Who can offer the answer with either in-car data, or even the math and theory, about the deceleration differences between an car and the same car with 15% additional weight. (say 3000lbs vs 3500lbs).

Assumptions. same tires, aero and speed... call it 100mph to 40mph.

which is going to slow faster (decel at a faster rate ) and why?
Or will they slow at the exact same rate? example..... If the light car can decel at 1g, can the heavier car decel a this same rate or is it something lower. if lower, how much lower.

show your work, or detail your experiences.

 

 

I have found that things with more mass react differently to an input than do objects with less mass.

But I'm in the early stages of my work and have yet to publish on the topic.

-Galileo

 

Considering that both friction force on the tires (normal force times the friction coefficient) and acceleration (a=f/m) are linear wrt mass, physics would say that they will decelerate at the same rate (assuming sufficient brake force). However, and that is a big however, the additional mass requires that the brakes dump more heat over the same period of time. So, if the rotors and pads can dump the heat without over temping and losing performance then the theory holds. Other wise....

 

Come on, Mark. You must know this.

The reason it takes longer to stop the heavier car than the lighter car, all other things being equal, is the same reason a heavier car does not corner as well as a lighter car, all other things being equal.

What is the reason? Even though the deceleration force a tire can generate does increase with increased vertical load, it is at a diminished rate. In other words, increasing the weight of a car, all other things being equal, always reduces the tires braking efficiency.

 

Who cares ?
Fact is, that it does.
Just make your car as light as you can physically or within the rule set that you race. On to bigger and better things.

What's the old quote again ........
“Never argue with MK. He will only bring you down to his level and beat you with experience.”

 

Mark,

The phenomenon is often called "tire load sensitivity". You can look here too:

https://en.wikipedia.org/wiki/Tire_load_sensitivity

Look at the "Example" section where Milliken is quoted. Notice that as vertical load increases, lateral acceleration decreases. It says the same is true for longitudinal forces as well.

 

Oh:

[/thread]

 

Do both cars have the same brake setup?

...and in this theoretical we're only discussing a single event. The difference may grow between the two cars (time / distance) as the heavier car begins to have an effect on the braking system after multiple events.

 

yes, this is two identical cars ... only change is to the weight (adding more)

yep, know about the load sensitivity.. the question is how much.
I was assuming that with 15% more weight, you might have 15% less deceleration rates... but according to some very general graphs, it looks like maybe only 5% difference.

Nice..... again, being the "d...." as usual. The question is often brought up. what about ballast weight? sometimes, with a given weight, redistributing it can help in a huge way too.

yes, the mu goes down slightly, so even though the force goes up, its not in direct proportion to the increase in weight. example, 1g of deceleration might go to .95g of deceleration for some level of increased weight.
so, in the end... what is that tire brakiing force efficiency loss for adding weight???



Thats the theory....... and sure, the brakes have to be able to take the "heat dump", and if not, obviously, the force created by the brakes would be lower especially as the mu goes up with the pads getting beyond their temp range.
I think the main factor here , if the brakes can take the heat (power dissipation rate) is that the mu changes with added weight, slightly. so the deceleration rate should always go down... all other things kept equal.

 

So you knew all about tire load sensitivity and how the mu goes down as vertical load on a tire increases...so why did you ask? I know...you were hoping nobody would know about that stuff so you show us all how smart you are. Sorry, that didn't work.

Just go buy "Race Car Vehicle Dynamics" by Milliken and Milliken.....

 

Winders, FTW!

I look at a variety of data for similar cars at different weight and I see little difference in decel rates, as measured in g's...

 

Peter,

"Similar" doesn't cut it. Mark asked about the same car except for almost 17% more weight. There is no doubt that the lighter version of the car has the ability to generate more longitudinal acceleration while braking which means it will stop more quickly.

This is all about tire load sensitivity and is why a heavier version of the car generates less lateral acceleration as well.

Or are you going to tell me weight doesn't make a difference in cornering? I didn't think so.....

A little bit of weight makes a small difference. A lot of weight makes a big difference. I am sure there are graphs out there that approximate the curve. Mark looks to have posted one but I am not sure of the source or the data.

Scott

 

Ford answered this in the 70's when they were advertising the bloated Pinto.. Handling was improved because of ground hugging weight

Portable bonfire was not a feature they advertised for some reason

 

can you just try to not be such a" D....K". I do know about load sensitivity.
but, those are general graphs.. it seemed to me, i heard and seem to experinences point to a 1-1 reduction of decel rates for increases on weight

thats interesting... we see heavy (CORRECTED! I meant lighter P2 cars obvioisly . Cup Cars ar near 2500lbs vs P2 at 2000lb as we all know) P2 cars FLY by the cup cars under decel. ignoring the speeds where downforce is a major component, it seems that the mu goes down with weight going up.

Yes, you and i are on the same page.. thats my general feeling as well. I suspect there is more going on for the same tire and more weight. how the efficiency goes down with more weight due to racing tire tread deformation ? more heat, reduces adhesion?

ha ha ha