Don't know.

 

I'm probably about to show my ignorance here (again) but if all the factors that you've said remain constants, surely they'd all stop in the same distance.

All of the cars above would have the capacity to lock the wheels under severe braking and assuming they all weigh the same, have the same ABS system and the driver reacts in eaxactly the same time, then they'd all pull up in the same time.

I believe that the argument for bigger and better brakes is one of consistency rather than a one-off perfromance.

 

The depth of tyre tread will also affect the braking distance A tread with only 3mm tread requires on average 30% further to stop than a tyre with 8mm tread.

 

Vic - good point - same tyres all round also then

Not many answers yet....
(Cheers Kell for your stab )

 

I'm probably about to show my ignorance here (again) but if all the factors that you've said remain constants, surely they'd all stop in the same distance.

All of the cars above would have the capacity to lock the wheels under severe braking and assuming they all weigh the same, have the same ABS system and the driver reacts in eaxactly the same time, then they'd all pull up in the same time.

I believe that the argument for bigger and better brakes is one of consistency rather than a one-off perfromance.

Not so, tyre size and pressures will make a difference as it determines the active area (nothing to do with coef of friction) - assumine tyre temp is a constant.

For the record a Porsche GT3 will stop from 100mph in 2/3 the official stopping distance for 70mph (proven on TopGear I believe)... which just shows how out of date the highway code is in some areas (then again, the stoopid factor needs to be included, so maybe they are right to keep them at the same distance for the last 50 years or so)

 

However I could be completely wrong... depends whether we are talking theory or practice...

see http://hyperphysics.phy-astr.gsu.edu/hbase/crstp.html for an interesting discussion...

or http://www.sdt.com.au/STOPPINGDISTANCE.htm

 

Would it be the car that can produce the most friction at the Disc/Drum and at the road surface, whilst disipating the heat the most efectivley in each of these areas. Also the car that handles the best under these conditions as anything that may step out at the back or front will totally throw the equation.

 

This is an amusing debate for first thing on a Monday morning.

I'm just about to head off on those nasty dangerous public roads so I'll leave you with a couple of things to think about.

A car with both front tyres fully deflated will stop in pretty much the same distance as a car with both tyres correctly inflated. The tyres make no real life difference.

If the car with the latest mega bucks super dooper all singing brakes could stop in a shorter distance the owner had better start stocking up with rear bumpers.

I'll catch up with this thread early tomorrow morning to see how it develops.

 

Irving - good links

The first implies that mass is not a factor (can't see how that is, but I've ruled it out of the test anyway).
The second includes a table of results....

Len - You are thinking the way I am

 

WHY?

Does more tread mean better stopping? Surely less tread means more rubber ( bigger footprint) on the road surface
:?

 

Oi John - start yer own thread! :lol:

The more rubber there is in contact with the road, the more force can be applied into stopping the car

Anyho - all my cars in the test have identical tyres, all at the same pressure and tread levels and temp, all carrying the same weight

I'm trying to focus purely on the braking system, ok?

 

DIRY OK so if the only difference is the braking system then front to rear bias will have some effect,and also how good the guy in the fiesta is at cadence braking.
By the way if a set of puny std discs can( in this test )get the abs to cut in then no advantage to big brakes BUT
If the test was from 120mph you would not have enough power in the brakes to get abs to cut in .
Also even at lower speeds with track tyres the same may happen.
And finally the all important overheating the brakes and boiling the fluid with multiple application, big brakes greatly reduce any chance of this.

 

Roland - er. thanks. I think!

Perhaps I need to refine the test even further. The Fiesta was only chucked for the 'extreem' factor really... (the fact that it was on drum brakes, not discs).

We are talking about a THEORETICAL situation here.

So. Let's level the field even further.

1 TT. Ability to swap to various different sets of brakes setups.
Test of braking from (say) 80 mph to zero.
Distance measured from when the brake pedal is hit (ie without reaction time) to when it comes to a halt.
All test done on the same stretch of road, same conditions, same tyres etc.

1) std 3.2 brake system (ie RS4 discs)
2) std 1.8 brake system
3) 'upgraded' system using the 'typical' bigger discs, green/red stuff pads
4) 'downgraded' system, using drum brakes from a '76 Fiesta (we will make them fit, ok?) Leaves TT ABS system in place (go with me on this!)

ONE stop per system - so ignoring all the good bits about reduced fade over the course of several stops - we'll cover that seperately

 

Wayne, can I ask why you need this info?

If the fiesta worked out better , would you install drum brakes all round ?

:evil: Devils advocado :evil:

 

DIRY Assuming all cars are capable of getting the abs to cut in all the way from the 70mph to zero, then all the same stopping distance.
This proves that in your experiment the brakes are up to the job.
In practice you can stop quicker if you apply slightly less pedal pressure so the abs doesnt quite cut in. One reason why rally drivers throw it in the bin.

 

Wayne, can I ask why you need this info?

If the fiesta worked out better , would you install drum brakes all round ?

:evil: Devils advocado :evil:

John.

According to what I thought I knew, then the 'upgraded' system would stop in a shorter distance - although that goes against what Roland has just posted above :?

I chucked the drum brake system in as an extreeme

my understanding was that the frictional co-ef of the pads to disk/drum and the heat dissapation properties had a large part to play in how quickly you stopped.

Going for larger discs, had (again, my understanding) 2 main advantages:
1) allowed the heat to be spread over a larger area (which, as it was larger, cooled better in the air stream aswell), and
2) allowed the use of a larger brake pad (or pads), increasing the amount of material that was in contact with the disc - thereby increasing the total co-ef.

To take things to an extreme in a different direction:

Take your TT (or more to the point - don't!).
Replace existing pads with 2 blocks of soap (frictional co-ef approaching zero).
Try to stop......

So now I'm more confused than ever.... :?

 

Busy morning then matey ? :roll:

 

DIRY Its simple your soap is not capable of getting the abs to cut in all the way from 70 to zero. If your fiesta drums are capable are capable of abs cut in from 70 to zero they will be just as good.
Your test is only one stomp remember.

 

Roland - very true.... The soap won't cut the ABS in! :lol:

But what about the rest of it - bigger discs / bigger pad area, giving larger frictional co-ef, giving shorter stopping distance?

Should we take ABS out of the equation too?
I realise (und understand) that once skidding (and assuming that driver does not cadence brake), it's all down to the co-ef of tyre vs road.

BUT - in the above 5 scenarios, surely each set up will result in a skid starting at different points (and probably speeds)?

Confusing myself even more now!

 

Interesting quote from the second of those links.... (sorry DIRY for going OT a little)

"This is why the tailgating car usually cannot stop, when the brake light came on in the car in front, this driver had already completed the perception, human and vehicle reaction periods. The following driver was perhaps 1 second to late in applying the brakes. At 100km/hr the car required 28 metres further to stop."

Which is why you have to look 2 or 3 cars ahead and why high-level brake lights have reduced accidents on motorways. Its also why I think rear tinted windows should be banned as well as the front tints... Its funny how otherwise intelligent people are blind to this fact... my Dad always used to argue that separation distance didnt need to be as big as stopping distance "because the guy in front isnt going to stop dead is he...." and no amount of argument about reaction times would change his mind.... just count myself lucky he never rear-ended someone when I was a kid...

 

Lots of people assume that bigger brakes mean better stopping.

But this is only true if the existing brakes are unable to get the wheels to lock. So in the example, the drum brakes might not be able to lock the wheels up - they have less friction than that between the road and the tyres - so they would never casue the ABS to cut in.

However, simply making the brakes bigger means that you make the friction of the brakes/pads greater than that of the road/tyres - then the car skids. Asssuming everything eles is constant, then any car capable of lcoking the wheels, will stop in the same time/distance as any other car capable of locking the wheels. The ABS would work the same on all of them, the tyres are the same etc etc. Seems to make sense to me, though I may be miles wide of the mark.

I have to admit though, I'm still confused as to why weight doesn't affect the stoppping distance surely more mass would be more difficult to bring to a halt?

 

Kell - I'm with you on that (mass)....

Ok....
refining things further in the pursuit of an answer...

Narrow it down to 2 systems.

One system has a friction co-of between pad and disc that is massively larger than that between tyre and road.

Other system has a friction co-ef which is just slightly higher than that of the tyre / road.

Would the ABS cut in at the same place?

 

Mass has everything to do with it , a caterham will pull up much quicker than a heavy salloon.

 

That's what I figured. Though I'm a track virgin, it also expalins why they can brake later going into corners.

 

:idea: Heres a thought....... :idea:

If so far.....every thing is equal, then everyone would stop at the same piont, I still think mass/inertia is going to creep in, and also....BHP/TOURQE as the engine is still pushing the car so now to equalise every thing again our man needs to step on the brakes and throw it into neutral. :?: :?