Useful calculator for inputting wheel and tyre size to get overall tyre diameter for old and new combination, giving percentage change in diameter and hence speedo error, revs at a fixed speed etc.:
www.tyresave.co.uk/tyresize.html
|
|
|
I guess the tyres rubbing on the inner arches would probably have a braking effect as well :)
Taking a typical warm hatch type car, anyone want to hazard a guess at exactly what sort of impact it might have on 0-60?
Just trying to get a feel for if it would be a noticable difference, i.e. I doubt that anyone would notice 0.01 sec off the 0-60, but 0.5 of a sec you would.
|
|
Depending on the gearing it might mean that you can hit 60 with one less gear change, and actually improve the time, although it would feel slower.
|
Depending on the gearing it might mean that you can hit 60 with one less gear change and actually improve the time although it would feel slower.
Very good point, well spotted!
Please don't tell me that dustbin size exhausts also improve performance, that would really spoil my day :)
|
|
|
|
Using my performance spreadsheet, and data for our 1.6 8 valve Astra
With standard wheels, the rolling radius is 0.3m and the 0-60 is predicted as 11.04
With wheels 2" bigger overall, the rolling radius is 0.326, and the 0-60 is predicted as 11.17
The spreadsheet doesn't include any change in Cd between the two cases, aerodynamic drag remains as per the standard car.
|
NC - thanks for doing the math!
I would consider 0.13 of a sec to be a significant impact on performance.
Could you also calculate what would happen if you now add in 20kg of subwoofers?
And just for interest, would the effect be increased or decreased with a higher performance car? Say something with 0-60 of around the 6 sec mark?
PS - is it math or maths?
|
Moonshine, unlike you I don't worry about 0.13 of a second from 0-60.
What causes me to worry is these blinged up cars effectivly being clocked. The wheel circumfercence figures from NC are
0.3m Rolling raduis for standard wheels ,With wheels 2" bigger overall, the rolling radius is 0.326.
That means that a car blinged up when new will have covered 32,600 miles when the odomiter says it has covered 30,000.
-clocking is illegal, so could we outlaw any uncalibrated odometers? It would get the Chav cars off the road!
|
mk124, unlike you I dont worry about chav cars being clocked. Chav cars will be sold on to other chavs further down the food chain.
Does anyone care if the odo on a chav corsa is out by a few thousand?
Unless it can be used the get them off the road... :)
Don't get me wrong - I don't have a problem with people modding their cars - each to their own. I just find it ironic that performance enhancing mods are sometimes anything but...
|
|
|
|
|
Moonshine: maths in English, math in American.
Edited by Lud on 04/09/2008 at 14:21
|
Moonshine: maths in English math in American.
I'll take the English version to be the correct version :)
Thanks Lud.
|
|
|
|
And just for interest would the effect be increased or decreased with a higher performance car? Say something with 0-60 of around the 6 sec mark?
The effect would be the same, but the significance of it would decrease.
PS - is it math or maths?
It is maths, unless you speak with a lithp.
|
Doubling engine power and torque from my previous example gives;
0.3m - 6.11s
0.326m - 6.19s
|
|
|
All completely irrelavent! Generally when increasing wheel sizes you drop tyre profile typically 1 inch on wheel size drops 10% in profile & overall rolling radius stays the same.
The big issues are wheel arch clearance & steering lock limitations. When you go manically big suspension & bearings start to give hassle but it has to be massively bigger.
|
>>All completely irrelavent!
No, the question asked by the OP included these words...
>>Assuming that the oversize wheels have increased the gearing
and that's what the answers have been based upon.
Yes, rolling radius can be kept constant, but, that's not always the case.
|
|
|
Regarding the exhaust scenario, if the exhaust were to be too big, the gases would be looser coming out due to the loss of presure in the bore.
So fitting a big cherry bomb silencer on a 1.1 Saxo would redeem it less powerfull unless it were to be tuned right.
Bigger wheels would mean it would take more weight, like sub woofers & more underage passengers etc...
|
|
|
|
|
So, should we all be driving around with little piddly wheels/tyres to improve performance then?
|
It depends by what you mean by the word performance.
|
|
I can't remember why - something to do with the brakes perhaps - but I changed the back axle on the Lada 1200 estate we had at one time for a s/h unit from a Lada 1500 saloon, which had a slightly higher final drive ratio. I can't say I noticed any real difference on the road - it was a car of such surpassing fundamental coarseness that subtle differences were hard to spot - but I suppose it must have been going a couple of miles an hour faster than previously for a given speedometer reading.
|
|
|
|
So should we all be driving around with little piddly wheels/tyres to improve performance then?
Good point- doesn't the predicted increase in 0-60 assume that the car isn't 'under geared'? That is, there must come a point at which decreasing the diameter of the wheels doesn't improve the 0-60 because of the engine's power/torque characteristics - at what sort of diameter would this happen?
|
I suppose the ideal would be to have a CVT where peak power/torque can be applied all the time. I don't think I fully understand your point though.
To get the best 0-60 time would it be best to gear low (wether by means of wheels, diff, gearbox or all 3) giving a top speed of 60mph in 5th gear? The down side would be additional gearchanges.
Or would you want to hit 60mph in 3rd gear just after peak power/torque?
Is that the point you are making?
|
>>I suppose the ideal would be to have a CVT where peak power/torque can be applied
If you have a CVT, and want the best acceleration, you need to keep the engine running at the speed for maximum power.
If you have a manual gearbox, in any particular gear, you get the best acceleration at the engine speed for peak torque.
|
If you have a CVT and want the best acceleration you need to keep the engine running at the speed for maximum power.
I had the use of an audi for a day with the CVT.
I had great fun flooring it off the lights. The revs went straight to around 5500rpm and stayed exactly there while the speedo went from zero to something naughty...
|
|
I don't think I fully understand your point though.
Not sure myself now...
I was thinking that we have said increasing the wheel diameter increases the 0-60 time. That implies decreasing it would decrease the 0-60 time. But presumably you can't keep doing that, keeping the transmission gearing the same (which we have assumed when increasing the wheel diameter), because you will run out of gears and won't be able to keep the engine in it's optimal rev range,
Or something like that.
|
If you reduced the gearing so much that the top speed was only 59mph then your 0-60 time would infinate.
|
|
|
A car being under or over geared relates purely to the relationship between the rpm for peak power, and the top speed of the vehicle.
For 0 to 60, it doesn't matter, and in all sensible cases (barring those regions where a gear change may be eliminated), reducing the overall gearing of the vehicle will result in a faster 0-60.
there must come a point at which decreasing the diameter of the wheels doesn't improve the 0-60
That's the point where 60 mph can't physically be reached any more!
The serious point is that cars aren't made solely to acheive good 0 to 60 times (this is what I was hinting at above when I asked about the definition of performance). The spacing of the intermediate gear ratios may be tweaked a bit to help the figures, but, in general, it's not a serious design aim.
|
That's the point where 60 mph can't physically be reached any more!
But if it topped out at exactly 60, those last few mph must take some time to reach, resulting in a slower 0-60 than if it topped out at say 70?
EDIT maybe not, because it's so low geared, engine revs to max pretty quickly
Edited by Focus {P} on 04/09/2008 at 15:15
|
I think the ideal would be to hit 60 just as the peak power starts to decline, say at 6000rpm rather than 7000rpm.
Or maybe you would want to be bouncing off the rev limiter? I'm not so sure now either...
|
>>EDIT maybe not
No, I think you're right, and I'm wrong - I went too far.
For practical purposes, most cars are quite a long way away from the optimum for 0-60.
|
A bit of playing about with the spreadsheet shows that increasing the overall gear ratio (same effect as making the wheels smaller) does consistently reduce 0-60 time, even when taken to extremes.
The only thing offsetting this reduction is the extra time spent changing gear. In the spreadsheet, I've assumed a failry leisurely 0.8 seconds for a gear change. Of course, with CVT this isn't an issue.
|
Competition drivers use different ratios for racing and hill-climbing, for obvious reasons, and in the more serious formulae they will have different ratios for different race circuits. I believe the Porsche 911 GT3 is specially designed to facilitate rapid gearbox changes.
I have always fancied a chain-drive Frazer Nash, with a big box of spare sprockets and chains.
|
|
What makes me laugh even more than boy racer Corsas with bling drug-dealer wheels is upmarket 4x4s fitted with them, especially if all sorts of off-road accessories like bull-bars and snorkels are fitted too! These fat tyres would be useless off-road. Maybe we're supposed to assume that the owner has a separate set of beefy off-road wheels and tyres!
|
I think it would probably not be good for the geartrain. On another forum some youngsters(I presume) are finding their gearboxes giving up the ghost at very low milages.
The manufacturers have their own "Number Crunchers" to work out the optimum ratios before they start production of any model. Play with these at your peril,and cost.
|
|
|
|
|
|
|
|
|
|
