please dont laugh if im talking crap!!
Discussion
right i was thinking if a car is fitted with 16inch wheels, and it can do, say 130mph, if you put 30 inch wheels on and keep the EXACT same car wont the car be quicker because the wheels are bigger and for every revolution of the wheel it will cover more of an area,
is this right or am i talking shit! please help!
is this right or am i talking shit! please help!
The tyre size has the final say. A diff/'box combination have a set final drive ratio. Change for a larger or smaller TYRE overal diameter will change this ratio. Just putting larger wheels but with a different profile tyre may even reduce the overal diameter of the tyre therefore the speed you will do at any revs.
Also, over gearing a car could result in it achieving a slower top speed if it does not have enough grunt to reach maximum revs with the higher ratio .......
I'll get me coat!
>> Edited by ultimapaul on Tuesday 18th June 21:09
Also, over gearing a car could result in it achieving a slower top speed if it does not have enough grunt to reach maximum revs with the higher ratio .......
I'll get me coat!
>> Edited by ultimapaul on Tuesday 18th June 21:09
I was assuming we were talking about one particular car with no other mods .... so as you say, the power would be the limiting factor unless you brought the gearing down so low it reached reached maximum revs at a lower speed or you over geared it so it could not reach maximum power!
>> Edited by ultimapaul on Tuesday 18th June 21:29
>> Edited by ultimapaul on Tuesday 18th June 21:29
The larger wheels will effectively increase all the gears. So it'll behave in 1st as though it were in 2nd (or 3rd or 2.5th or whatever) and in 2nd as if in 3rd etc etc.
If in normal top gear the engine hits the rev limiter at its top speed, then increasing the gears like this (or alternatively by changing the final drive which would look less silly
) may well increase the top speed by dropping the engine speed towards its peak power.
However if it reaches top speed below top rpm, or at peak power, then it almost certainly wouldn't have the beef to push the higher gears any faster.
If in normal top gear the engine hits the rev limiter at its top speed, then increasing the gears like this (or alternatively by changing the final drive which would look less silly
) may well increase the top speed by dropping the engine speed towards its peak power. However if it reaches top speed below top rpm, or at peak power, then it almost certainly wouldn't have the beef to push the higher gears any faster.
The first car I ever had was a 1000 cc Mini. It was all the rage at the time to take out the mini engine and substitute it with the Austin 1100 engine and gear box which would fit quite well.
It took me quite while to work out why it wouldn't stay in 1st gear but would accelerate form 0 to 60 in an incredibly short time.
The penny dropped one day when I stopped next to an Austin 1100 at some red lights. His wheels were substatially bigger than mine. The gearbox was designed for the larger wheels not mini wheels. There was so much torque in the drive in 1st that it could not cope.
It took me quite while to work out why it wouldn't stay in 1st gear but would accelerate form 0 to 60 in an incredibly short time.
The penny dropped one day when I stopped next to an Austin 1100 at some red lights. His wheels were substatially bigger than mine. The gearbox was designed for the larger wheels not mini wheels. There was so much torque in the drive in 1st that it could not cope.
quote:
right i was thinking if a car is fitted with 16inch wheels, and it can do, say 130mph, if you put 30 inch wheels on and keep the EXACT same car wont the car be quicker because the wheels are bigger and for every revolution of the wheel it will cover more of an area,
is this right or am i talking shit! please help!
remember those old sturmey archer gears you got on your old pedal bike ?? well, Slapping 30 inch rims on you rmotor will have the same effect ( sort of ) of pedaling in sturmeys top gear. You see, its easy to pedal in 1st gear, but when you step up a few, into 3rd!!! its gets harder to push. Anyway, although the wheels will spin faster, it will be harder work for the engine to spin em, and so, they cancel each other out and you get exactly the same speed. Bummer aint it
Whats needed is bigger wheels and a bigger engine, thats what i say. Then all this confusion will be long lost and we will all be happy. What does everyone else think ? >> Edited by RobM on Wednesday 19th June 02:13
I agree with Rob M, The engine doesn't know what size wheels are on the car so th gearbox thingy will spin the axle at the same speed regardless of what wheel size. Or at least it will try tobecause it needs more power to compensate for the weight increase.
However; if your 16 inch wheels weighed the same as the 30 inch wheels then i'm presuming the axle would turn at the same rate. If it turns at the same rate it can't be laying down more tarmac, so, the speed remains the same.......Teds right!
However; if your 16 inch wheels weighed the same as the 30 inch wheels then i'm presuming the axle would turn at the same rate. If it turns at the same rate it can't be laying down more tarmac, so, the speed remains the same.......Teds right!
Petrol Ted is right power is the limiter on top speed. Cars are generally geared by use of gearbox, diff and wheel size (rolling circumference) to achieve the top speed at peak power in top gear. Change the wheel size and you alter the overall gearing affecting acceleration and possible top speed. I had a modified 240Z at one time, the gearbox blew and the only spare I had was an American spec 4 speed item. This was normally used with a different ratio diff. Top speed was impressive (160 mph thanks to lots more power than a standard engine) but acceleration was poorer, the car could actually achieve 65 mph in first, and was difficult to get moving. I found a UK spec 5 speed gearbox asap! Back to around 5 sec to 60 and a 152 mph top speed.
I would've thought the critical limiting factor, other than engine speed, is the wind resistance of the car. If you could get your car to work in a vacuum, your speed would be limited to the peak rpm of the motor and the gear and diff ratios - and doubling wheel size would double your speed. However, air drag increases exponentially with speed so that is where all the power goes. The other thing to note is that the speedo will generally give you the speed that you'd be doing at that rpm with factory tyres, so it would read 30 while you're doing 60.
JohnL's descrption last night was absolutely correct.
If you bounce off the rev limiter in top gear then you could go faster with longer gearing, and one way of achieving this is to increase the external circumfrence of the tyre (i.e. the bit in contact with the road). End of story.
In practice you can't normally hit the rev limiter in top. As has been said, your air resistance normally balances your engine's power at some point lower in the rev range. In these circumstances you can either reduce drag or increase available power (more powerful engine, or maybe even shorten the gear ratios so you are further up the power band in top gear).
If you bounce off the rev limiter in top gear then you could go faster with longer gearing, and one way of achieving this is to increase the external circumfrence of the tyre (i.e. the bit in contact with the road). End of story.
In practice you can't normally hit the rev limiter in top. As has been said, your air resistance normally balances your engine's power at some point lower in the rev range. In these circumstances you can either reduce drag or increase available power (more powerful engine, or maybe even shorten the gear ratios so you are further up the power band in top gear).
Teds right, power is the limiting factor (though a pedant might argue its actually the torque).
I once converted a Yamaha RD400 to watercooling, using Shepherd 440cc racing cylinders ('twas featured in 1985 Bike mag as it happens.) This thing had at least twice the power of the base engine, and despite revving to 13,000 rather than 8,000, it needed gearing up somewhat. Couldn't fit larger wheels/tyres, but bikes have the advantage of being able to change sprockets to alter gearing. A combination of tiny drive sprocket and huge rear sprocket helped, which is kind of analagous to putting bigger wheels on, which is the reason for this ramble!
It still wheelied like crazy. I never did max it out though, and standard speedo only read to 120 - went off that by some margin. Unfortunately it had a habit of eating pistons (race two strokes don't like idling, piston skirts keep fouling in bucket sized ports) so didn't last long as a road bike.
I once converted a Yamaha RD400 to watercooling, using Shepherd 440cc racing cylinders ('twas featured in 1985 Bike mag as it happens.) This thing had at least twice the power of the base engine, and despite revving to 13,000 rather than 8,000, it needed gearing up somewhat. Couldn't fit larger wheels/tyres, but bikes have the advantage of being able to change sprockets to alter gearing. A combination of tiny drive sprocket and huge rear sprocket helped, which is kind of analagous to putting bigger wheels on, which is the reason for this ramble!
It still wheelied like crazy. I never did max it out though, and standard speedo only read to 120 - went off that by some margin. Unfortunately it had a habit of eating pistons (race two strokes don't like idling, piston skirts keep fouling in bucket sized ports) so didn't last long as a road bike.
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