JRP3
Hyperactive Member
You can see that effect on drag racing tires as they spin up and get taller and narrower. Something similar is probably happening on a much smaller scale with regular tires.
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Regen vs. Coasting
- Thread starter Todd Burch
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- Driving Dynamics Model S
physicsfita
Member
I was just thinking about the contact patch size, and doing so just before I took a nap (always dangerous). This effect is very obvious in the rear tires of a top-fuel dragster, but they engineer a very thin sidewall to achieve it. Having gotten up from my nap, I'm inclined to agree that the effect would be smaller for a passenger tire, and that heating of the air inside is probably a bigger deal. Still, when you're talking about going between degrees of being out-of-round, it doesn't take much to have an effect.
I agree with your rolling friction model -- that's what I learned, too.
I agree with your rolling friction model -- that's what I learned, too.
Interesting.
Classic!
Interesting. I don't know anything about dragsters - why do they want to engineer their tires to achieve this effect?
JRP3
Hyperactive Member
My guess is there may be two reasons. One, it's a side effect of building a very soft flexible tire for maximum grip, and two, I think it acts something like a gear change, smaller tire for better gearing at takeoff, shifting to a taller tire/gearing for speed.
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Not true. The contact patch is almost entirely based on the load divided by the air pressure. A tire that has 1000 lbs of load on it inflated to 50 psi will have 20 sq.in. of contact patch area.
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Dragsters tend to have a lot of power and little weight. The idea is to get the biggest contact patch possible for maximum grip on the track's surface. The more flexible the sidewall the closer the tire comes to the formula (load divided by pressure). They also run very low psi and have to screw the beads to the wheel to prevent slippage.
JRP3
Hyperactive Member
True, because as the dragster example shows when the tire gets taller it also gets narrower, so the contact patch probably remains the same.
Radial tires don't get larger in diameter, the steel belts prevent that (bias-ply tires actually do get bigger in diameter). What happens at speed is that the tires deflect less, primarily because they have less time to do the deflection, but some because of the increased pressure as the tires warm up. The faster the vehicle goes, the less time the tire has to deflect. For some tires the manufacturer will publish both a static loaded radius and a dynamic loaded radius.
JRP3
Hyperactive Member
That actually doesn't happen because the steel belts dictate the tire's RPMs (except for a small amount of flexing of the tread compound). All that really happens is that the tire deflects less than it would if the car was stopped. (Drag racing tires are a whole 'nuther ballgame).
if the tires deflect less, how can the power required to overcome rolling resistance be linear in speed? Or are you saying that although this power is approximately linear in speed in reality there small deviations from strict linearity (which is certainly believable)?
ThosEM
Space Weatherman
I've read most of this, and I believe it can all be summarized for practical purposes as:
0. Use the brakes only when absolutely necessary.
1. Keep the power meter low as possible (including negative numbers) while maintaining target traveling speed.
(this is what cruise control does)
Did I miss anything? ;=)
0. Use the brakes only when absolutely necessary.
1. Keep the power meter low as possible (including negative numbers) while maintaining target traveling speed.
(this is what cruise control does)
Did I miss anything? ;=)
ThosEM
Space Weatherman
How you can say that? Cruise control adds only the power needed to maintain speed, and it uses regen as much as possible to prevent the speed from rising above the target speed, recapturing energy when that is possible. Am I misunderstanding your statement?
Incidentally, here's a little calculation of the effect of regen on driving through mountainous country. I haven't yet tried this out carefully on a mountain trip, but have seen reports that driving through the Alps increases average consumption by only about 5% compared with driving on the level, assuming the average is taken between points at the same altitude. That's very impressive and here is how I think it is achieved:
-- Total energy for trip with altitude gain/loss
M:=5000/2.2:; --mass, loaded
M:2.27e+03--kg
g:=9.8:;--m/sec^2
Cflat:=180:;--wh/km rated consumption
Cup:= M*g*1000/3600:;--wh/km
Cup:6.19e+03;-- Wh/km altitude gain
Cdn=Cup*0.85
D:=200:;--distance in km
upAlt:=2.000:;--altitude gain
dnAlt:=2.000:;--altitude loss
netAlt=upAlt-dnAlt;
netAlt:0;-- km
Eflat=Cflat*D;
Eflat:3.6e+04; --kWh
Ehill=Eflat + upAlt*Cup - dnAlt*Cdn
Ehill:3.79e+04; --kWh
Ehill/Eflat:1.05
To summarize, for a 200 km trip with 2000 m of altitude gain and loss, assuming 85% recovery of the energy used climbing when descending, we expect only 5% increase in the total energy use compared with a flat 200 km trip. Without regen (Cdn=0), this would be a 34% increase instead of 5%. Coasting wouldn't help at all, though the downhill parts of trip might be more exciting.
Incidentally, here's a little calculation of the effect of regen on driving through mountainous country. I haven't yet tried this out carefully on a mountain trip, but have seen reports that driving through the Alps increases average consumption by only about 5% compared with driving on the level, assuming the average is taken between points at the same altitude. That's very impressive and here is how I think it is achieved:
-- Total energy for trip with altitude gain/loss
M:=5000/2.2:; --mass, loaded
M:2.27e+03--kg
g:=9.8:;--m/sec^2
Cflat:=180:;--wh/km rated consumption
Cup:= M*g*1000/3600:;--wh/km
Cup:6.19e+03;-- Wh/km altitude gain
Cdn=Cup*0.85
D:=200:;--distance in km
upAlt:=2.000:;--altitude gain
dnAlt:=2.000:;--altitude loss
netAlt=upAlt-dnAlt;
netAlt:0;-- km
Eflat=Cflat*D;
Eflat:3.6e+04; --kWh
Ehill=Eflat + upAlt*Cup - dnAlt*Cdn
Ehill:3.79e+04; --kWh
Ehill/Eflat:1.05
To summarize, for a 200 km trip with 2000 m of altitude gain and loss, assuming 85% recovery of the energy used climbing when descending, we expect only 5% increase in the total energy use compared with a flat 200 km trip. Without regen (Cdn=0), this would be a 34% increase instead of 5%. Coasting wouldn't help at all, though the downhill parts of trip might be more exciting.
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The problem is that it adds way too much power and way too much regeneration. The best efficiency happens when you slow down going up hills a bit and speed up a bit (not a lot because at some point aerodynamic losses exceed the heat generation) going downhill. Regeneration takes kinetic energy and turns it into electricity and heat. Acceleration takes stored energy and turns it into power and heat. Ideally, you want to keep the power use graph as flat as possible for best efficiency, but cruise control keeps the speed within about one mph and will get very aggressive to maintain that speed.
Historically, the focus of cruise control has been (exclusively?) on speed maintenance. With the advent of EVs, efficiency around that speed maintenance becomes both interesting and important.
We're due for the advent of ECC (efficient cruise control) technology. I started a post eons ago as a starting point for discussion somewhere on TMC. I'd love to see Tesla focus on this area "just a little bit". I think they could get it pretty decent very quickly.
Edit:
Here's the post: Efficiency Miser Mode
2013-09-02 - just over a year ago.
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