Daftest stuff said on PH which isn't really true

[Super Sonic]

mikey_b said:

You push the inside handlebar slightly - so if it's a left turn, you nudge gently forwards on the left bar grip. This of course twists the handlebars, and with it the front wheel, slightly to the right, but the bike then drops down leftwards and turns that way. Hence the term 'counter' steering.

It works because the only thing in contact with the road is the very bottom of the tyres. If you turn the bars slightly to the right, then the grip on the road causes the bike to initially steer that way, but because of inertia from the mass of the bike and rider, the rest of the bike tries to stay where it is - so effectively you pull the bottom of the bike out to the right, and gravity means the bike starts leaning to the left. Once leaning left, the geometry of the steering system means the bars are then pulled to the left and the front tyre leads the bike around the corner turned the way you'd expect. So, a quick and gentle nudge on the 'wrong' side of the bars causes it to drop down and turn towards that side. It's an extremely effective way of initiating a turn.

If you look down at a motorcycles steering from the top, you'll notice that the two fork tubes are set forwards of the headstock, which is where they attach to the front of the frame. This means that if you follow the line of the forks downwards, the point at which that line hits the ground is slightly ahead of the lowest point of the tyre. This distance, called the trail, is what forces the steering to turn left when leaning the bike to the left. The more trail the greater the effect, and is one of the most fundamental parts of the geometry of a bike frame and suspension. You see the same on a bicycle, without front suspension there is a forwards curve in the front forks which serves the same purpose, and if there is front suspension you'll see the top forms a sort of shallow triangle.

You're spot on about the countersteering.
Trail, however, is measured from the steering axis ( a line through the centre of the headstock or head tube) to the front tyre contact patch. The curve on bicycle forks is to provide a flex. Not all rigid bicycle forks have this, Kona P2s for example. Bending the fork forwards actually shortens the trail.

[GroundEffect]

That vehicle mass affects braking distances.

[julian64]

GroundEffect said:

That vehicle mass affects braking distances.

You will have to explain that as I'm totally thrown if that's not the case. It may not be the only factor, but surely its a factor

[Randy Winkman]

julian64 said:

GroundEffect said:

That vehicle mass affects braking distances.

You will have to explain that as I'm totally thrown if that's not the case. It may not be the only factor, but surely its a factor

Am glad you asked.

[deadtom]

Randy Winkman said:

julian64 said:

GroundEffect said:

That vehicle mass affects braking distances.

You will have to explain that as I'm totally thrown if that's not the case. It may not be the only factor, but surely its a factor

Am glad you asked.

me three, I await the explanation for this one with interest

[RizzoTheRat]

While Isaac Newton would disagree with the original statement, there are some mitigating factors. More mass means more grip (F= μR) so if the brakes are able to cope a heavier vehicle can generate a greater braking force before the tyres lose grip

I would say the statement ought to be "Vehicle mass doesn't effect braking distance as much as you'd expect", but not sure how big the tradeoff is.


ETA: - No I'm wrong, it's not just a tradeoff...

Braking force = Mass x Deceleration (F=ma)

"Grip" = coefficient of friction x reaction force (F=μR)

reaction force is the weight of the vehicle pressing down on the tyres, ie R=mg

If the limit of braking is grip, then ma= μmg so mass cancels out.


This assumes you can brake on the absolute limit of tyre grip though,

Edited by RizzoTheRat on Wednesday 6th December 09:43

[deadtom]

RizzoTheRat said:

- engineering stuff -

This assumes you can brake on the absolute limit of tyre grip though,

Edited by RizzoTheRat on Wednesday 6th December 09:43

'This assumes you can brake on the absolute limit of tyre grip though,' is the critical part here.

re-do the calculation to see how vehicle mass affects the friction force between the brake pads and the brake disc

[RizzoTheRat]

That's part of it, but I'd be surprised if many modern cars can't lock up the tyres. The problem is that as soon as you lock them you have less grip, and then on most cars the traction control will start cutting in which will do a decent job but will still be below maximum braking force.

On a motorbike the maximum braking force possible is with your back wheel just off the ground so that's even harder to get right

[Biker's Nemesis]

Zarco said:

Counter steering doesn't exist (when riding a motorcycle).

Do you want the full hour you cheese eating surrender monkey.

[deadtom]

RizzoTheRat said:

That's part of it, but I'd be surprised if many modern cars can't lock up the tyres. The problem is that as soon as you lock them you have less grip, and then on most cars the traction control will start cutting in which will do a decent job but will still be below maximum braking force.

On a motorbike the maximum braking force possible is with your back wheel just off the ground so that's even harder to get right

likewise mountain bikes, something which I have got wrong a few times and made a rapid exit out the front door, as they say.

But while I suspect you're right that most modern cars would be able to lock the brakes even when loaded to their max, I think that accusing people who think that 'vehicle mass affects brake distance' of being daft, is itself, daft.

[GroundEffect]

deadtom said:

Randy Winkman said:

julian64 said:

GroundEffect said:

That vehicle mass affects braking distances.

You will have to explain that as I'm totally thrown if that's not the case. It may not be the only factor, but surely its a factor

Am glad you asked.

me three, I await the explanation for this one with interest

I guess it feels intricically "right" that a higher mass should take more effort to stop through Newton's second law (it does but it's only one side of the equation). That a heavier vehicle needs more effort to slow it, therefore must take longer to stop. But the key question is what limits our braking effort? I think most of us with cars without ABS know that all too well - grip (or, static friction with the road surface).

Looking at static friction force of an object then:



You'll see it is the normal force, not just mass that's affects how much grip you get. Assuming friction coefficient stays the same (same tyre compound, same tyre pressures) then we can increase braking effort potential by increasing the load on the tyre.*

I think everyone has at least heard of Weight Transfer, but this is the function of the force seen on a given axle change as acceleration is seen on the body. Under braking, more normal force is seen on the front tyres. Would that give us more braking capacity on the front axle? Yep! Why do you think front brakes are bigger than rears? They need to be able handle the higher duty cycle of the increased capability on the front axle.

So if we look at weight transfer:



Where a is the longitudinal acceleration of the vehicle, h is height above ground of Centre of Mass, b is wheelbase and m is vehicle mass.There's that mass again. So as mass increases, so does weight transfer to the front axle. Which means going back to the earlier equation, that as mass of the vehicle increases the available grip capacity increases. Which means we can push the brakes harder before lockup.

So if you related friction force and weight transfer, you would find that as you increase the mass of the vehicle and keep other things constant (like centre of gravity, wheelbase, tyre type). Vehicle mass is NOT a factor.

You can look at some examples:

From 60mph:

718 Cayman GT4: 30.5m (Motortrend)
Tesla Model S Plaid: 31.7m (Motortrend)
Tesla Model 3 Performance: 30.2m (Motortrend)

There is some difference in the result because of run to run differences, and not helped by the very low CoG of the Tesla, but they're basically the same despite 700kg more.

Or to reduce the mass a bit:

Alpine A110R: 33.1m (Sport Auto)
Lotus Elise 111R: 35.1m (Road & Track back in the day - older tyres)

Braking performance is more determined by the amount of weight transfer you can get. When I was designing Formula Student cars way back in the day, for the acceleration test it was all about maximising weight transfer, but rearward this time. Get the CoG height quite high and get the tyres as grippy as possible.

• These lead to another rabbit hole that larger brakes don't give you more braking power. They are there because they can handle more heat, therefore their performance is more stable over use. It's also semi-related to tyres too - wider tyres don't mean more grip; there is a relationship here unlike brakes but again it's not so obvious.

[TheJimi]

Would be tempted to suggest that your first post is a tad misleading, or maybe click-baity

I think Rizzo's wording is more accurate.

[deadtom]

GroundEffect said:

engineering stuff

detail, engineering rigour; 8/10

timeliness: 0/10



I suppose it entirely depends on what you consider the original statement to be saying (eg light performance car vs heavy performance car? unladen tuktuk vs fully laden 44 tonne artic? 30 year old estate car empty vs 30 year old estate car packed to the gunwales?) but as you point out in the first line of your reply it is entirely reasonable to think that heavier cars would take longer to stop, and people who would ridicule that are this guy:

Edited by deadtom on Wednesday 6th December 10:38

[GroundEffect]

Rizzo said mass cancels out. I said mass cancels out.

[RizzoTheRat]

It genuinely surprised me, quite counter intuitive.

[Gecko1978]

TwigtheWonderkid said:

Berger 3rd said:

Yes, you can insure anything for the right price,

You actually can't. There's a vast range of stuff that's just not insurable, at any cost.

You can't insure agaist a certainty hence life assurance cos we all die one day but genuinely interested are there "things" you can't insure but which I mean solid real objects cars etc.

Are some uninsulated beyond you being 19 living in towerhamlets and wanting to park your lambo on the street

[TheJimi]

GroundEffect said:

Rizzo said mass cancels out. I said mass cancels out.

I meant his alternative wording for your original post -

RizzoTheRat said:

I would say the statement ought to be "Vehicle mass doesn't effect braking distance as much as you'd expect"

[GroundEffect]

Maybe, but consider that a 2.2 tonne Tesla can brake from 60mph to 0 faster than a Lotus Elise might be considered quite alarming? So "less than you expect" isn't really telling the story.

And it gets even worse. A Caterham 620R, the ultimate Caterham, can brake from 60mph to 0 in 33.6m. Slower than the Tesla too, and the Tesla is 4x the weight!

Mass is the least important variable. More important is height of CoG and tyre grip.

[RizzoTheRat]

The CoG thing is counter intuitive too, despite me being quite aware of the effect it has on a motorbike. You'd think the transfer of weight to the front increasing the front grip would be offset but the rear grip being reduced. Is it that the rear brakes are generally less powerful so not braking at maximum force, or do you gain more grip from the front than you lose from the rear?

[Mr Pointy]

GroundEffect said:

Maybe, but consider that a 2.2 tonne Tesla can brake from 60mph to 0 faster than a Lotus Elise might be considered quite alarming? So "less than you expect" isn't really telling the story.

And it gets even worse. A Caterham 620R, the ultimate Caterham, can brake from 60mph to 0 in 33.6m. Slower than the Tesla too, and the Tesla is 4x the weight!

Mass is the least important variable. More important is height of CoG and tyre grip.

Are you saying that if you have one person in a car, say an Octavia, with no load & you brake from 70 to a stop & then you repeat it with four Samoan rubgy players on board & luggage up to the maximum vehicle weight limit that it will stop in the same distance?