Power to weight vs power and weight
Discussion
Hi,
I realise that there is no answer to this and that it will vary from circuit to circuit, lots of other factors like grip, traction, wind resistance etc will have a big impact but...
Imagine you have 2 cars. One weighs 750kg and has 150bhp, the other 1250kg and has 250bhp. They both have 200bhp/tonne. They have about the same amount of drag.
So, assuming traction wasn't an issue they should both accelerate at the same rate, except that drag will have an impact so the heavy car would be quite a lot faster in a straight line, espcially at the end of long straights. But it would have to brake and turn all that extra mass.
Would the light car have a hope? What about at Mallory Park or similar?
Your thoughs / expirience please
How do you think they would compare?
I realise that there is no answer to this and that it will vary from circuit to circuit, lots of other factors like grip, traction, wind resistance etc will have a big impact but...
Imagine you have 2 cars. One weighs 750kg and has 150bhp, the other 1250kg and has 250bhp. They both have 200bhp/tonne. They have about the same amount of drag.
So, assuming traction wasn't an issue they should both accelerate at the same rate, except that drag will have an impact so the heavy car would be quite a lot faster in a straight line, espcially at the end of long straights. But it would have to brake and turn all that extra mass.
Would the light car have a hope? What about at Mallory Park or similar?
Your thoughs / expirience please
How do you think they would compare?
BertBert said:
why would the heavier car be faster down the straight with the same power to weight ratio?
Because they both have the same amount of drag - acceleration is proportional to (available power - power required to overcome drag) / weight. (150 - x) / 750 is only equal to (250 - x) / 1250 when x = 0.
It would depend on the circuit, Mallory park doesn't have massive straights but it doesn't have that much in the way of corners either so it's a hard one to call. It would probably all depend on how fast the two cars could take the big first corner as elsewhere the corners and straights would probably equal out more or less. Somewhere like Cadwell Park would play to the lighter car but Snetterton's long straights would favour the powerful car apart from the infield.
The lighter car would have lower loads on the tyres and that would likely result in higher lateral acceleration as doubling the normal force doesn't double the lateral force. It would likely be faster in grip-limited areas, whereas the heavier car would be faster where power-limited.
radical78 said:
could do theories and senarioes all night but on a real track the light cars are always faster
You can lap faster in a lighter car but in circuit racing if your slow on the straights and get held up in the corners by the heavier cars your light weight isn't much use to you.Dan
otolith said:
Because they both have the same amount of drag - acceleration is proportional to (available power - power required to overcome drag) / weight.
(150 - x) / 750 is only equal to (250 - x) / 1250 when x = 0.
What about air resistance being proportional to the square of the velocity? It's not linear.(150 - x) / 750 is only equal to (250 - x) / 1250 when x = 0.
Bert
radical78 said:
could do theories and senarioes all night but on a real track the light cars are always faster
Funny that because the Porsche Club Championship has classes in which several different models compete against each other on a balanced power to weight ratio basis (not exact as various factors are taken into account). In either of the two main classes people always go for a model with more power and weight for the simple reason that its much easier for a powerful car which may be slightly worse on the brakes/cornering to defend position than it is for a light car to get passed a powerful one. The old expression 'Backing em up" in the corners proves the point really, if the guy in front can open a gap on the straights you need a huge braking and cornering advantage to get past which doesn't tend to happen in practice.NJH said:
radical78 said:
could do theories and senarioes all night but on a real track the light cars are always faster
Funny that because the Porsche Club Championship has classes in which several different models compete against each other on a balanced power to weight ratio basis (not exact as various factors are taken into account). In either of the two main classes people always go for a model with more power and weight for the simple reason that its much easier for a powerful car which may be slightly worse on the brakes/cornering to defend position than it is for a light car to get passed a powerful one. The old expression 'Backing em up" in the corners proves the point really, if the guy in front can open a gap on the straights you need a huge braking and cornering advantage to get past which doesn't tend to happen in practice.As soon as I saw the 2014 regs it was obvious you had to have a 996 to win. The 996 weighed in at 1380kg with 300BHP and the lightest car (Boxster 986) came in at 1275kg with a max 252BHP. The boxster had a better power to weight ratio but honestly I think the boxster would need to be 1100kg (which is virtually impossible without spending mega bucks) to be able to compete and my money still would be on the 996 (easier car to drive as well). When a car is that heavy and no downforce per say to begin with - taken out 100kg doesn't make that much difference.
to give you an example at brands hatch indy race
Fastest 996: 50.967
Fastest Boxster 52.205 !!!
In F1 an extra 10kg will make on average a difference 3/10ths slower (OK an F1 car weighs just over half that of a Boxster 691kgs) but it's the downforce that makes the difference.
Someone clever will come up with a proper equation for this !
woof said:
In F1 an extra 10kg will make on average a difference 3/10ths slower (OK an F1 car weighs just over half that of a Boxster 691kgs) but it's the downforce that makes the difference.
But unless I am missing something, that's not the question being asked. As the scenario has equal power to weight ratios, not a (10kg) heavier car with the same power.Bert
BertBert said:
otolith said:
Because they both have the same amount of drag - acceleration is proportional to (available power - power required to overcome drag) / weight.
(150 - x) / 750 is only equal to (250 - x) / 1250 when x = 0.
What about air resistance being proportional to the square of the velocity? It's not linear.(150 - x) / 750 is only equal to (250 - x) / 1250 when x = 0.
radical78 said:
could do theories and senarioes all night but on a real track the light cars are always faster
That's normally because the lighter cars have a higher power/weight and less drag too (they tend to be smaller). Of course, when it comes to cars with downforce, the lighter car needs less downforce for the same cornering performance, which sheds drag and so helps to equalise the straight line performance. It's worth noting that a car that carries a higher corner speed has a speed advantage coming out of the corner, so the faster car in a straight line continues to lose ground after the corner until it regains that speed defecit from better acceleration when it can start to catch the car ahead.Thansk for the replies. What I was really asking was...
The more powerful car will be faster on the straights and the lighter car will be able to brake later and corner faster. Would being lighter by 500kgs be enough extra braking and cornering performance to counter the 100bhp advantage of the more powerful car?
To me 100bhp seems like a lot!
I guess the Lotus Cortina / Ford Galaxy battles of the 60's or the mk2 Jag / Austin A40 suggest that at times there will be a balance between a lighter car and a heavier but more powerful car. I also take on board the passing on the straights, blocking in the corners argument. I remember exactly this happening when lighter 400cc GP bikes were allowed in with full fat 500s in the 90's. The light bikes were fairly competitive in qualifying but didn't get to use their cornereing advantage in the race.
The more powerful car will be faster on the straights and the lighter car will be able to brake later and corner faster. Would being lighter by 500kgs be enough extra braking and cornering performance to counter the 100bhp advantage of the more powerful car?
To me 100bhp seems like a lot!
I guess the Lotus Cortina / Ford Galaxy battles of the 60's or the mk2 Jag / Austin A40 suggest that at times there will be a balance between a lighter car and a heavier but more powerful car. I also take on board the passing on the straights, blocking in the corners argument. I remember exactly this happening when lighter 400cc GP bikes were allowed in with full fat 500s in the 90's. The light bikes were fairly competitive in qualifying but didn't get to use their cornereing advantage in the race.
Woof it was obvious as soon as the 996 was introduced a few years back that it was the car to have. Then Hartech went and won the thing with a boxster turning all expectations on the head, same when the 968 was allowed into class 2, they might be much heavier than an S2 in the regs and in theory very little difference in straight line performance but everyone quickly felt that the two most powerful allowed cars (boxster 2.7 and 968) became the cars to have.
Something else is that these comparisons are very dependant on the circuit, I don't like driving my car so much on the GP circuit at Silverstone as it just feels utterly gutless along those straights, a much more powerful car like a 964 or 993 or 996 just romps off into the distance once in front. The gear ratios also don't work there as you keep hitting the top of 3rd in the bends whereas the more powerful cars have the longer gears which work better there (gears must be standard in regs same as for lots of production car based racing).
One thing is fairly certain though, the heavier cars always cost a lot more to run.
Something else is that these comparisons are very dependant on the circuit, I don't like driving my car so much on the GP circuit at Silverstone as it just feels utterly gutless along those straights, a much more powerful car like a 964 or 993 or 996 just romps off into the distance once in front. The gear ratios also don't work there as you keep hitting the top of 3rd in the bends whereas the more powerful cars have the longer gears which work better there (gears must be standard in regs same as for lots of production car based racing).
One thing is fairly certain though, the heavier cars always cost a lot more to run.
An interesting case study into the differences in power & weight (and even if irrelevant, it's good fun to watch)
http://youtu.be/t3ppTh3TYtA
http://youtu.be/t3ppTh3TYtA
Gassing Station | UK Club Motorsport | Top of Page | What's New | My Stuff