Calculating your cornering G force
Dave Baker
round a tight 90 bend at 60 mph in the wet in his crapmobile I've been doing
some calcs on the cornering power of my own Focus 2.0 ESP. Using
multimap.com to measure the corner radii and knowing the exact speed because
my speedo has been calibrated I've come up with some fascinating results.
You can easily do the same.
Firstly the equations again.
Cornering G force = speed (mph) squared / corner radius (ft) / 14.957
The first bend is the roundabout at Beaconsfield at the junction of the A40
and A355. Multimap ref
http://www.multimap.com/maps/?hloc=GB|51.60105,%20-0.62404
Zooming right in and using a ruler against the scale on the map I measure
the corner radius of the outside lane to be exactly 200 ft. The road has
Shellgrip on it so you stick to it like that stuff to a blanket and the
speed is low enough to not quite exceed the limits of my bravery so I can
take the car right to the edge. Speed is 55 mph indicated which on my speedo
is 51 mph true. Cornering G force calculates to be
51 x 51 / 200 / 14.957 = 0.87G
That seems pretty damn fair for the road surface and the capabilities of the
car. Ford quote 0.82G for the Focus ST but of course road surface and tyre
brand have a massive influence. Shellgrip must be worth 0.05G I reckon. The
Tirerack site in the states regularly gets 0.9G plus from their test BMWs on
top rubber on grippy test track tarmac.
Second corner is the offramp from the M25 clockwise onto the M40 eastbound.
It's a higher speed corner and the consequences of losing it at those speeds
mean I can't quite get to the tyre squeal stage. There's also a nasty bit
right at the end where the bend actually tightens so when you're on the
limit in the first bit you don't half get a touch of brown trouser syndrome
when you hit the kink. If you zoom in you can even see the kink on the map.
It's a bleedin' good corner for getting your own back on the straight line
hero who was up your arse in the fast lane but starts shitting himself at 60
indicated in the bend and then you power round the outside of him at 70
plus. Definitely moistens the palms every time I really give it some round
there but very nice for the ego.
http://www.multimap.com/maps/?hloc=GB|51.56225,%20-0.53506
Corner radius measures at exactly 400 ft and my best speed on the speedo so
far is 75 indicated which is 70 true. G force is
70 x 70 / 400 / 14.957 = 0.82G
Again seems spot on for standard tarmac and a Focus on good 205/50s.
I'm amazed that A) this seems to all work out spot on and B) I've never
thought of doing it before. I keep meaning to put the G Pro meter in the car
and see what it says but haven't got round to it.
You do need to know accurate speeds though. Just going by the speedo is a
waste of time if you don't know the error given that there's a square law in
the calculation and speedos can be up to 10% fast.
Anyone else fancy a try with their favourite corner?
--
Dave Baker
Puma Race Engines
adder1969
>
> Corner radius measures at exactly 400 ft and my best speed on the speedo so
> far is 75 indicated which is 70 true. G force is
>
> 70 x 70 / 400 / 14.957 = 0.82G
>
> Again seems spot on for standard tarmac and a Focus on good 205/50s.
What effect does that have on your microwave meal for one in the
boot? :-)
Steve B
"Dave Baker" <Nu...@null.com> wrote in message
news:fkgfof$o93$1...@news.datemas.de...
You may get more accurate roundabout dimensions from Flash Earth using the
Microsoft VE mapping option.....www.flashearth.com/ as there are much larger
images but sadly there's no distance readout, so the longitude/latitude will
have to be used as a guide and recalculated. There's calculators at
http://jan.ucc.nau.edu/~cvm/latlongdist.html and
http://www.csgnetwork.com/latlongdistcalc.html which in north Kent gives a
readout of 61.9ft per minute of longitude, 62.9ft in Beaconsfield. I make
your roundabout 214ft radius not 200ft, but check it for yourself.
I have a safe test 30mph-ish roundabout where nothing has ever gone faster
than me when I've been on it. I'll look at my speedo next time and plug the
figures in. Interesting.
Peter Hill
Messy.
You know you are having fun when inanimate objects in the car become
excitedly animate. I've had stuff placed on pass seat bounce off side
glass and even sunroof before I've batted them down into pass
footwell.
--
Peter Hill
Spamtrap reply domain as per NNTP-Posting-Host in header
Can of worms - what every fisherman wants.
Can of worms - what every PC owner gets!
Steve B
My first test of this seems a bit off at lower speeds, but i'm wondering if
it's due to the slight road camber or perhaps at lower speeds the formula
isn't accurate. This is in damp 5°C conditions on a 40mph speed limit
roundabout in Canterbury at 51°16.43N 1°04.24E. 35mph indicated (real
33mph) and still very safe on the inside lane, no drifting at all. I make
the radius at my outer tyre 76ft. That gives a 33x33/76/15 figure of
G=0.96. Speedo has been calibrated only by stop watch and motorway
red/blue 1/10km markers at 50mph 60mph and 70mph. It seems to be a constant
4% out as far as I can tell. Driving a 1997 Primera 2.0 with 205/55R15
Continental PremiumContact tyres.
Dave Baker
"Steve B" <sbradsPUTTW...@ukfsn.org> wrote in message
news:fl59b3$2445$1...@energise.enta.net...
> My first test of this seems a bit off at lower speeds, but i'm wondering
> if it's due to the slight road camber or perhaps at lower speeds the
> formula isn't accurate. This is in damp 5°C conditions on a 40mph speed
> limit roundabout in Canterbury at 51°16.43N 1°04.24E. 35mph indicated
> (real 33mph) and still very safe on the inside lane, no drifting at all.
> I make the radius at my outer tyre 76ft. That gives a 33x33/76/15 figure
> of G=0.96. Speedo has been calibrated only by stop watch and motorway
> red/blue 1/10km markers at 50mph 60mph and 70mph. It seems to be a
> constant 4% out as far as I can tell. Driving a 1997 Primera 2.0 with
> 205/55R15 Continental PremiumContact tyres.
The formula is a mathematical constant but at low speeds it only takes a
small numerical error in speed or radius to make a large difference in the
result. 32 mph would be 0.9g and 31 mph 0.85g. Speedos usually have a larger
percentage inaccuracy at low speeds than high. Mine for instance is doing a
true 26.8 mph at indicated 30 (11.7% error), true 36.4 at ind 40 (9.8%
error) and settles to about 6% fast above 70 mph. In fact the error is
closer to a constant 4 mph than to a percentage until it reaches high
speeds.
Unless you've calibrated the speedo at the speeds you are actually cornering
at the results are too imprecise to be relied on.
The effects of camber mean ploughing through a lot of rather unpleasant
equations. I've just spent the last hour doing that and at the sort of grip
levels normal cars experience every incremental 1 degree of camber increases
the potential speed of the car by about 1.8%. So a 5 degree camber would
increase the potential speed by 1.018^5 = 1.093. A 31 mph curve would become
a 33.9 mph one.
This relationship stays fairly linear up to cambers of about 25 degrees so
more than enough for anything you'd see on a public road.
If you add the one or two possible mph speedo error to the few mph possible
camber error you can obtain G forces varying by a huge amount. You need to
try the experiment at higher speeds and in the range your speedo has been
calibrated at.
Peter Hill
wrote:
You need to survey the curve with a sprit level to find the camber. If
you wind up as road kill it's not my fault.
Any slight grade will change it too. Unless it's dead flat you are
effectively accelerating or decelerating which changes the traction
available for cornering.
vb2...@gmail.com
DavidR
news:2b195ad9-4f65-45af...@googlegroups.com...
a number.
Tim Watts
uk.rec.cars.maintenance:
To go around a corner (well, to take a path along a circular arc to be
exactly) requires a certain force to be applied to the car perpendicular
to the direction of travel (centripedal force).
That force must be balanced *exactly* (Newton's 3rd law) by the forces
applied to the mass of the car by the tyres and possibly by other
sources, eg aerodynamics and gravity.
We can probably ignore aerodynamics unless the car has movable fins.
Gravity will assist if the road is banked. Once that's been taken off,
the rest of the force must be provided by the tyres - not necessarily in
equal amounts.
--
Tim Watts Personal Blog: http://squiddy.blog.dionic.net/
http://www.sensorly.com/ Crowd mapping of 2G/3G/4G mobile signal
coverage
adrian...@gmail.com
Peter Hill
It's the conversion of mph to ft/sec to be consistent with the units of
radius in feet and G in ft/sec^2.
g acceleration = ( (v mile / hour ) (5280 feet/mile) (1 hour / 3600
secs) )^2 / ( (radius (feet)) * (32.174 feet / sec^2)
Reference
https://www.physicsforums.com/threads/help-calculating-the-g-force-in-a-corner.727746/
(5280 / 3600)^2 / 32.174 = 2.151 / 32.174 = 14.957.
50 mph 1/20th mile radius
0.632
In metric, as G = 9.81 m/sec^2 have to convert km/h to m/sec.
G's = V^2 (km/h) * (1000/3600)^2 / R (m) / 9.81
G = V^2 / (R * 127.1)
80.45 km/h 80.45m radius
0.632