Change in Tyre diameter after prolonged running.

[Ciro Pabón]

PNSD, nice movie. Frankly, this is the first time I use the "youtube" tag that Tomba added. I think it looks cool: just to remind everybody that it can be used the same way you use the "image" tag, use the same adress PNSD gave of the movie in YouTube, that's it:

[youtube]http://www.youtube.com/watch?v=wL_6dDCCHlg[/youtube]

Thanks again, J-Tom.

Now that you mention the "pretty serious amount of power" you need to move a car at 300 kph, I have here something that intrigues me, perhaps somebody can help (I should open another thread, I know, but...)

Here you have some other approximations I made of the power of an F1 car, given the rough figures I have for 0-100 kph times, to estimate the power to the wheels delivered by an F1 car. Please, don't laugh too hard...

In red the figures I assumed (from what I've read), in blue the "Net power" (the one you need to impulse the car), "Drag power" (to move through the air) and "Rolling power" (for the resistance of bearings and wheels).



IF I made no gross mistakes, I get that you only need 333 hp, that is you only need 43% of the "nominal power" applied to the wheels. I thought, well, that's why you need launch power limiters.

Now, if I try to use Reca's figures for acceleration, that he extracted from a sound analysis of the engine, given here,



then I get this:



Why? I get AGAIN that the car delivers only 361 hp, only 47% of "nominal" power. I gladly concede that:

a- I might have made a mistake in the formulas (here is the Excel file, just in case)
b- Maybe the drag calculation is extremely simple.

Or what?

[Jersey Tom]

350hp might be a reasonable number. Keep in mind thats AVERAGE power delivered to the wheels. F1 cars make say 800hp peak. Average power through the rev range is much lower, maybe by a factor of 25-40%.

For example look at this power curve for a sportbike (high RPM, naturally aspirated engine)

http://www.areapnolimits.com/images/pro ... no_big.gif

Peak is about ~100hp. Average from 6-14k rpm though is only ~70.

From a standing start you'll have a much lower average power delivered than peak engine power.. losing some because the tires can't deliver all that power, or because at some point you have to be low in the revs, gears arent perfect, etc.

[Miguel]

Dear Ciro,

I fear you've made a mistake in the distance calculation. If we suppose constant acceleration, distance should be
s=v1*t + 1/2*a*t^2

In any case, distance required to accelerate from v1 to v2 has to be larger than min(v)*t, which is 375m in the second case. Another way to check the power required just for the acceleration would be
P=dW/dt~(E2-E1)/t=1/2*m*(v2^2-v1^2)/t

I'd also say the drag formula is underestimating the power required. Although it's a good approximation for the average power needed, you still need 1/2*rho*Cd*A*v^3 watts to overcome drag at final speed.

With these changes, the first scenario would require just 183 HP and the second one would need 523HP. While the larger number seems reasonable, the smaller one is indeed striking. I'd suppose that taking into accound wheel inertia is needed to obtain a more sensible number. I fear that obtaining 330HP in both cases was just a coincidence. In any case, Jersey Tom made a good point: at ~290 km/h F1 cars are operating near the redline (and thus P ~770HP), while on a start, much less power would be available.

BTW: At what speeds do F1 cars usually change to 2nd gear?

Sorry if my first post here offended anyone.

[CMSMJ1]

ref the change in diameter at speed, or the growth question.

I recall michelin inrtoduced some "Zero growth Technology" tyres into F1 to negate this issue. IIRC they were developed from tyres used on the Concorde.

My gut feeling is that the effect of growth and the effect of wear are both so small as to matter not in the overall performance of the tyre itself.

[Ciro Pabón]

Thanks, Miguel, I think I used (V1-V0)^2 and I should have used (V1^2 - V0^2). I'll post the corrections as soon as possible. You mention I should use 1/2*rho*Cd*A*v^3, but why? And why could anyone be offended by your post?

[Miguel]

You mention I should use 1/2*rho*Cd*A*v^3, but why?

From the original formula, we'd be calculating the power needed to overcome drag at the average speed between v1 and v2. However, if we want to move at v2, we do need to overcome the drag at v2.

In any case, let's look at the original formula, with the average speed. If we started from a lower v1, average speed would be lower and thus the (drag) power needed would actually be smaller. This seems unintuitive. Keep in mind it's true that the average power used is smaller, though. Now, let's split the car's movement into a piecewise function. We'd have that the last piece needs drag power very close to whatever*v2^3.

I'm also a bit dubious about units in Rolling friction power, but I actually don't have a clue how one usually models rolling resistance.

And why could anyone be offended by your post?

It feels slightly bad writing one's first post to correct a respected member, which is why I apologised beforehand.

[rjsa]

You mention I should use 1/2*rho*Cd*A*v^3, but why? And why could anyone be offended by your post?

V^2 Gives you a force (N) The formula as stated above is resulting W, or N*(m/s)

[Ciro Pabón]

Thanks for your kindness, Miguel, but I believe that the only respected members around here are the ones that can learn from others and/or can say what nobody says, for some true team work. In our modest environment, if you have no "peer-review", you have no science at all, hence no technics, so criticism of posts is always welcome. Either that or you get only a bunch of opinions instead of a thread.

Thanks, rjsa, you're totally right.

Following the lead by CMSMJ1 I read a little on Zero Growth tyres for the Concorde airplane: they do not expand too much, but they do because they use more rigid materials. The article mentions that the carcass of a normal tyre expands up to 6%, but these tyres expand only 3%.

http://www.sae.org/aeromag/techupdate/08-2001/


The tires were developed in 2002, because of the Concorde accident (accident that was caused by a tyre that blew and broke a tank in the plane).

In 2005, the Concorde not flying anymore, they launched another technology called Infinicoil: the wires in the tyre are continuously woven and more strong. I think is a repackaging of the Zero Growth, but I'm not sure:

http://www.tirereview.com/default.aspx? ... &item=2789

Apparently, the tire wears more evenly and last longer, because it doesn't deform so much. I don't know if this is good or bad for racing, but definitely is not what dragsters builders want.

If we follow the original idea of callmekart, what could be developed is the exact opposite: a "Constant Growth Tyre". You could imagine several ways for the tyre to lose rigidity as it wears, so it expands and compensate loss of height because of wear.

[rjsa]

Thanks for your kindness, Miguel, but I believe that the only respected members around here are the ones that can learn from others and/or can say what nobody says, for some true team work. In our modest environment, if you have no "peer-review", you have no science at all, hence no technics, so criticism of posts is always welcome. Either that or you get only a bunch of opinions instead of a thread.

I should add to this that we are non professionals (me at least), rusty long had been engineers (me, for sure) having lots of fun re-thinking the math and theories that once where learned and understood as clear daylight.

My response to Ciro above started as a pointing finger to a (supposedly wrong) formula, which I thanks god understood before hitting post.

The fact that it might be possible that F1 teams have spotters around here and that this messed up some guy's head is by no means conclusive that any challenging response is to be taken personally.

[Miguel]

Following the lead by CMSMJ1 I read a little on Zero Growth tyres for the Concorde airplane: they do not expand too much, but they do because they use more rigid materials. The article mentions that the carcass of a normal tyre expands up to 6%, but these tyres expand only 3%.

[shortened for the sake or reading]

Apparently, the tire wears more evenly and last longer, because it doesn't deform so much. I don't know if this is good or bad for racing, but definitely is not what dragsters builders want.

If we follow the original idea of callmekart, what could be developed is the exact opposite: a "Constant Growth Tyre". You could imagine several ways for the tyre to lose rigidity as it wears, so it expands and compensate loss of height because of wear.

I fear using these tires would be counterproductive in racing. After all, putting lots of metallic coils in the peripheria of the tire surely adds to the tire's inertia. But, again, I don't have a quick estimate for the inertia of a tire. It would also fall in unsprung weight category. It's possible, though, the extra rigidity also improves the behaviour of the tire, and then my reasoning does not apply.