The theory of braking
Discussion
I was recently suprised by some statements on www.ridedrive.co.uk/tippoffs03b.ht which covers brakes and
upgrades to them. From a theorectical point of view I belive he(Julian Smith)is saying that the weight and tyre width of a vehicle does NOT make any differance to its stopping distance! He also says that upgrading your disks/pads has little effect, except that you don't need to shove the pedal so hard!.
surely the theory is that MASS X ([Velocity]x2)=energy so a light car using up to limiting fricton will stop in a shorter distance than a heavy car with the same brakes,or is my physics flawed.
Can any expert confirm my thoughts
P.S. by limiting friction I mean that the tyre is not skidding.
John
upgrades to them. From a theorectical point of view I belive he(Julian Smith)is saying that the weight and tyre width of a vehicle does NOT make any differance to its stopping distance! He also says that upgrading your disks/pads has little effect, except that you don't need to shove the pedal so hard!.
surely the theory is that MASS X ([Velocity]x2)=energy so a light car using up to limiting fricton will stop in a shorter distance than a heavy car with the same brakes,or is my physics flawed.
Can any expert confirm my thoughts
P.S. by limiting friction I mean that the tyre is not skidding.
John
johnem said:
is my physics flawed.
Yes your physics is flawed. If your brakes are perfect then the limiting factor is the coefficient of friction between the tyre and the road. Although I don't fully agree with Julian's view on brake upgrades, because brake systems are not actually perfect in practice.
Unless you've got a Wedge you can probably lock the wheels at any speed. So why would you need better brakes?
Feel. How much feed back do the brakes give you when you are approaching the transition between stick and slip and how well can you control the braking effort here.
Fade. Better cooling or resistance to heat, required because vehicle is being used outside the original design specification e.g. track days. or stopping from in excess of 40mph twice in a wedge!
Flashy. You might be that chav who's bought the elcheapo super heavy extra large wheels and want something big in the middle to light up with the blue LEDs!
Other things to consider (but that don't begin with F)are vehicle weight transfer under braking, and brake balance.
Also if the design was flawed or technology has improved since the original design date, improvements could be made!
It seems foolish to think that the OEM would deliberately design poorer brakes than are needed. On the other hand they are designing to a specific cost and market and compromises have to be made. You may be prepared to spend a bit more and put up with extra noise in order to get other advantages! Something the OEM wasn't allowed to do!
There is always the law of diminishing returns and spending twice as much, as the original system, on an upgrage wont get you twice the stopping performance no matter how you quantify it!
Feel. How much feed back do the brakes give you when you are approaching the transition between stick and slip and how well can you control the braking effort here.
Fade. Better cooling or resistance to heat, required because vehicle is being used outside the original design specification e.g. track days. or stopping from in excess of 40mph twice in a wedge!
Flashy. You might be that chav who's bought the elcheapo super heavy extra large wheels and want something big in the middle to light up with the blue LEDs!
Other things to consider (but that don't begin with F)are vehicle weight transfer under braking, and brake balance.
Also if the design was flawed or technology has improved since the original design date, improvements could be made!
It seems foolish to think that the OEM would deliberately design poorer brakes than are needed. On the other hand they are designing to a specific cost and market and compromises have to be made. You may be prepared to spend a bit more and put up with extra noise in order to get other advantages! Something the OEM wasn't allowed to do!
There is always the law of diminishing returns and spending twice as much, as the original system, on an upgrage wont get you twice the stopping performance no matter how you quantify it!
I had a quick look through the articles concerned. He seems to have absolutely unshakeable faith in his assertion that everything will stop in the same distance on locked wheels! I think this is the mentality that gave us the stopping distances in the highway code! There is, of course, more than a grain of truth in what he says - but as with many things, life ain't that simple in reality. Tyres aren't linear devices. His argument that the grip goes up as the pressure on the contact patch goes up is true - but only up to a point. Stick a 40 tonne artic on space-savers and try stopping it quickly! Similarly, it's pretty much impossible to lock all the wheels simultaneously so the rear wheels tend to still be rotating when the fronts have locked. These can obviously do some useful work still. Also, I'm perplexed at the notion that skidding on dry tarmac melts the bitumen. I can't quite see how this happens because the locked wheel is always moving on to new bitumen as the car slides - whereas the contact patch is stationary. I might be wrong here but it was always my understanding that the tyre slid on a cushion of molten rubber when it locked - not tarmac! That's why you can "flat-spot" tyres by locking the wheels but you rarely see grooves in the tarmac under the skid marks!
I'd also like to see him stop a Discovery in the same distance as a 911! The Porker stops well because all 4 wheels are doing a more even share of the work. The Porsche has a MUCH lower centre of gravity so the weight transfer when you stand on the brakes is much less than in the Land Rover. That being the case, the Porsche can exert a much bigger retardation with its rear tyres than the Landie can. In fact, the Landrover's rear brakes won't be doing very much stopping at all at 0.8G! The Porsche is also helped by the fact that it has a lot of its weight behind the rear wheels anyway. That being the case, when the weight transfers to the front during braking, there is still a fair bit of it left on the rear axle - which means that the rear tyres can grip better.
In my view, tyre technology is one of the biggest areas of improvement in the whole of car design. Again, difficult to quantify but having driven a 1936 Alvis with drum brakes that were powerful enough to lock the wheels and having driven a modern family saloon whose brakes were also powerful enough to lock the wheels, I can honestly say that there is absolutely no comparison!
I do, however, agree that all these things needs to be kept in proportion. Yes, of COURSE you can go daft in tyre size selection for the weight of any given car but, like brakes, manufacturers have costs to consider and they fit the best tyres and brakes that they can afford to fit within the cost of the car (and the legal requirements). My wife's 1.8 Alfa 156 has solid front discs but the 2 litre has vented ones. I can just about start them fading on a long downhill descent near my house if I abuse them. OK, I accept that one can alter one's driving style to work within the limitations of a particular car but there's no denying that my mate's 2 litre doesn't do this under the same circumstances.
In my view, the fitting of much wider tyres than standard should be discouraged for loads of reasons but I do think that correctly selected and tested, better tyres and brakes can reduce stopping distances (whether or not the wheels are locked) but obviously rather more if the wheels are NOT locked (which was the reason for wanting to fit the stuff in the first place, presumably)!
I'd also like to see him stop a Discovery in the same distance as a 911! The Porker stops well because all 4 wheels are doing a more even share of the work. The Porsche has a MUCH lower centre of gravity so the weight transfer when you stand on the brakes is much less than in the Land Rover. That being the case, the Porsche can exert a much bigger retardation with its rear tyres than the Landie can. In fact, the Landrover's rear brakes won't be doing very much stopping at all at 0.8G! The Porsche is also helped by the fact that it has a lot of its weight behind the rear wheels anyway. That being the case, when the weight transfers to the front during braking, there is still a fair bit of it left on the rear axle - which means that the rear tyres can grip better.
In my view, tyre technology is one of the biggest areas of improvement in the whole of car design. Again, difficult to quantify but having driven a 1936 Alvis with drum brakes that were powerful enough to lock the wheels and having driven a modern family saloon whose brakes were also powerful enough to lock the wheels, I can honestly say that there is absolutely no comparison!
I do, however, agree that all these things needs to be kept in proportion. Yes, of COURSE you can go daft in tyre size selection for the weight of any given car but, like brakes, manufacturers have costs to consider and they fit the best tyres and brakes that they can afford to fit within the cost of the car (and the legal requirements). My wife's 1.8 Alfa 156 has solid front discs but the 2 litre has vented ones. I can just about start them fading on a long downhill descent near my house if I abuse them. OK, I accept that one can alter one's driving style to work within the limitations of a particular car but there's no denying that my mate's 2 litre doesn't do this under the same circumstances.
In my view, the fitting of much wider tyres than standard should be discouraged for loads of reasons but I do think that correctly selected and tested, better tyres and brakes can reduce stopping distances (whether or not the wheels are locked) but obviously rather more if the wheels are NOT locked (which was the reason for wanting to fit the stuff in the first place, presumably)!
I'm pretty sure if you do manage to lock all the wheels he is right, and your Disco v 911 argument only holds true if you haven't locked all the wheels. In that situation the rubber behaves in a pretty linear fashion and the size of tyre, mass etc all cancel out, you might get minor variations in friction constant between tyres but nothing hugely significant.
I guess he's just trying to discourage the large number of badly thought out brake upgrades you seem to see. For example - only the front brakes do much work, so I'll fit bigger discs and calipers to those and leave the rears standard. Well done.. you've just increased your stopping distances in one easy step.
Or even worse, one that I saw recently where someone had swapped rear drums for rear discs from a higher spec car because the discs looked nicer. Of course the higher spec car had bigger front brakes too, but they didn't take those. So now they've moved the brake balance backwards which doesn't strike me as a clever move. There's no telling some people though...
I guess he's just trying to discourage the large number of badly thought out brake upgrades you seem to see. For example - only the front brakes do much work, so I'll fit bigger discs and calipers to those and leave the rears standard. Well done.. you've just increased your stopping distances in one easy step.
Or even worse, one that I saw recently where someone had swapped rear drums for rear discs from a higher spec car because the discs looked nicer. Of course the higher spec car had bigger front brakes too, but they didn't take those. So now they've moved the brake balance backwards which doesn't strike me as a clever move. There's no telling some people though...
OK, I'm not in favour of people messing about with brakes - there's a lot more to them than meets the eye, I agree. This is especially true of more modern cars where they don't just have ABS but Electronic Stability Control too. It's far too easy to make one thing a bit better but upset a load of others!
I'm not sure why you think the 911 / Landrover argument doesn't work though? Even if all 4 wheels are locked on both cars (highly unlikely) there is still a LOT less weight transfer off the rear tyres and on to the fronts with the 911 so what braking force there is will be much more evenly shared between all 4 tyres.
Surely, the "bottom line" is that if nothing makes any difference and everything takes the same distance to stop, why aren't we all still driving about on skinny crossplies and drum brakes?
I'm not sure why you think the 911 / Landrover argument doesn't work though? Even if all 4 wheels are locked on both cars (highly unlikely) there is still a LOT less weight transfer off the rear tyres and on to the fronts with the 911 so what braking force there is will be much more evenly shared between all 4 tyres.
Surely, the "bottom line" is that if nothing makes any difference and everything takes the same distance to stop, why aren't we all still driving about on skinny crossplies and drum brakes?
Avocet said:
Surely, the "bottom line" is that if nothing makes any difference and everything takes the same distance to stop, why aren't we all still driving about on skinny crossplies and drum brakes?
It’s the friction between tyre and road that makes the difference and better (modern tech) tyres produce far more grip/friction. Brakes have improved for all the reasons given and of course to better cope with the extra grip of the new tyres.
One aspect of OEM brakes that can sometimes be improved (from what I read – I’m not in the trade) is the balance or bias. To keep things safe for the wide range of driving abilities, OEM systems can sometimes be overly front biased. Adding a bit more rear (in those cases) can reduce stopping distances. That’s why d-man is spot on, I think, when he says adding big front discs can easily increase stopping distances.
Thanks to you all, I was trying to get my head round the Physics and think I have now!
As Julian said the weight (Mass) of differant cars cancells out as its in both formulae,Kinetic energy and Friction. so the difference comes from the effectiveness of the braking system. I would hope that a Porsche, capable of high speeds, would have a better braking system than that of a Landie, also better tyres.
One reason the theory did not seem correct is that when the car is fully ladened the brake pedal needs a harder shove.
Thanks again John
As Julian said the weight (Mass) of differant cars cancells out as its in both formulae,Kinetic energy and Friction. so the difference comes from the effectiveness of the braking system. I would hope that a Porsche, capable of high speeds, would have a better braking system than that of a Landie, also better tyres.
One reason the theory did not seem correct is that when the car is fully ladened the brake pedal needs a harder shove.
Thanks again John
The bottom line is that whilst in theoreticaly the amount of rubber in contact with the road makes no difference (as are goes down, pressure goes up so frictional force should remain the same), in practice tyres do not behave in this manner. The frictional force between road and tyre is quite non-linear, and reduces as a pressure increases, so more rubber on the ground=less pressure=more grip.
The old argument about if you have enough braking force to lock the wheels then improving the brakes is pointless is also complete falacy. With the wheels locked, the brakes are doing no work at all, all the vehicles energy is being dissipated by converting solid rubber to a nice black line on the road.
The old argument about if you have enough braking force to lock the wheels then improving the brakes is pointless is also complete falacy. With the wheels locked, the brakes are doing no work at all, all the vehicles energy is being dissipated by converting solid rubber to a nice black line on the road.
Absolutely! It's all about turning the kinetic energy into heat AND then getting rid of the heat as fast as possible. Cars with drum brakes can lock the wheels so why bother fitting discs? In fairness to the original article though, he was actually talking about a locked wheel and I do accept that there is probably less difference once the tyre is busy melting - but that's a situation that just can't happen in reality these days. For a start, most cars have ABS and even those that don't hardly ever (if properly maintained) lock their rear wheels completely even when the front ones are locked so the rear wheels are always doing some useful retarding - which is where the non-linearity of the tyre (and its compound) comes in.
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