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Discussion in 'Multimedia' started by smee, Feb 13, 2016.
Interesting comparison, both have ABS, I'd like to see how a bike without ABS would fare.
... commenting with caution. I've ridden quite a few bikes with ABS, of various vintages, but I've never owned one.
If you step back 20 years, I'd have predicted a result very much like this one. That is, on a known surface, with everything new and working right, with an experienced driver / rider...
Real world experience is a little different. Both bikes and cars are susceptible to have their braking and handling and control badly compromised by circumstances or combinations of circumstances, and cars generally have some degree of advantage, and the more difficult and challenging and complex you make the circumstances, the bigger advantage the cars have.
I have a long and sad history of getting angry at car shows who do 'demonstrations' where a car thumps a bike, at anything, because usually they're set up that way. They're not really a fair comparison. This sort of is, and sort of isn't. This shows pretty much what I would expect, under ideal conditions. But out in the real world, when sh1t suddenly goes pear shaped, how often are the conditions ideal?
You can't really say bikes are better than cars, although they can and do sometimes stop better. A car is much more forgiving. That's somewhat less true when everything's got ABS on it, but the same trend remains.
I drove cabs for a long time, many of which had ABS on them, and generally once they were more than about a year old, it malfunctioned and never quite worked right again. Now whether that is representative of cars and bikes in general, I don't know, but it was sure as hell true of all cabs older than about a year...
I may add this as an afterthought... There's one case where bikes stop very much better than cars. You can do one hard stop in a car from about 100 km/h, and everything will work as advertised. But if you find a few (cough) 'closed off streets' and decide to start driving like you are on a race track, the brakes will overheat and start to fade very quickly. This can and does also happen to bikes, but you have to be trying a whole lot harder. Exactly how fast the brakes on a car will start to go away and how quickly that becomes a problem, varies a fair bit, but every road car I have ever tried to drive quickly for any length of time, runs out of brakes after a few minutes, perhaps after only one. I have had brake fade happen on a motorbike, but very rarely and usually only when there was something very wrong with the brakes. Motorbikes resist brake fade through overheating an AWFUL lot better than cars. That is not normally an issue, unless you are having some naughty fun... The main time this might become an issue, is let's say you are trying to get away from or out run somebody...
so they are saying bike braked with average coefficient of friction of 1.25 from 100, and 1.53 from 60?
and car braked with average coefficient of friction of 1.075 from 100, and 1.14 from 60?
methinks their tests were either shit, or deliberately biased. ABS doesn't change physics..
Auto Stopping Distance
Motorcycle Tire/Road Friction
What are you talking about?
He is saying the reported stopping distance for the bike from 60 is suspiciously good, and from 100 it's pretty much impossible. The distances reported for the car, are pretty much what you would expect.
Or did he just have an issue with the friction coefficient changing?.... There's plenty of reasons for that. Reaction times of driver and vehicles
physics.... just didn't seem to add up.. with the first site you can calculate average coefficient of friction required to brake in that distance.
typical road coefficient with typical car tyres is more like 0.8 coefficient of friction.
race tyres (think F1 and moto GP) when hot can probably maintain up to 1.5 or so .. this also means g force of stopping unless there is extra downforce (ie F1)
on that "typical" country road... with less than perfect surface, and downhill to boot in some of the vid...
even assuming 100% weight transfer to the front wheel, will a Kawasaki ER6N be able to brake at an average 1.5g's?? or even 1.25g's ?
remarkably unlikely, which is why I checked.
basically.. it's dodgy comparison.... numbers make no sense so why measure to 2 decimal places
they were either going a fair bit slower than they claimed, or started braking earlier.
think about this... a bike with wheelbase of about 1.4m has it's COG around the middle of wheelbase.. lets say 70cm.
the height of the COG, with rider, is maybe 50-70cm from the ground.
if COG was 70cm from front wheel, and 70cm from the ground, what do you think happens at >1g braking??? (ie, greater than 1.0 coefficient of friction)
it means that COG is pushed backwards AND upwards at >1g ... in other words.. the COG has a force pushing up that is greater than weight of bike, and you go OTB
make than 1.5g's average braking force... either the rider has very heavy calves to lower the COG dramatically or... the claimed stopping distance is not feasible
start on page 99 here for some calcs of OTB forces etc Motorcycle Dynamics
not the change, the actual stopping distances they showed
if you're going to make a comparison, at least use numbers that are physically possible
my cage with good tyres and good brakes can probably average around 1g on a fast stop on a good surface.. certainly much faster than a Holden Cruze with standard tyres and standard brakes.
arguably, bike tyres have better compounds and can maintain a higher rate of deceleration due to higher coefficient of friction, but physics still limits how fast a bike can decelerate due to going over the bars, whereas car is pretty much only limited by tyre/road traction.
the cops and RTA's say about 20m stopping distance on dry road, which is an average CoF of 0.7.. which is not far off for many "average" cars, but not good cars or bikes.. but 1.5? or even 1.25? a little optimistic
Agreed. I think the car did pretty much what it said, but I think the bike started braking at (or slightly before) the marker in the one test, and well before it in the other. Or maybe he was not doing the speed - how would we know? But something doesn't add up!
Also interesting, that is an odd coarse asphalt. That's likely to give increased grip to a car, but reduce the grip on a bike, which doesn't seem to correspond to the stated results at all...
btw,the full story at RideAdvice is here Can a Motorcycle Brake Faster Than a Car? -
What all this shows is that we can cancel the use of the variable m as it appears on both sides of the equation. In simple terms, it means that mass has no bearing whatsoever on stopping distances. The equation to thus show the maximum possible deceleration is
a = μg
So (negative) acceleration equals the coefficient of friction of the tires, multiplied by the gravitational force.
So, that light bike you have? When it comes to braking, it’s irrelevant. It’s down to the tires. And because a car has four of them instead of two (and they’re wider), it points towards a car being able to stop quicker. So, cars stop faster than tires. Case closed. Well, not quite…
The reason it’s not so straightforward is that an average bike has far higher performance characteristics than an average car. In general, an average sports bike will have higher quality brakes and better tires than an average hatchback or a family saloon. So your Yamaha R6 or Triumph Street Triple will probably out brake Miss Daisy in her Toyota Camry. But as soon as you get a car that’s remotely sporty, say a Golf GTI, the car will begin to stop in shorter distance than a bike. Physics will win and with those four tires, the equation of a = μg will see the car pull up quicker.
As our video shows, an average bike will outperform an average car. In the future though we’ll put the same bike up against a hot hatch or similar to show how the reverse becomes true.
cars COG is so much lower than half it's wheelbase that it is not an issue.
a bikes maximum deceleration will be approx. (half wheelbase) / (height of COG). eg if COG is 60cm and half wheelbase is 70cm, then max decel before OTB is 70/60 =~ 1.2g's regardless if tyres have higher CoF..
here is a fairer comparison from 160.. bike (S1000RR) lifts the rear end a bit.. and gets smashed by braking of the M135 (or maybe bike rider wasn't trying as hard as they could?)
some track braking tests of nice bikes.. none managed over 1G, even with ABS and slicks on the S1000RR
ABS Comparison Test | Absolutely Brilliant Stopping
car and bike 100-0 braking distances
Motorcycle braking distances • /r/motorcycles
S1000RR and ZX10R are 129ft = 39m
the RA videos ER6N..... 102.1ft = 31.13m
Yeah. This is partly why they seriously don't like to put bikes and cars out on the same track at the same time. MOST of the time, a car will stop better and later than a bike, and enter the corner faster and carry more corner speed. The bike will accelerate a whole lot better. Compared to some faster cars, that advantage will diminish quite a bit, and even sometimes (Ferraris and McLarens and some tuner cars) slope the other way, at very high speeds. But most of time, bikes are fast up the straights and get in the way through corners.
That's on a road circuit, or under fairly normal road conditions.
Go to a modern well maintained circuit, that's billiard table smooth and has special high friction hotmix, and throw in tyre warmers and slicks, on racebikes, and the corner speed thing changes. A racebike on slicks, on a good grippy surface, can pull about 1.1 ~ 1.2g. A roadbike on road tyres, is very unlikely to get much over 1. It is usually rather less than that.
But your bike is pretty much never going to pull more than 1 g stopping. A normal roadgoing car is about the same, perhaps very slightly better.
A good car on slicks, without any downforce, can pull about the same lat as a bike. But anything you'd think of as a 'racing car' has downforce, often much more than you think. As one example, an Aus V8 supercar can pull about 1.1g through a slow hairpin, but they can pull a bit over 2 through the Hayshed and Caltex Chase. How much downforce do you think they make? It's a bloody lot more than you think...
Remember......the coefficient of friction is independent of mass as it is a variable tied to the surface and tire interaction alone, but the frictional force exerted by tire depends entirely on the normal force (down force or mass). And in what we're talking about here, the frictional force is what matters as that stops the vehicle.
I would hazard a guess that the transfer of weight over the front tire of the motorcycle drastically reduces lateral forces, increasing the Normal force where the coefficient of friction is acting effectively drastically increasing the friction force. I also wonder if the softer round 'shape changing' nature of the motorcycle tire changes the relationship of static and kinetic coefficient's of friction......they tire moves more with the road it would make me think it would grip more effectively than a flat hard tire.
My basic maths goes like this....
Car = 1,500kgs
Bike = 280kgs (with rider)
To stop a vehicle, the associated energy needs to be removed. These vehicles travelling at 100kmhr present the following energy.
E(car) = mv^2 = 1500x(27.78^2) = 1157592J....roughly
E(bike) = mv^2 = 250x(27.78^2) = 192932J
Coefficient of friction is the same (assume as the coefficient is relative to the interaction between tire and road), but that doesn't change the age old rule that every action has an equal and opposite reaction.
Assume the same contact area between a car and bike tire.......CA. But as a bike brakes say 100% of the contact area is removed from the rear tire. Say half a tire's worth of contact patch is removed from a car when decelerating.
Bike = 1 x CA
Car = 3.5 x CA
Frictional force = the coefficient of friction x the normal force (the force acting perpendicular to the surface).
So disregarding wind resistance etc, the only force acting to slow the vehicle or dissipate the energy is the frictional force.
Frictional force car = Cof x N x 3.5 = Cof x Mass x Gravity = Cof x 1500 x 9.8 x 3.5 = 51540xCof
Frictional force bike = Cof x N = Cof x Mass x Gravity = Cof x 250 x 9.8 = 2450xCof
So basically the car has 21x more frictional force available to remove basically 6 times the energy.
So, I'm guessing things like chasis dynamics and how that effects the Normal force + the bike having a higher Cof with a softer 'better' tire plus extras like better brakes, increased awareness etc makes the argument more confusing giving the appearance the bike stops faster.
Also things like this to consider....
Of course it was early when I started this.....up waaay too early on a Sunday morning. I could be completely off the planet and I wouldn't realise at this point.
Amazed the 135i out drags the bike on pull off in the race at the end. That's not right!! I had a 335i chipped and it was quick as, but no way it was going to outpull an S1000RR. What the.....?
She mentioned launch control I that bike, could any riders tell me why that would slow it down so much? Supposed to perfect the launch isn't it?
135i is claimed 4.9 0-100
S1000rr is claimed 3.06, but anything sub 4sec is dependent on many factors tho?
methinks the bike let the car pull away for the first corner, to completely blow it away on the next straight (for TV).. it had a hell of a runup on the next straight too
a 10m or so difference in braking (from 100) seems reasonable
yup, coefficient of friction is a combination of (at least) three things.. pure friction, chemical bonding to road surface, and surface roughness intermesh.
the latter is partly negated by the rubber compounds chosen. a lighter bike has a muuuuuch softer rubber compared to even a good (road) car tyre, as the rubber has to be stronger to withstand the higher forces from the weight. tyre company knows there is no point having more give than is required to achieve optimal intermesh with road surface.
you're on the right track with frictional force, but (respectfully) a little offtrack as well.
- the energy is dissipated by the brakes, not the tyres.
- good car at maximum braking will have almost complete front wheel weight transfer also (not that it matters much, and not that contact area matters much either, until cohesive failure)
- the coefficient of friction of tyre will slightly increase as normal force increases (intermesh), until such point as the rubber starts shearing it's surface layer (cohesive failure)
-until the point of cohesive failure, frictional force is still related to normal force by CoF, so more normal force = more frictional force
the equations for deceleration all come down to coefficient of friction, which is determined by road surface and tyre properties alone (assuming brakes are good enough to potentially lock a tyre).
the weight transfer issue is not really an issue for a car, as any tyre with load (front or rear) can provide more braking force, and all the load of a vehicle is distributed among it's wheels, so the total normal force is always same.
weight transfer is only an issue for the bike doing stoppies
the physical diameter of the bikes brakes, plus low vehicle weight, contribute to the illusion, but physics disagrees
but we don't want people thinking their superbike can outbrake anything more than a Camry
(but of course a superbike rider can probably outbrake a Camry driver, if they are not hitting ABS)
NOT directly related or anything, but slightly entertaining...
Do note, this is a suspiciously YOUNG Jeremy Clarkson... I think he starred in Snow White or Sleeping Beauty or something the year AFTER this was made...
so.. a Lexus (rebadged Camry) brakes faster than a 911? (the first time... probably not the second or third tho)
Theoretically mass, and centre of gravity etc don't change the theoretical minimum stopping distance.
However, the more mass, the more force, the more heat..... Which will change the friction coefficient of your pads, and tyres and throw theory out the window.
I'm not so interested in the comparison. But in how they got it wrong...
Something I think may have been worth noting... Cars can brake and turn at the same time. Easily.
I am going to very respectfully and politely disagree with you there. On a car, almost any car, this is largely true, but on a bike, especially a short and tall sports bike or race bike, the limit of grip is not actually the limiting factor. You can do a stoppie, just like a wheelie under power, and the permanent argument amongst the factories and teams who develop racing bikes, is do we make it longer and lower, to improve braking and acceleration, or do we make it shorter and taller to improve cornering?
Yamaha have a bit of a history of going short and tall, especially since Rossi and Burgess arrived and showed them their first 4 stroke effort was way too long and low. Honda have a long low history of making long low bikes. The NSR Gardner and Doohan raced was long and low, but they had a fella there who'd just developed the 900 Fireblade, and they gave the first 5 cylinder RCV to him, and he made it quite short and tall. Since then, the damn things have been getting longer and lower again. M&M has become pretty good at staying on them, but many other Honda punters have Bee-atched that the front end grip is no good, and they wash out and fall down. But they're good under brakes and power...
The main problem they're having with it, right now, is their latest engine is a bit of an angry beast. The last one also was, and they were supposed to fix that. They sort of did, but it depended on doing clever digital demon stuff with the electronics. But this year, they don't make the electronics. It gets given to them. They can program it, a bit, but they have to deal with the Webber Marelli Italian striped barber's Pole and ... dispensations from the Pope and Mr Blatta at the Soccer club... The people who are in the box seat with the Italian Electrics, are Ducati.