Daftest stuff said on PH which isn't really true
Discussion
Super Sonic said:
If mass doesn't affect braking distance, why do lorries take longer to stop?
Exactly.The calc is;
F X d= 1/2 x mxv2
F=1/2 x mxv2 over d
F= 1/2 x 36000 (kilos)x22 meters per second squared /100
Braking force for 36 tonne truck is 87,000 newtons
The brakes and tyres cannot produce that braking force within a shorter distance
Trevor555 said:
I used a similar statement when I was buying used cars off the public.
I'd bid them a price to buy.
They'd often come back to me saying, so in so said it was worth this much, garage down the road says this much, bloke in the pub said is was this much.
My simple answer was "did none of them get their cheque book out then?"
Or "your car is only worth what someone will actually pay for it today"
When I was working in a corporate function for a big automotive supplier based very much in the Westmidlands in the 1990s, we were funding a series of PHD students at various universities, in the subject of design for manufacture, assembly reliability, maintainability, recycling, whole life cost etc.I'd bid them a price to buy.
They'd often come back to me saying, so in so said it was worth this much, garage down the road says this much, bloke in the pub said is was this much.
My simple answer was "did none of them get their cheque book out then?"
Or "your car is only worth what someone will actually pay for it today"
In one steering meeting we spent 2 hours defining "Market Price"
I can sum up what the grey beards decided.
In a competing market, where the customer has a choice, and the supplier of the goods has a choice, and there are no monopolies on supply, market price is the figure that both parties agree to trade at.
Note that 5 minutes later that figure could be different.
The market price of a watch is £20 by the way.
Most tyres seem to have a limit of around 2000 newtons of brake torque before you lock up, this is where Whizzo’s argument meets its limit, you can see from altering the numbers in the equation above for the vehicle mass that the braking distance will increase with mass.
The tyre cannot accept more brake force regardless of the added friction once it reaches its limit. GT3’s, GTRs etc all tend to stop from 70 mph in around 128 feet (shortest ever recorded GT3rs on sticky tyres with ceramics by road and track) to about 138feet less well braked supercars. They are already in the 2000 to 3000 newtons maximums of the tyre, if you add more mass you won’t get any more brake torque.
The figures above are not made up. The formulas are 100% accepted and supported by newtons laws, you can play with the numbers and get the same outcomes. (Change feet to meters)
The tyre cannot accept more brake force regardless of the added friction once it reaches its limit. GT3’s, GTRs etc all tend to stop from 70 mph in around 128 feet (shortest ever recorded GT3rs on sticky tyres with ceramics by road and track) to about 138feet less well braked supercars. They are already in the 2000 to 3000 newtons maximums of the tyre, if you add more mass you won’t get any more brake torque.
The figures above are not made up. The formulas are 100% accepted and supported by newtons laws, you can play with the numbers and get the same outcomes. (Change feet to meters)
Edited by Caddyshack on Friday 8th December 12:54
julian64 said:
This thread is about stuff that gets said regularly on PH as if the poster thinks its an undeniable truth. When in reality when you think about it there is little sense in it
I nominate "something is worth what someone is prepared to pay for it."
Its been said on numerous watch threads when people comment on the price an expensive watch. Its been used a few times on the Ferrari GTO thread that's currently running, and almost always appears on any thread to do with the sale of paintings, and multiple times in past threads. People use the phase as if its an unassailable fact.
But is it? When I think about it the phase seems to be one of the daftest things its possible to say.
Personally I've just bought a bicycle brand new for 2.5K. Its probably worth half that when its delivered to my front door and worth even less the minute I ride it. Nothing has actually changed on the bike throughout that time, its still the same bike it was in the showroom. The truth is that was only worth that price to me at the time I bought it and I wouldn't expect anyone else to buy it off me for the same price even if it was still in its wrapping.
Take any auction. If I was completely bonkers and went into an auction I saw some beaten up mini rusted through being pushed onto the stage with a guide price of £200 and I immediately bid £10K its doesn't make the car worth that because I paid that money for it.
There are probably many other examples where probably only two or three people i the world would regarding something as having a high value when the rest of the world would call them crazy. And the thing I hate most about the saying is it assumes if a few people are being nutters we all have to regard what they are doing as valid and sensible.
I nominate "something is worth what someone is prepared to pay for it."
Its been said on numerous watch threads when people comment on the price an expensive watch. Its been used a few times on the Ferrari GTO thread that's currently running, and almost always appears on any thread to do with the sale of paintings, and multiple times in past threads. People use the phase as if its an unassailable fact.
But is it? When I think about it the phase seems to be one of the daftest things its possible to say.
Personally I've just bought a bicycle brand new for 2.5K. Its probably worth half that when its delivered to my front door and worth even less the minute I ride it. Nothing has actually changed on the bike throughout that time, its still the same bike it was in the showroom. The truth is that was only worth that price to me at the time I bought it and I wouldn't expect anyone else to buy it off me for the same price even if it was still in its wrapping.
Take any auction. If I was completely bonkers and went into an auction I saw some beaten up mini rusted through being pushed onto the stage with a guide price of £200 and I immediately bid £10K its doesn't make the car worth that because I paid that money for it.
There are probably many other examples where probably only two or three people i the world would regarding something as having a high value when the rest of the world would call them crazy. And the thing I hate most about the saying is it assumes if a few people are being nutters we all have to regard what they are doing as valid and sensible.
wolfracesonic said:
Rizzo, Caddy, oto, sonic, seeing as you have hijacked the thread, what are your demands, a flight to Cuba? Also how much retardation will be needed to bring the plane to a standstill on the runway, assuming no conveyor belts?
Sorry, I find it fascinating though as elements of it are totally counter intuitive. As I mentioned earlier, look up the Monty Hall problem if you really mess with your intuitive answers 
Caddyshack said:
Ok, first remove the feet and rest it on cardboard so nothing digs in the floor.
And you'll still find the fridge is harder to move as it has more friction with the floor, you need to provide enough force to overcome that friction before it will move. The lighter box will be a lot easier to move as you have less friction to overcome. Without friction or air resistance you could apply a tiny force and still cause it to move, albeit with a very small acceleration. It doesn't mater if it's from stationary or already doing 100 mph, the acceleration from the supplied force will be the same. That's why I say momentum's irrelevant, as the speed is irrelevant, it's all about the force, mass and acceleration.Caddyshack said:
Most tyres seem to have a limit of around 2000 newtons of brake torque before you lock up, this is where Whizzo’s argument meets its limit, you can see from altering the numbers in the equation above for the vehicle mass that the braking distance will increase with mass.
The tyre cannot accept more brake force regardless of the added friction once it reaches its limit. GT3’s, GTRs etc all tend to stop from 70 mph in around 128 feet (shortest ever recorded GT3rs on sticky tyres with ceramics by road and track) to about 138feet less well braked supercars. They are already in the 2000 to 3000 newtons maximums of the tyre, if you add more mass you won’t get any more brake torque.
The figures above are not made up. The formulas are 100% accepted and supported by newtons laws, you can play with the numbers and get the same outcomes. (Change feet to meters)
So if it's braking at the limit of the tyre already, if you put less sticky road tyres on it (ie decrease mu), it will take longer to stop as the tyres will still be the limiting factor but will have less grip. Adding more weight will increase the grip.The tyre cannot accept more brake force regardless of the added friction once it reaches its limit. GT3’s, GTRs etc all tend to stop from 70 mph in around 128 feet (shortest ever recorded GT3rs on sticky tyres with ceramics by road and track) to about 138feet less well braked supercars. They are already in the 2000 to 3000 newtons maximums of the tyre, if you add more mass you won’t get any more brake torque.
The figures above are not made up. The formulas are 100% accepted and supported by newtons laws, you can play with the numbers and get the same outcomes. (Change feet to meters)
I agree there will be a point at which the tyre really can't take it, and will break up or delaminate, but I would hope that's one of the reasons tyres have load limits.
Edited by RizzoTheRat on Friday 8th December 13:25
Caddyshack said:
Most tyres seem to have a limit of around 2000 newtons of brake torque before you lock up
It seems to me that this is an attribute of the vehicle rather than the tyre, and all other things being equal is determined by the mass of the vehicle. You're quoting a number in N rather than Nm, so I assume that you mean the force at the contact patch rather than the torque applied to the wheel? If so, I think that would only be enough to brake a car with an 80:20 front:rear brake split at 1g if it weighed 500kg. A 2190kg Tesla can pull more than 1g under braking. Where is the number coming from?
otolith said:
Caddyshack said:
Most tyres seem to have a limit of around 2000 newtons of brake torque before you lock up
It seems to me that this is an attribute of the vehicle rather than the tyre, and all other things being equal is determined by the mass of the vehicle. You're quoting a number in N rather than Nm, so I assume that you mean the force at the contact patch rather than the torque applied to the wheel? If so, I think that would only be enough to brake a car with an 80:20 front:rear brake split at 1g if it weighed 500kg. A 2190kg Tesla can pull more than 1g under braking. Where is the number coming from?
Otolith, are you able to do the calcs based on nm and n using the formulae above using the truck and then work it back to the 133ft or 40.5m for a gt3 at max stop from 70mph?
My maths can’t work it out
In theory we should get close ish to 133feet using the calcs although obviously the variable would be the driver
The gt3 was 1420kg as tested.
My maths can’t work it out
In theory we should get close ish to 133feet using the calcs although obviously the variable would be the driver
The gt3 was 1420kg as tested.
Caddyshack said:
otolith said:
Caddyshack said:
Most tyres seem to have a limit of around 2000 newtons of brake torque before you lock up
It seems to me that this is an attribute of the vehicle rather than the tyre, and all other things being equal is determined by the mass of the vehicle. You're quoting a number in N rather than Nm, so I assume that you mean the force at the contact patch rather than the torque applied to the wheel? If so, I think that would only be enough to brake a car with an 80:20 front:rear brake split at 1g if it weighed 500kg. A 2190kg Tesla can pull more than 1g under braking. Where is the number coming from?
Rizzo, you can add as much weight as you like but the tyre will not exceed 3000 newtons (ish - prob much less) and the brakes will not run more than 80bar of pressure at the pad.
The momentum increases with mass by an unlimited factor but the friction does not go up in equal amounts as it hits a limit at which point the car skids.
Double the mass you double the distance.
The momentum increases with mass by an unlimited factor but the friction does not go up in equal amounts as it hits a limit at which point the car skids.
Double the mass you double the distance.
otolith said:
Caddyshack said:
otolith said:
Caddyshack said:
Most tyres seem to have a limit of around 2000 newtons of brake torque before you lock up
It seems to me that this is an attribute of the vehicle rather than the tyre, and all other things being equal is determined by the mass of the vehicle. You're quoting a number in N rather than Nm, so I assume that you mean the force at the contact patch rather than the torque applied to the wheel? If so, I think that would only be enough to brake a car with an 80:20 front:rear brake split at 1g if it weighed 500kg. A 2190kg Tesla can pull more than 1g under braking. Where is the number coming from?
Also, try a different car as I think the 80/20 may not be accurate with a low c of g and even weight of Tesla that can massage the regen across the tyres. ( I may be wrong here)
Can you get the calcs to roughly match the 33m from 60 that was tested? (For the Tesla)
Edited by Caddyshack on Friday 8th December 14:05
Edited by Caddyshack on Friday 8th December 14:08
Caddyshack said:
Rizzo, you can add as much weight as you like but the tyre will not exceed 3000 newtons (ish - prob much less) and the brakes will not run more than 80bar of pressure at the pad.
The momentum increases with mass by an unlimited factor but the friction does not go up in equal amounts as it hits a limit at which point the car skids.
Double the mass you double the distance.
Double the mass you double the distance if the braking force remains the sameThe momentum increases with mass by an unlimited factor but the friction does not go up in equal amounts as it hits a limit at which point the car skids.
Double the mass you double the distance.
If you're managing 3000 N with a sticky track tyre, a less grippy tyre on a less grippy surface will do way less than that. But we already know that the tyre construction can take 3000 N, and that you can increase the force a tyre can generate by either increasing the coefficient of friction (sticky rubber on a grippy track) or increasing the reaction force pressing the tyre in to the road.
There's 2 ways to change that reaction force (assuming gravity is constant), increasing the mass or increasing the downforce, and nobody has a problem believing that adding downforce to a car gives it more grip. If the brakes are powerful enough, a car with downforce can brake harder at 70mph than one without, obviously that downforce drops off with speed, but it starts with more force so in braking from 70 to 0 the car with lots of downforce will stop quicker (admittedly some of that will be due to the extra drag). The extra reaction force on the tyres has decreased the braking distance because it has more grip.
If you accept that a car with downforce has more grip and is therefore able to produce more braking force, then a car with extra mass can clearly produce more braking force so it's not a linear relationship between mass and braking distance.
Taken to an extreme, an F1 car can generate around 750kg worth of downforce at 100 mph. I don't know what it's stopping distance is from 100 mph but I bet it would be a lot longer if it didn't have the downforce.
The f1 car would also take longer to stop with double the mass…that is what this whole discussion has been about.
Double the mass would not cancel out due to the friction benefit to an equal amount so the stopping distance would still increase.
The F1 car is not at 100 for long so the downforce would decay very quickly. It wouldn’t be producing much down force in the 50-0
f1 cars stop quickly due to low mass
Double the mass would not cancel out due to the friction benefit to an equal amount so the stopping distance would still increase.
The F1 car is not at 100 for long so the downforce would decay very quickly. It wouldn’t be producing much down force in the 50-0
f1 cars stop quickly due to low mass
Edited by Caddyshack on Friday 8th December 15:07
Edited by Caddyshack on Friday 8th December 15:07
Edited by Caddyshack on Friday 8th December 15:10
A real world test by a science teacher:
Funny that you bring this up just last week I did a demonstration for my students on this subject.
The weight of a vehicle affects the braking distance dramatically, to the point where an overloaded vehicle is very unsafe. It’s not so noticeable in a car with extra weight in it or even going up to the weight of a SUV or even a mini Van, these are all considered light vehicles.
So to answer this question correctly I need to use some terminology that are mostly only used in the heavy vehicle industry, however is it the best way to describe what happens and why it’s so dangerous. And sorry the answer will be long, I will include some picture to show the different vehicles, hopefully you will be able to identify .
So first the terminology mostly so I can type the abbreviation rather than the full description.
GVM / GVWR = Gross Vehicle Mass / Gross Vehicle Weight Rating = maximum weight allowable of the vehicle and it’s load also the weight that the brakes are able to stop safely.
GCM = and Gross Combination Mass = maximum weight of the vehicle and trailer and the loads.
Tare = Tare Weight = the weight of the vehicle without load, often called the kerb weight especially in light vehicles.
Load = the weight that the vehicle is carrying, the weight of the people are included in the load figures , also this is the difference between the tare and GVM.
In light vehicles the maximum load is a percentage of the tare therefore the load is a small part of the total weight so doesn’t effect the braking distance too much.
I have just bought a car like this car below
It’s a 2006 Holden Crewman Cross 8 so it has a 6.1L V8 and 5 Speed Automatic
Crewman Cross 8 kerb mass (Tare) (kg): 1942
Crewman Cross 8 Gross Vehicle Mass (GVM) (kg): 2685
Crewman Cross 8 Braked Towing Mass (kg): 2100
Crewman Cross 8 Gross Combination Mass (GCM) (kg): 4785
These Figures come from the Holden Website
From these figures we can calculate that the maximum load is 2685–1942 = 743kg
So after putting 4 Big guys inside 743 - 400 = 343kg for stuff and luggage.
I’m going to be using this with a trailer like this all the time
This trailer
Trailer kerb mass (Tare) (kg): 1234
Trailer Gross Vehicle Mass (GVM) (kg): 2500
The trailer has cab operated brakes and will be used to the maximum weight of the towing vehicle all the time.
To demonstrate the difference in braking I did a test with this combination to show my students what happens. On a blocked off straight road near my base at 80 kilometres per Hour emergency stopping to stationary, after every run we topped up the fuel tank.
Vehicle and Driver only = 32 Metres
Vehicle, Driver only and 300 kg load = 38 Metres
Vehicle, Driver and 3 Passengers = 39 Metres
Vehicle, Driver, 3 Passengers and 300kg load = 45 Metres
Vehicle, Driver and empty Trailer = 38 Meters
Vehicle, Driver, 300kg load and empty Trailer = 41 Meters
Vehicle, Driver, empty Trailer and 3 Passengers = 41 Metres
Vehicle, Driver, 300 kg load, empty Trailer and 3 Passengers = 43 Metres
Vehicle, Driver and 800 kg loaded Trailer = 48 Meters
Vehicle, Driver, 300 kg load and 800 kg loaded Trailer = 52 Meters
Vehicle, Driver, 800 kg loaded Trailer and 3 Passengers = 52 Metres
Vehicle, Driver, 300 kg load, 800 kg loaded Trailer and 3 Passengers = 58 M
This is real world testing with a car that had been fully serviced with new tyres, new brake pads and discs. everything was working to specification on the car. These results are the average of 5 different drivers. we had warmed the tyres before we started, the road was dry and nice warm sunny weather.
Now this vehicle below is very different but physics are the same
Tare = 19,440 kg
Load = 23,000 kg
GVM = 42,440 kg
Top speed = 60 kmh
stopping distance at top speed when empty = 67 metres
stopping distance fully loaded = 93 metres
Funny that you bring this up just last week I did a demonstration for my students on this subject.
The weight of a vehicle affects the braking distance dramatically, to the point where an overloaded vehicle is very unsafe. It’s not so noticeable in a car with extra weight in it or even going up to the weight of a SUV or even a mini Van, these are all considered light vehicles.
So to answer this question correctly I need to use some terminology that are mostly only used in the heavy vehicle industry, however is it the best way to describe what happens and why it’s so dangerous. And sorry the answer will be long, I will include some picture to show the different vehicles, hopefully you will be able to identify .
So first the terminology mostly so I can type the abbreviation rather than the full description.
GVM / GVWR = Gross Vehicle Mass / Gross Vehicle Weight Rating = maximum weight allowable of the vehicle and it’s load also the weight that the brakes are able to stop safely.
GCM = and Gross Combination Mass = maximum weight of the vehicle and trailer and the loads.
Tare = Tare Weight = the weight of the vehicle without load, often called the kerb weight especially in light vehicles.
Load = the weight that the vehicle is carrying, the weight of the people are included in the load figures , also this is the difference between the tare and GVM.
In light vehicles the maximum load is a percentage of the tare therefore the load is a small part of the total weight so doesn’t effect the braking distance too much.
I have just bought a car like this car below
It’s a 2006 Holden Crewman Cross 8 so it has a 6.1L V8 and 5 Speed Automatic
Crewman Cross 8 kerb mass (Tare) (kg): 1942
Crewman Cross 8 Gross Vehicle Mass (GVM) (kg): 2685
Crewman Cross 8 Braked Towing Mass (kg): 2100
Crewman Cross 8 Gross Combination Mass (GCM) (kg): 4785
These Figures come from the Holden Website
From these figures we can calculate that the maximum load is 2685–1942 = 743kg
So after putting 4 Big guys inside 743 - 400 = 343kg for stuff and luggage.
I’m going to be using this with a trailer like this all the time
This trailer
Trailer kerb mass (Tare) (kg): 1234
Trailer Gross Vehicle Mass (GVM) (kg): 2500
The trailer has cab operated brakes and will be used to the maximum weight of the towing vehicle all the time.
To demonstrate the difference in braking I did a test with this combination to show my students what happens. On a blocked off straight road near my base at 80 kilometres per Hour emergency stopping to stationary, after every run we topped up the fuel tank.
Vehicle and Driver only = 32 Metres
Vehicle, Driver only and 300 kg load = 38 Metres
Vehicle, Driver and 3 Passengers = 39 Metres
Vehicle, Driver, 3 Passengers and 300kg load = 45 Metres
Vehicle, Driver and empty Trailer = 38 Meters
Vehicle, Driver, 300kg load and empty Trailer = 41 Meters
Vehicle, Driver, empty Trailer and 3 Passengers = 41 Metres
Vehicle, Driver, 300 kg load, empty Trailer and 3 Passengers = 43 Metres
Vehicle, Driver and 800 kg loaded Trailer = 48 Meters
Vehicle, Driver, 300 kg load and 800 kg loaded Trailer = 52 Meters
Vehicle, Driver, 800 kg loaded Trailer and 3 Passengers = 52 Metres
Vehicle, Driver, 300 kg load, 800 kg loaded Trailer and 3 Passengers = 58 M
This is real world testing with a car that had been fully serviced with new tyres, new brake pads and discs. everything was working to specification on the car. These results are the average of 5 different drivers. we had warmed the tyres before we started, the road was dry and nice warm sunny weather.
Now this vehicle below is very different but physics are the same
Tare = 19,440 kg
Load = 23,000 kg
GVM = 42,440 kg
Top speed = 60 kmh
stopping distance at top speed when empty = 67 metres
stopping distance fully loaded = 93 metres
32 up to 45 meters for the car with driver and then with passenger and 300kg load.
Go measure 13meter and look at it.
This is a real world test that supports the theory and the science.
30-40% more weight gave 40% more braking distance
Double and the weight and double the distance. The extra weight did not cancel itself out in added friction. The only variable that changed was the weight.
Go measure 13meter and look at it.
This is a real world test that supports the theory and the science.
30-40% more weight gave 40% more braking distance
Double and the weight and double the distance. The extra weight did not cancel itself out in added friction. The only variable that changed was the weight.
Edited by Caddyshack on Friday 8th December 16:06
Caddyshack said:
The f1 car would also take longer to stop with double the mass…that is what this whole discussion has been about.
Double the mass would not cancel out due to the friction benefit to an equal amount so the stopping distance would still increase.
The F1 car is not at 100 for long so the downforce would decay very quickly. It wouldn’t be producing much down force in the 50-0
f1 cars stop quickly due to low mass
Mass makes more of a difference to cars with downforce because the increase in mass to decelerate does not match the increase in how hard the tyres are pushed against the road.Double the mass would not cancel out due to the friction benefit to an equal amount so the stopping distance would still increase.
The F1 car is not at 100 for long so the downforce would decay very quickly. It wouldn’t be producing much down force in the 50-0
f1 cars stop quickly due to low mass
e.g. a 750kg car making 750kg of downforce has the available absolute grip of a 1500kg car and the mass of a 750kg one and can thus decelerate twice as hard as if it had no downforce at all. If you add 150kg to it, it has 20% more mass to decelerate but only 10% more grip with which to do so.
If it doesn't have any downforce, then adding 150kg makes it a 900kg car with the grip of a 900kg car. Both mass needing to be decelerated and grip available to do so have increased by the same factor.
otolith said:
Caddyshack said:
The f1 car would also take longer to stop with double the mass…that is what this whole discussion has been about.
Double the mass would not cancel out due to the friction benefit to an equal amount so the stopping distance would still increase.
The F1 car is not at 100 for long so the downforce would decay very quickly. It wouldn’t be producing much down force in the 50-0
f1 cars stop quickly due to low mass
Mass makes more of a difference to cars with downforce because the increase in mass to decelerate does not match the increase in how hard the tyres are pushed against the road.Double the mass would not cancel out due to the friction benefit to an equal amount so the stopping distance would still increase.
The F1 car is not at 100 for long so the downforce would decay very quickly. It wouldn’t be producing much down force in the 50-0
f1 cars stop quickly due to low mass
e.g. a 750kg car making 750kg of downforce has the available absolute grip of a 1500kg car and the mass of a 750kg one and can thus decelerate twice as hard as if it had no downforce at all. If you add 150kg to it, it has 20% more mass to decelerate but only 10% more grip with which to do so.
If it doesn't have any downforce, then adding 150kg makes it a 900kg car with the grip of a 900kg car. Both mass needing to be decelerated and grip available to do so have increased by the same factor.
Where does the downforce become ineffective? I have driven wings and slicks at 150mph and they make superb grip, you just go after and faster but below a certain speed they become like normal cars. I think 60-100 mph might be the speed but might be 80-100, my point was that once the car drops to say 70-10 braking I doubt the downforce is doing much at all?
Edit to add that 50mph starts to benefit aero but I don’t think it’s much.
Edited by Caddyshack on Friday 8th December 19:50
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