How many G's does a car pull under acceleration?

How many G's does a car pull under acceleration?

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topless_mx5

Original Poster:

2,763 posts

217 months

Wednesday 16th June 2010
quotequote all
My mate has a G-force recording app on his iPhone. It records the acceleration and deceleration G's during a journey. He has so far managed a maximum of 0.6 G acceleration on his CBR600 whilst nailing it in 2nd.

Is there somewhere I can find out the max G's various cars can pull? Would be interesting to see how it stacks up against supercars etc/.

V8mate

45,899 posts

188 months

Wednesday 16th June 2010
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Aren't G-forces 'turning forces'?

kambites

67,460 posts

220 months

Wednesday 16th June 2010
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No, G-forces are usually used as a measure of acceleration where "1" is acceleration due to gravity.

Obviously how much a car will pull depends on how fast it's going at the time. Off the line, you're generally governed by traction, at higher speed by power to weight and power to drag ratio.

Edited by kambites on Wednesday 16th June 09:45

Thudd

3,100 posts

206 months

Wednesday 16th June 2010
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G forces are acceleration, deceleration, or turning - these are all types of acceleration (forward, negative, and centripetal).

jon-

16,496 posts

215 months

Wednesday 16th June 2010
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Probably about the same for a fast car, up to about 1.2G under braking.

Worth noting those "G apps" on the iphone are pretty useless. The GPS only runs at about 1htz, so only updates once a second and then has to calculate the mean G.

The Wookie

13,909 posts

227 months

Wednesday 16th June 2010
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A relatively quick RWD mid engined car I'd expect about 0.75G at launch, and that's limited by the traction of the tyres

A quick AWD car you might see over a G on launch.

It tails off after that

MX5 Withdrawal

677 posts

247 months

Wednesday 16th June 2010
quotequote all
G-forces can be felt in all planes of movement.

It is a comparison to gravitational force, hence why astronauts pull 5g or so going straight up to escape the atmosphere.

The easiest way to generate g-force is by turning tight circles quickly hence the training in a centrifuge undertaken by astronauts.

I seem to remember a big hoha associated with a car pulling 1g in straight acceleration but cant remember which one....

grahamr88

421 posts

172 months

Wednesday 16th June 2010
quotequote all
jon- said:
Worth noting those "G apps" on the iphone are pretty useless. The GPS only runs at about 1htz, so only updates once a second and then has to calculate the mean G.
Do they not use the accelerometer?

FasterFreddy

8,577 posts

236 months

Wednesday 16th June 2010
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It depends on the type of car, it's power to weight ratio, gearing, type of tyre, road surface, conditions etc. etc. but a road car would be hard pressed to pull more then about .5G under acceleration and 1G under braking.

The built-in accelerometers, sound level meters etc. in phones are getting better, but they're not accurate enough to replace a specialised dedicated data logger.

Fine for a quick comparison between different cars but I wouldn't rely on it to do anything serious like testing tyres, brakes etc.

MX5 Withdrawal

677 posts

247 months

Wednesday 16th June 2010
quotequote all
My fag packet calcs show that you would need to be pulling about a 9 second quarter mile with constant and smooth acceleration to be pulling 1g throughout the process.

clarkey

1,365 posts

283 months

Wednesday 16th June 2010
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I had a Caterham R500 that was 1g up to about 60 miles an hour. An Impreza was 1g up to about 30mph.

The Wookie

13,909 posts

227 months

Wednesday 16th June 2010
quotequote all
MX5 Withdrawal said:
My fag packet calcs show that you would need to be pulling about a 9 second quarter mile with constant and smooth acceleration to be pulling 1g throughout the process.
Close enough

MX5 Withdrawal

677 posts

247 months

Wednesday 16th June 2010
quotequote all
The Wookie said:
MX5 Withdrawal said:
My fag packet calcs show that you would need to be pulling about a 9 second quarter mile with constant and smooth acceleration to be pulling 1g throughout the process.
Close enough
Phew!

otolith

55,899 posts

203 months

Wednesday 16th June 2010
quotequote all
The iPhone accelerometer apps I've seen need to be firmly mounted in a fixed orientation to give accurate results, so not so many people have recorded accurate figures on bikes. The other issue is that the maximum instantaneous acceleration recorded may have only occurred for a fraction of a second and not be representative of what the vehicle can actually sustain.

The highest peak longitudinal acceleration I recorded for the Elise was 0.81g, and for the RX-8, 0.61g, but those are brief peaks in 1st gear, you'd get a better idea of what sort of sustained acceleration a car can pull from the road test figures. To get the average acceleration in g from 0-60, divide 2.74 by the 0-60 time (or 2.83 by the 0-62 time).

(Edit for reciprocal)



Edited by otolith on Wednesday 16th June 10:30

RobM77

35,349 posts

233 months

Wednesday 16th June 2010
quotequote all
A g force is an acceleration force. Because the feeling of gravity is the same as the feeling of acceleration (Einstein's Equivalence Principle), when we're sat on the surface of the earth (as we all are now), we're said to feel "1g". This is equivalent to an acceleration of about 10m/s/s (so if you accelerate from rest to 10m/s in 1 second, you feel "1G", or perhaps to 30m/s in 3 seconds, 40m/s in 4 seconds etc). Incidentally, the usual analogy of the equivalence principle is that a man in deep space accelerating upwards in a lift at 10m/s/s wouldn't feel any different to a man sat still on the earth. Using this, you can easily work out the "g force" of any acceleration (it's actually 9.81m/s/s if you want to be more accurate), by just dividing the change in speed in m/s by how long it took, and then to get the g force, divide by 10, or 9.81. Don't quote me on this, but I think 0-60 in about 3 seconds is 1g (edited to say: stated below as 2.74s). Most road cars will pull approaching 1g in braking and cornering, because that's the frictional limit of your average road tyre against your average tarmac (probably the reason most road cars don't dip below 3 seconds 0-60...). Typically, a normal road car or motorbike will get up to about 0.7-0.8g, and something like an Elise may just about breach 1G. That's in cornering and braking. American magazines test this for cars, so you can find this info on the web. Junior level single seaters such as I race will get around 2-3g, thanks to very soft slick tyres (they give their best after about 40-50 miles...) and modest downforce. Incidentally with only 350-400bhp/tonne (the same as a Caterham R400), a Formula Renault will do 0-100mph in 4.9 seconds, such is the grip on offer!). Formula One cars can pull up to 5G. When g is experienced downwards or upwards, such as when banking hard in a fighter jet, the blood rushes away from or towards your head, causing black-out or red-out. I think the limit for most fighter pilots is about 7-8g, but correct me if I'm wrong. People vary enormously in their tolerance of vertical g, and the soviets actually select their pilots early on in training based on this criteria. I'm terrible, and blackout if I stand up too fast! Obviously the cars I race though only pull g from side to side and front to back, so I'm ok, provided my neck muscles are strong enough. Personally, I love lateral g; there's nothing quite like the feeling of cornering at over 100mph - at Snet I'm taking Coram flat at about 115mph which is great smile I'd love to try a higher downforce car like an F3.

HTH

Edited by RobM77 on Wednesday 16th June 11:06

McSam

6,753 posts

174 months

Wednesday 16th June 2010
quotequote all
otolith said:
The iPhone accelerometer apps I've seen need to be firmly mounted in a fixed orientation to give accurate results, so not so many people have recorded accurate figures on bikes. The other issue is that the maximum instantaneous acceleration recorded may have only occurred for a fraction of a second and not be representative of what the vehicle can actually sustain.

The highest peak longitudinal acceleration I recorded for the Elise was 0.81g, and for the RX-8, 0.61g, but those are brief peaks in 1st gear, you'd get a better idea of what sort of sustained acceleration a car can pull from the road test figures. To get the average acceleration in g from 0-60, divide the 0-60 time by 2.74 (or by 2.83 for a 0-62 time).
^ This.

You can also glean from this that, to have an average acceleration of 1.0G (9.80665 m/s^2) all the way to 62mph, you need to get there in about 2.826 seconds - pretty quick, in other words wink

The rest of what's been said here covers pretty much everything, except to say that if you want to find out how much G a particular (longitudinal) acceleration generates, you should convert it from a speed change in miles/kilometres per hour to a change in metres per second, divide by the time the acceleration took, then divide that by 9.81 (which is approx. the value of g at the Earth's surface).

Miles per hour, you'd do this by multiplying by 1.609, dividing by 3.6, then by the time, then by 9.81
Kilometres per hour, you just need to divide by 3.6 and the time before doing dividing by 9.81.

For example, if my car reaches 62mph in 8.5 seconds, the average acceleration over that 8.5 seconds is:
Change: 62mph 

= (62*1.609) kph
= 99.758kph

= (99.758/3.6) m/s
= 27.711m/s

= (27.711/8.5) m/s^2
= 3.260m/s^2

= (3.260/9.81) G
= 0.332G


To cut corners, divide the change in speed in miles per hour by ~21.949 and the time and you're there smile

EDIT - Also good stuff from RobM77!

Edited by McSam on Wednesday 16th June 10:36

esuuv

1,306 posts

204 months

Wednesday 16th June 2010
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So - anything that can do 0-60 in less than 2.74 seconds will be doing an average of 1g

So that puts you in Veyron territory

mat205125

17,790 posts

212 months

Wednesday 16th June 2010
quotequote all
otolith said:
The other issue is that the maximum instantaneous acceleration recorded may have only occurred for a fraction of a second and not be representative of what the vehicle can actually sustain.
yes

Some of the frightfully controlled gear changes, that one of the chaps I lift share to work with, performs would generate instantaneous G that would put superbikes and Veyrons to shame.

FlyInMySoup

81 posts

183 months

Wednesday 16th June 2010
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Caterham 7 race car 185bhp on Avon slicks 0 - 60 in 4.3 secs 0.82g peak for first second. 0.58 just before 60. According to my datalogger.

Same car on road tyres 0 - 60 in 5.3 secs peak of 0.71g 3 secs in.

monthefish

20,439 posts

230 months

Wednesday 16th June 2010
quotequote all
grahamr88 said:
jon- said:
Worth noting those "G apps" on the iphone are pretty useless. The GPS only runs at about 1htz, so only updates once a second and then has to calculate the mean G.
Do they not use the accelerometer?
Are all i-phones equipped with an accelerometer just in case someone downloads a G app?