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I have found a formula to calculate braking distance in feet : S= v²/64.f
v = velocity
f = tyre adhesion (e.g. 0.5 or 0.8 in respectively wet and dry).
This formula does not incorporate vehicle weight (or vehicle mass for that matter). I assumed that weight has to be taken into account as well. A lighter car can accelerate faster because it doesnt have to haul all that weight around, consequently it should also decelerate faster. So, why does this formula ignore vehicle weight?
These are APPROXIMATIONS so you can avoid many of the fine details.
So ansering at the grossest level ...
Extra weight means extra traction for the tyres.
The extra GRIP means you can acheive greater retarddation.
And so can slow the ghigher mass in rougly a similar time/distance
MANY assumption - ie brakes able to stop the extra weight :)
As an example that many folks have seen and experienced and seldom thought through ... an empty truck takes longer to stop than a loaded one. The reason is many of the tyres have insufficient weight on them to match necessary traction to stop and so lock up VERY early adn provide zero retardation. So instead of 8 rubber patches slowign teh vehicle there are only 4 and hence longer braking distance
I have looked this up earlier on and Carroll Smith's book says that braking is governed by the same factors as acceleration simply turned around. he states gross vehicle weight as one of the factors.
Or does he refer to braking force, which needs to be higher for heavy vehicles? Thus could it be that braking force is proportional to weight and braking distance isn't?
I think a car would stop quicker than my racing bicylce doing 50 kph:)
[quote=henk4]I think a car would stop quicker than my racing bicylce doing 50 kph:)[/quote]
then you have bad brakes :) my bike takes 3 metres when doing 40-45km/h :) the brakes are seriously hot then.when knowing i am going fast, i therefor take a small botlle of water with me to cool some stuff off, otherwise, it is unstoppable for a while ;)
i must say, that i can really feel some g-forces then, bt probably not really much.
[QUOTE=drakkie]then you have bad brakes :) my bike takes 3 metres when doing 40-45km/h :) the brakes are seriously hot then.when knowing i am going fast, i therefor take a small botlle of water with me to cool some stuff off, otherwise, it is unstoppable for a while ;)
i must say, that i can really feel some g-forces then, bt probably not really much.[/QUOTE]
my bike does it in less then a meter, my body on the other hand goes a little bit further then ;)
[quote=PoRsChEnUt]my bike does it in less then a meter, my body on the other hand goes a little bit further then ;)[/quote]
depends offcourse on the speed. Lean way back and put your legs on the bagage carrier ;)And offcourse, brace yourself :)
So Matra, what is there an ideal weight according to this firmula?
[QUOTE=Matra et Alpine]
As an example that many folks have seen and experienced and seldom thought through ... an empty truck takes longer to stop than a loaded one. The reason is many of the tyres have insufficient weight on them to match necessary traction to stop and so lock up VERY early adn provide zero retardation. So instead of 8 rubber patches slowign teh vehicle there are only 4 and hence longer braking distance[/QUOTE]
Maybe I misunderstood the OP, but isn’t this a bit misleading in regards to his question?
He is asking what other parameters affect “threshold braking”, is he not?
Perhaps this is something that the OP needs to clarify. Because if you are only looking at the static (rolling) coefficient of friction doubling the weight will not double your tractive force. However, if the OP is concerned with dynamic (sliding) coefficient of friction then yes, weight could have the impact you mentioned above.
Keep in mind that as the load on the tires goes up the coef. of friction goes down. This is why balancing the weight between all four tires is important for good braking.
The following link was written by a 7 time SCCA national champ. He's also a senior engineer at Firestone.
[url]http://neohio-scca.org/comp_clinic/hand_out_reprints/Adv%20car%20prep%20Handout%202005a.pdf[/url]
Nice piece of information! The way I get it: lateral load does not increase at the same rate as the vertical load (the curve is not linear), so if you manage to reduce roll, and thus weight transfer, a heavy car could generate higher cornering forces than a light one?
[QUOTE=Cedric]Nice piece of information! The way I get it: lateral load does not increase at the same rate as the vertical load (the curve is not linear), so if you manage to reduce roll, and thus weight transfer, a heavy car could generate higher cornering forces than a light one?[/QUOTE]
Certainly a better balanced heavier car could corner harder than an other wise similar but lighter car. I wouldn't associate a reduction in roll with increased grip. In my conversations with engineers who set up racecars I know the relationship is more complex than that. If you just make the anti-roll very stiff you end up transferring more load to the outside tires thus reduce grip.
[QUOTE=culver]If you just make the anti-roll very stiff you end up transferring more load to the outside tires thus reduce grip.[/QUOTE]
How does that happen exactly? I always assumed a lack of anti-roll bar made for massive load transfer from the inside to the outside wheel.
Matra, I'm afraid that's rubbish. Again. The bit about the truck may be true but you could hardly say their suspension is fine tuned! Think about how stiff it would be unladen if it has to support 20tonnes loaded up. Of course the tyres will slide. In a road car where mass is constant (although the weight is not static) it is possible to optimise the settings for certain applications. Weight kills everything: braking, handling, acceleration, the car.
An anti-roll bar controls the weight transfer across the car.
As a general rule of thumb:
Soft cars will break grip earlier and more progressively
Stiff cars will hand on longer but break grip more suddenly
A lighter car will brake quicker than a heavy one (if all other things are equal). Think about it, lowering a weight to the ground is no easier than picking it up!
[QUOTE=Cedric]Nice piece of information! The way I get it: lateral load does not increase at the same rate as the vertical load (the curve is not linear), so if you manage to reduce roll, and thus weight transfer, a heavy car could generate higher cornering forces than a light one?[/QUOTE]
I think you are confusing body roll accompanied by [u]weight transfer[/u] with the overall [u]load transfer[/u].
[b]WEIGHT TRANSFER[/b]
Any time the center of gravity (CoG) of the vehicle moves the static weight distribution of the vehicle moves. When the body (more specifically the “sprung mass”) either rolls or pitches the CoG will also move slightly. The results will be a redistribution of the total vehicle weight/mass between the wheels, this is referred to as weight transfer. As well, something like fuel consumption will cause weight transfer. For weight transfer to occur, mass has to move from one place to another.
[b]LOAD TRANSFER[/b]
The load transfer represents the total load on the tires at any given time, and does not necessary require any mass to move. This is because the forces that accelerate the vehicle do not act though the vehicle’s CoG, so a [i]moment[/i] is generated on the vehicle. In order to balance this moment the force or load on the tires is redistributed between the each tire. When cornering more load is carried by the outside tire, when braking the front tires carry more load.
Load transfer is generally more significant than weight transfer from corner to corner.
Also, keep in mind that a perfectly ridged car without a suspension system would still undergo load transfer. The best method to reduce load transfer is the lower the CoG, increase the track width, or increase the wheel base.
One of the reasons for running stiff springs isn’t so much to reduce weight transfer as it is to preserve suspension geometry. It is difficult to maintain the desired suspension geometry over larger suspension travel, so stiffer springs help to reduce suspension travel. Conversely the stiffer spring makes it harder for the tires to follow the contour of the road, and if the tire isn’t touching the road it doesn’t matter how optimized the geometry is.
It is indeed a complex system.