32 replies to this topic

[The_Mistro]

ok, i tried to do a experiment, a legit one, not just being a chav in tesco car park, for an assignment at uni! the experiment was to see how average acceleration changes over set distances, the set distances are 100, 150 and 200 metres. now, tesco car park in swansea is pretty big, but not big enough for me to accelerate flat out and then brake (i did this while it was empty by the way) so i could only do the 100 metre sprints, i can make up the rest of the sprits, as the average acceleration rate will get lower over the longer distances with the air resistance buliding up and having to change gear more times etc, but what i need to know is, will a mini acclerate faster from a rolling start, at a constant speed with low revs, than from standing? i personally think it will be slower, some people agree with me and some people dont, so i was just wondering what you guys think? any feedback will be very very helpful! cheers guys!

[captainjack15]

From personal experience, I think it will be lower

That's about all I can put in I'm afraid

[R1minimagic]

Fastest acceleration will be with the car already moving but at a low speed...if that helps.

You could just plot out speed vs time in each gear and see which has the highest gradient.

[The_Mistro]

From personal experience, I think it will be lower

That's about all I can put in I'm afraid

thankyou! im glad im not the only one who thinks this! thanks jack!

[captainjack15]

S'alright

[The_Mistro]

Fastest acceleration will be with the car already moving but at a low speed...if that helps.

You could just plot out speed vs time in each gear and see which has the highest gradient.

but if the car is at low revs, it wont have the power to accelerate quicker, where as the car from standing had the revs set, so it was almost "launching" of the line, would this make any difference?

[Ethel]

It's quite simple really

Force = mass x acceleration

the mass is a constant so you only need to tot up the forces on the car:

The faster you go the greater the wind resistance, rolling resistance, etc pushing you in the wrong direction

But the engine will produce more power at higher rpm.

Any power the engine produces has to be transmitted through the front tyres, so if you can rip them off the road (wheelspin) you were producing the maximum usable power just before - unless you really complicate things & factor in aerodynamic downforce, the fastest acceleration is at the lowest speed where wheelspin can be induced.

[R1minimagic]

If you plotted it all out you would find the answer!!

For example, car already moving but at high revs in low gear could be accelerating faster than a standing start from high revs which needs some inertia to get everything moving...

The only way to find out is to run a test!

Edited by R1minimagic, 25 November 2009 - 01:09 PM.

[Nelson92]

your lower gears are where the most acceleration is surely?
but as said, if you're rolling slowly and not laying down massive amounts of power; a rolling start from 10mph will accelerate hardest.
but do you have to put figures on this to prove it?
theres an iphone app you can use; or get someone with a stop watch to time your runs to 40mph [gps not speedo]
run one standstill to 40
run two rolling start to 40. and then work it out.

[daemonchild]

What tyres are we talking about? What road surface?
Is it windy? Which test are you doing first as the car will get lighter as the fuel gets used up?

Forget all the above rubbish I just wrote. MAKE A SIMPLE MODEL.
And, as said, plot it out.

Good project.

[The_Mistro]

the rolling start is at 15m/s which is 33.55 mph, so even at this speed, at low revs, would the car accelerate faster than from standing? as the gearing is higher and the power isnt there to make the wheels spin, unlike at a standing start?

i am trying to plot it out, but i dont have enough data to do it, as i havent got a large enough flat area to do the experiment to get the data! (im in swansea, and there are no big flat patches where i can do this without doing it illegally or getting caught by speed cameras) i can work it out using the SUVAT equations/equations of motion, theres no trouble there! i have the figures for 100m sprint from standing which is:

100m -> Time = 8.4s -> acceleration = 2.83m/s^2

Edited by The_Mistro, 25 November 2009 - 01:33 PM.

[Ethel]

even at 30mph the wind resistance will be a big factor. Low speed wins, it's only a matter of engine revs...
1st gear x best rpm for power - if you weren't already spinning the tyres. I don't think a standing start matters much, the tyres are always stationary as far as the road surface is concerned so it's only the static friction in a few bearings and the brakes. Inertia has no effect on acceleration: Newton's 1st.

I bet you can find plenty of free fall plots online.

[Nelson92]

the rolling start is at 15m/s which is 33.55 mph, so even at this speed, at low revs, would the car accelerate faster than from standing? as the gearing is higher and the power isnt there to make the wheels spin, unlike at a standing start?

i am trying to plot it out, but i dont have enough data to do it, as i havent got a large enough flat area to do the experiment to get the data! (im in swansea, and there are no big flat patches anywhere, where i can do this without doing it illegally or getting caught by speed cameras) i can work it out using the SUVAT equations/equations of motion, theres no trouble there! i have the figures for 100m sprint from standing which is:

100m -> Time = 8.4s -> acceleration = 2.83m/s^2

then you'll have to lower the rolling start in order to do it at sensible speeds.
if you have grippy tyres and a carefull clutch foot you can do this without spinning the tyres at all.
you did the 100meter run in a car park right? can you not start ten meters further back just to be rolling and then somehow adjust the calculation to take that into account? you won't be going that fast when you pass the start line but with a gps and a few goes i'm sure someone can put a figure to it.

[The_Mistro]

yeah i agree that wind resistance will be a big factor, even at the low speeds, especially as it is accelerating, thankyou everyone for your help and input!

[The_Mistro]

the rolling start is at 15m/s which is 33.55 mph, so even at this speed, at low revs, would the car accelerate faster than from standing? as the gearing is higher and the power isnt there to make the wheels spin, unlike at a standing start?

i am trying to plot it out, but i dont have enough data to do it, as i havent got a large enough flat area to do the experiment to get the data! (im in swansea, and there are no big flat patches anywhere, where i can do this without doing it illegally or getting caught by speed cameras) i can work it out using the SUVAT equations/equations of motion, theres no trouble there! i have the figures for 100m sprint from standing which is:

100m -> Time = 8.4s -> acceleration = 2.83m/s^2

then you'll have to lower the rolling start in order to do it at sensible speeds.
if you have grippy tyres and a carefull clutch foot you can do this without spinning the tyres at all.
you did the 100meter run in a car park right? can you not start ten meters further back just to be rolling and then somehow adjust the calculation to take that into account? you won't be going that fast when you pass the start line but with a gps and a few goes i'm sure someone can put a figure to it.

yeah, but then F=MA comes into it, the car will already have the momentum behind it accelerating, i want to inviestigate from constant initial velocities, i.e from standing the initial velocity is 0mph and the rolling would be from 33.55mph not accelerating from 20mph to cross the start line at 33.55 mph, as the momentum would be helping the car to accelerate!

when i accelerated from standing i wasnt spinning the wheels on all of the runs, just the first couple, but you cant spin the wheels while rolling at 33.55mph even if you push the accelerator through the floor, so surely this suggests the acceleration is better from standing?