Whats the answer, who knows?

[Peter103]

Imagine you have two identical vehicles same power, torque, gearbox etc.All four wheels on one, have a greater diameter of 6 inches over the other.
Would the vehicle with the big wheels, 1. Have a slower rate of acceleration? 2. Have a higher top speed, when both are at their limit? 3. Have better fuel efficiency, ie. MPG?
Who knows? :

 

[Deleted member 113659]

Yes. I've wondered this in the past. It's an engineering question for sure. Physics

I think one may outweigh the other.

My transit van had 15" steal wheels when I bought it new. The sat nav said I was doing 66.5mph and my speedo read 70mph. Then I bought some (mid life crisis) 18" alloys. Now the sat nav reads 71mph and the speedo 70mph.

Hell, I kinda bored myself then. Not sure what I'm trying to say.

Ant.

 

[streethawk]

Imagine you have two identical vehicles same power, torque, gearbox etc.All four wheels on one, have a greater diameter of 6 inches over the other.
Would the vehicle with the big wheels, 1. Have a slower rate of acceleration? 2. Have a higher top speed, when both are at their limit? 3. Have better fuel efficiency, ie. MPG?
Who knows? :

I'd say question 1 the larger wheel would have a fast acceleration as it's all about surface speed ie larger diameter would travel further on one rotation than the smaller one so that should answer questions 2 and 3 with a yes.

 

[Deleted member 113659]

I'd say question 1 the larger wheel would have a fast acceleration as it's all about surface speed ie larger diameter would travel further on one rotation than the smaller one so that should answer questions 2 and 3 with a yes.

I'm wondering if the smaller wheel would have quicker acceleration. The larger wheel would be more difficult for the engine to turn.

Imagine yourself trying to speed away on a penny farthing with that massive driven wheel.

It's all about ratios.

I don't bloody know.

Ant

 

[dozypillock]

Smaller wheels would have faster acceleration. Try pulling away using only 5th gear, instead of 1st, and see what a difference it makes.
The other 2 questions are debatable as bigger wheels will not make the car faster, unless sufficient power is available, and MPG will be shot if engine is working its nuts off trying to maintain a speed.

Neil

 

[Deleted member 113659]

if the engine was powerful enough to reach the max revs on both cars only then would the larger wheel give highest top speed.

I have no idea about mpg.

Ant

 

[renault12ts]

Assuming you mean the overall diameter, including the tyres is greater, then: slower acceleration, faster top speed, better fuel economy.

 

[Deleted member 113659]

Imagine you have two identical vehicles same power, torque, gearbox etc.All four wheels on one, have a greater diameter of 6 inches over the other.
Would the vehicle with the big wheels, 1. Have a slower rate of acceleration? 2. Have a higher top speed, when both are at their limit? 3. Have better fuel efficiency, ie. MPG?
Who knows? :

Great question Peter. How the hell do you expect me to sleep now??? I'll be dreaming of scrolling numbers now!

Ant

 

[renault12ts]

if the engine was powerful enough to reach the max revs on both cars only then would the larger wheel give highest top speed.

I have no idea about mpg.

Ant

Yes, higher top speed.

 

[Deleted member 113659]

Assuming you mean the overall diameter, including the tyres is greater, then: slower acceleration, faster top speed, better fuel economy.

I knew you'd know about this. You're my Jonny Ball (think of a number) are you a school teacher???

Ant

 

[streethawk]

Smaller wheels would have faster acceleration. Try pulling away using only 5th gear, instead of 1st, and see what a difference it makes.
The other 2 questions are debatable as bigger wheels will not make the car faster, unless sufficient power is available, and MPG will be shot if engine is working its nuts off trying to maintain a speed.

Neil

Do you really think 6inch on diameter would make that much difference to how much torque is required. Wonder why drag cars have large rear wheels.

 

[flango]

Assuming you mean the overall diameter, including the tyres is greater, then: slower acceleration, faster top speed, better fuel economy.

Spot on, give that man a coconut lol

 

[Deleted member 113659]

Spot on, give that man a coconut lol

Now then, what would be the optimal way to open a coconut without loosing its contents? Answers on a postcard.

Ant

 

[camerafodder]

Do you really think 6inch on diameter would make that much difference to how much torque is required. Wonder why drag cars have large rear wheels.

Top fuel dragsters have no gears and immense power and torque. The large wheels help attain the high speed, in fact the tyres expand to increase overall diameter providing a crude form of gearing. Smaller wheels would make for easier faster acceleration but top speed would be limited so the size of wheels on a dragster are the best compromise between acceleration and top speed on a car with no gears and BHP measured in 5 figures!

 

[streethawk]

Top fuel dragsters have no gears and immense power and torque. The large wheels help attain the high speed, in fact the tyres expand to increase overall diameter providing a crude form of gearing. Smaller wheels would make for easier faster acceleration but top speed would be limited so the size of wheels on a dragster are the best compromise between acceleration and top speed on a car with no gears and BHP measured in 5 figures!

Point taken.

 

[camerafodder]

Point taken.

This vid illustrates the awesomeness of the whole thing. Not much time for a gear change there!

[YOUTUBE]Tji8QAoaW9Q[/YOUTUBE]

 

[Scooby_Doo]

I think you're barking up the wrong tree.
It could well be that the two different tyres have a slightly different circumference , the same as if you're comparing a new tyre with 8mm tread and a worn tyre with 2mm tread. Although the speedo should read the same you're actually travelling further per revolution of the wheel with the newer tyre. I assume the sat nav calculates your speed differently.

 

[grober]

While gearing may indeed influence top speed usually as speed builds its aerodynamic drag that dictates the cars final speed. While modern cars are undoubtedly more aerodynamic than in days of yore years ago an oft quoted figure was " above 100mph every 1mph increase requires 10bhp"
Power
Note that the power needed to push an object through a fluid increases as the cube of the velocity. A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7.5 kW) to overcome air drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW). With a doubling of speed the drag (force) quadruples per the formula. Exerting four times the force over a fixed distance produces four times as much work. At twice the speed the work (resulting in displacement over a fixed distance) is done twice as fast. Since power is the rate of doing work, four times the work done in half the time requires eight times the power.

 

[Deleted member 113659]

The sat nav is the more accurate for speed as the van is tracked as it moves.

I think you misunderstood. 15" steal wheels with 75 sidewall tyres are much smaller than 18" alloys with 50 sidewalls.

We're not talking just a difference of tread depth.

Ant.

 

[Scooby_Doo]

I think you misunderstood. 15" steal wheels with 75 sidewall tyres are much smaller than 18" alloys with 50 sidewalls.

We're not talking just a difference of tread depth

Ant, I understood the original post, and I haven't a clue what the answer is, he then asked why his speedo and sat nav were giving different readings with the new wheels, which what my answer referred to.