kurt M

I think time is a factor too. A longer race might favor a car that has a better weight to tire patch ratio other aspects being close. Brian hit it first. Easy to repass down the straight after the lighter car worked hard to get by you in the corner. Traffic favors the HP car as well.

The vipers seem to go like hell until they run out of tire.


< I like lighter cars, more fun to drive, less $ to maintain.

Bryan Watts

At least in the case of GTS, there are no limits on tire width or body work. Heavier cars can simply strap on wider tires for a more favorable tire patch shape (not size). That will solve most of the problem, even for an extended enduro.

FredC

I don't understand what you wrote.

mark kibort

in otherwords, its all about HP to weight. so,even if a car you were facing had more hp, if its HP to weight was less than you had, (at a lighter weight car), you would run around him (him being the heavier, more hp car, but less on the ratio of HP/weight ratio)

claykos

Don't forget about HP to Drag. This will dominate at high speeds. It's why a Lotus elise at 2000 lbs with 200 hp runs into a wall at about 130 mph, but a 4000lb 400hp car will have a much higher top speed...

mark kibort

not nessarily. you are betting that the heavier car has a different HP/drag ratio based on more drag and that is an assumption you really cant make. we have to keep some level of constistancy to discuss this logically. (e.g. putting balast in the same car). Also, in GTS, there are quite a few smaller , heavier cars with even more power, so thats where that assumption falls apart. (i.e. 911 turbo with 500whp vs BMW with 400whp)

Now, if the heavier car is larger (frontal area), AND has a higher total drag, then yes, that could be a dominant factor , at tracks with very high speed straights. some same cars have different weights where this wouldnt be a factor. So, coming out of most corners, the speeds are low enough for the drag factors to be rounding errors at best.

Acceleration= power/(mass x velocity) (all other things being equal)
or if you prefer, Acceleration =F/M.
In both cases were talking net force or power, where drag would be factored in.

FredC

Hopefully you are kidding... Because this is not correct (over-simplified) even when HP/WT is the same. Acceleration depends on more variables. In the case I was referring to, the other cars had a better HP/WT ratio than my car however (higher class).

claykos

I am assuming equal drag for both cars. 2 identical cars, one with a 1000 lb lead plate bolted inside and commensurately more power. The heavier car with more power has better HP/Drag and will accelerate faster in a straight line.

T/m-D/m=a

T/m is the same for both cars. D is the same for both cars. With larger m, I am subtracting a smaller number and a is thus bigger.

Or you can write it this way

1-D/T=(m/T)a

hold m/T constant. Hold D=constant. More thrust means bigger a.

mark kibort

Ive alreay talked about Hp/drag a few times. I agree with you , BUT, again, more assumptions that might be misleading

your example of a lotus elise. Do you know the drag coefficient. lets say, both are about Cd 0.4 with aero. both have 10:1 hp weight ratios.
But, what about the frontal area, if one is half the other. if the drag force of the larger car requires 400hp at 200mph, it will be 50hp (edit: corrected) at 100mph. if the smaller car has half the frontal area, then it would have half the drag forces, and the power required would be half, which would keep things even between the two. in this example, even at 100mph, both would have the same net power for acceleration. scaling in aerodynamics is very predictable, and in this example, both cars could close to the same in most areas.

claykos

Mark,

I have an MS in Aerospace Engineering. I know quite a bit about this topic.

There are indeed many variables.

Note that power required is proportional to speed cubed (drag proportionally to speed squared).

All I am saying is that cars with equal TOTAL DRAG - and equal power to weight ratio, the heavier, higher hp car will accelerate faster.

Back to the original topic: If I have time in the next few days I will run some Bosch LapSim cases of cars with the same power to weight but one heavier/more hp and show laptimes for different types of tracks.

mark kibort

Then if you know the variables, then you know the dangers of the assumptions you had made to make a comparison.

Yes, the as the speed goes up the power will be cubed. agreed there as well.
I think I mis spoke with the comparisons with HP required at 100mph. but anyway, double the speed, 8x the power. It all factors to be the same if power and frontal area are halved, right? Now, you made the comparison of the 2000lb/200hp Elise and a 4000lb 400hp car, right?

So, to your point, if two cars have the same total drag, yes, the heavier car will accelerate faster in the higher speed ranges due to drag. BUT, two cars having the same drag but different HP is true for all one type of 911s or cars, and that might be your point, but I thought we were refering to different types of cars, like your example of the Viper vs the 911, where in fact, the viper will have a lot more frontal area vs the 911, with near the same Cd. In this case, the drag to hp ratios might be near the same and their acceleration forces at speed might be as well.


your simulation will address same cars with different weights and equal hp/weight ratios. I guess that could be an issue for a guy that wants to maximize his performance on a real fast track like Cal speedway. It will be interesting to see if that drag costing power, overcomes the losses in stopping and cornering, with non drag acceleration areas being realtively the same.

Look forward to seeing the results of that simulation.

mk

mark kibort

But the discussion here was about same HP/weight cars. when HP/weight is the same, acceleration is the same. (sans the drag component which could be counteracted by proportial changes in frontal area, but yes, it was a simplification). I know you were talking about other classes passing you.

kurt M

You just said what the others have been saying as a rebutal to their posts. You can't just cross out the drag from the issue at hand.

True or False. It is harder to pass another car in corners than it is to pass on a straight.

mark kibort

No, what i have been saying is that it depends on the frontal area of the car (and drag coefficient). if we are talking same car, then yes. But, others have been comparing (Lotus vs heavy 4000lb cars) or examples at the track where a 911 was going against Vipers and claiming advantages or disadvantages. in those cases where the frontal area are different, it could very well be that the straight, high speed acceleration would be the same for both cars if the size of the car was proportional to the hp advantage AND the HP/weight ratios were constant. This is precicely why the 911 can compete against the Viper in pro racing when the HP to weight ratios are near the same, even on very fast tracks. sure, there are more other factors than you can shake a stick out, so we are discussing this in general terms.

so, to your question below, that also depends. personally, I think its always easier to pass under braking, or in "the corners", unless you have a hp to weight advantage, where the extra power advantage can allow for an easier time to pass in the corner.

mark kibort

A real world data point.

A porsche 928 has 115% greater Cd x Area than a RS 911. ( approx = 2.0 vs 1.7 sq-ft and .34 to .32 Cd)

What this means is that on the track, the 928 can have 350hp , and the RS could have 300hp. based on their Cd and frontal area, if both cars had the same HP to weight, there would be no advantage at any speed, strictly based on HP to weight and drag factors for the greater hp car. quite possibly, the lighter car would have an advantage due to braking and handling capabilities. down the straights, it might even be faster due to faster corner exit speeds, but then that is an area that is riddled with factors as well.