higher the grip. Area surprisingly has nothing in theory to do with grip!

Why – because grip is simply weight x coefficient of friction.

The formula for friction (grip) is     Fr = μN

  • Fr is the force on the tyre caused by cornering, and brakes or accelerating. When this exceeds μN – the tyre slides away.
  • μ is the coefficient of friction between the tyre and road.
  • N is the force pushing the tyre directly down onto the road, which is mostly equal to the bike and rider’s combined weight. (This varies a bit with undulations in the road and suspension movement, which is why a well set up bike handles much better – but you will likely potentially lose grip over bumps which is obvious to most riders).

The inevitable question is then why do we have such wide tyres? The answer is threefold we believe – to have a carcass that can take the torque from the engine, braking or cornering forces ; to dissipate heat; to reduce wear so softer grippier compounds can be made to last.

Rear tyres are wider as they have to cope with a higher sustained loads than a front tyre. You accelerate hard as you can all the way down the straight, but only brake usually for the last couple of hundred metres when almost all the load will be on the front. The maximum load is limited, as bikes can flip when braking and loop when accelerating (unlike cars).  However cornering speed is mainly dependent on tyre grip, which is the obvious payback for having grippier tyres, and hence the development of dual compound tyres with softer shoulders. Hope this makes sense.

Old bikes with far lower power had similar sized tyres front and back, and really skinny ones if anyone has ever seen a 50cc Kreidler close up.

Tyre Grip - Mohr's Circle

Mohr’s Circle in it’s simplest form shows the theory of tyre grip. You use grip up on cornering, and also when braking or accelerating at the same time, but as long as the combined forces stay inside the circle you have grip, outside the circle you don’t. Clearly in this example the red arrow showing the effect of a high lean angle where the cornering force is nearly at the edge, so any serious braking or accelerating and the tyre will slide. With good modern tyres, the grip ends between 50 and 60 degrees of lean (assuming you have the ground clearance and more with race slicks), but Mohr’s theory does indicate that you can accelerate and brake quite hard at medium lean angles, but this ability reduces quickly as you get towards the limit.

A second myth is that you get more grip at 45 degrees than you do upright. You don’t, as above grip is coefficient of friction x weight, and weight is a constant (bike and rider). It might feel as if you have more grip, but that is just the horizontal forces compressing the suspension at high lean angles (which some riders also confuse with tyre slip).

The third myth is ‘standing it up onto the fat part of the tyre’ when coming out of corners, it’s just that the more upright you can get the more grip is available to accelerate rather than corner. This technique requires an initial slower exit as a trade off with getting on the gas a bit harder and earlier.

Tyres are designed to slip (drift not slide) before they let go, which is where rider feel comes in. Slip angles are typically up to about 6 degrees in ‘normal’ riding, and either tyre or both can drift sideways in the same way a car oversteers or understeers. You need a lot of experience and confidence to get this far, and it would be only sensible on a track. Watch closely how Josh Brooks often rides when he is really on it.

Tyres also distort significantly under load, looking kidney shaped at high lean angles, or ‘s’ shaped when cornering and braking or accelerating, with a steep temperature gradient from front to back of contact patch, which can also move forwards and backwards as the tyres distort, slowing the steering marginally, and ‘shortening’ the effective wheelbase.

There is more information in Motorcycle Dynamics by Vittore Cossalter ISBN-13: 978-1430308614, which you can get on Amazon and elsewhere. Chapter 2 is on tyres. Great book – but heavy on the maths.

It is usually safer to just use the rear brake if you need to slow on corners, as this stabilises the bike and is recoverable if you lock the rear wheel. In theory the front brake would be more effective, but it can sit the bike up and make you run wide, and if you lock it when leant over you’ll very likely drop it, so needs a higher degree of skill and practise, which is risky on the road.

Motorcycle Braking Distances from High Speed.

Braking from high speed is very different from braking from road legal speeds.

It is quite difficult at first to judge braking distances. We were supporting a sponsored day at RAF Waddington a couple of years ago, and the first two riders both went straight on at the end of the main straight (which was a mile or so long down the runway). They were nowhere near to getting stopped in time.

Braking follows a ‘square law’. Twice the speed = 4 times the stopping distance, so stopping from 60 mph takes 180 feet (using the usual parameters but ignoring reaction time), from 120 mph takes 720 feet, and on the same basis a whopping 1620 feet from 180 mph. Reaction time at this speed at an average 0.7 seconds equals another 200 feet on its own, and it still takes time on a track from deciding to brake, and pulling on the lever – it’s not instant.

Also braking hard from high speed requires a degree of control to keep the bike straight.

Modern bikes using track tyres can do better, but it means the forces on your body will exceed 1 g – i.e. worse than performing a handstand. We noticed James Ellison’s BSB Kawasaki had a carbon fibre ‘nose’ on the back of the tank to get wedged behind.  Many modern bikes have cut outs for rider’s legs so you can hang off before a corner and get your thigh wedged behind the tank to take some weight off your arms.

Stomp grips are a good idea and also help when cornering. Club racers used to stick seat foam on the back of the tank before these were invented. Modern ‘tank pads’ can be a bit slippy, although some are made in soft rubber.

Road riders can grip the tank with their knees to take some of their weight off their arms. Even braking hard from 70 mph takes a fair bit of upper body strength, and many trackday riders wear themselves out early on by braking hard at every corner, rather then working on higher corner speeds and corner exit.

Braking distances are worked out using the formula (Newton’s Law) :-

braking equation

Where:

d = distance in feet

v = speed in feet/sec

u = Coefficient of friction between tyre and road (0.7 as used in the Highway Code – which is conservative)

g = force of gravity or 32ft/sec/sec

Braking graph 5

We’ve also added 0.7 seconds thinking (reaction) time which again is an average figure – you still need time to react to braking markers even on the track, so you need to anticipate where to start braking.

Judging braking distances on the road at high speed can be difficult and very easy to underestimate, which often gets sportsbike riders into trouble. You would probably survive a 30 mph impact, probably not a 40 mph one, so the margin is very small in terms of being able to slow in time.

Road or track you need to be looking well ahead.