Ok this might seem to be one of those stupid ones but anyway here goes…
In a car the engine basically produces power to turn the axles which then rotate the wheels, right? Now if we increase the size of the wheels wouldnt the car go further with the same power because of the increased circumference of the wheels? Wouldnt we get more distance per rotation? I know i must be missing something…Can someone tell me what?
Ok this might seem to be one of those stupid ones but anyway here goes…
Larger diamter wheels=greater distance traveled per engine revolution (all else being equal); you’re right about that. But that bigger wheels will take more power to turn at the same rpm.
Take an extreme example: say you have wheels with a thousand-mile radius. You could cross the continental US in less than one rotation, but think how much power it would take to get them moving at all.
Also, don’t forget that the radius of the wheels is part of the gear ratio.
I’ve always assumed the answer to this question is, yes. Kinda. Yea, you’ll go further, but not with the same power. (Duh) What I mean is, I’ve always assumed you’d getter better mileage, but I think the point of diminishing return wouldn’t take long to cross. So to extend this question I’m I wrong to snicker at pimped out cars with little tiny wheels. I’ve always assumed they must get terrible milage with those.
iirc, the ‘big wheel’ idea in the OP was the main reason why there was a huge front wheel on those old-school (really old school, 1800’s old school) bicycles; gears weren’t used on bicycles yet, so the best way to increase speed per turn of the ‘crank’ was to increase wheel size (if anyone here doesn’t know what kind of bike I’m talking about, it is the kind with like a 6 foot front wheel, a tiny back wheel, and pedals directly connected to the front wheel hub rather than using a crank, gears and chain). I’m having trouble backing that up with a cite, but I’m sure I was told that factoid, I just don’t know if the person telling me that info was accurate. Makes sense to me.
Penny-farthings. Yep, that was the reasoning behind those. Unfortunately, if yours came to a sudden stop for whatever reason you had a nasty fall ahead of you.
Sort of related….
Why are new cars being sold with 16” and 17” wheels instead of the standard 15”? Is the overall height of the wheel/tire any bigger, or are we using lower profile tires?
I understand that a larger diameter tire will have a larger rubber/road contact area, but somehow, I doubt this is the reason.
Is it purely cosmetic?
I suspect it’s part of the SUV movement - people want large cars, which are heavier, and also now most cars have ample power to turn those large wheels.
For most cars the tire sidewall is shorter for roughly the same overall diameter. A shorter sidewall will distort less during hard cornering for better steering feel and handling (one of many factors of course). On the other hand a shorter sidewall does not absorb bumps as well so the ride is rougher. Larger rims also allow larger brakes.
Of course 90% of the time it is stricly a style/cosmetic thing.
As to the OP, you could do the same thing with gearing with out a ton of extra rotating mass. When you go farther per revolution you are effectivly increasing the gear ratio and the engine will need to make more power. (TANSTAAFL) The already mentioned penny Farthings used this method before gears where invented.
You can never second-guess what changing a car’s gear ratios will do to performance or gas mileage without knowing the particulars of the car.
It would seem like giving a Ford Mustang a drag racing-style (numerically high) rear end gear ratio would lower its top speed, increase its acceleration and lower its gas mileage.
In fact, I saw a guy in a Mustang forum who did it, and proceeded to get BETTER gas mileage. The reason? In around town driving, overdrive kicked in earlier, so most of the time he was driving the rear end ratio actually LOWERED his RPM versus running the factory gear ratio.
If the vehicle’s drive is primarily highway, the best thing to do is calculate the car’s RPM at cruising speed in top gear. You will usually get a factory number between 1900 and 3200 RPM. The more underpowered or race-oriented the car, the higher the number will tend to be. Getting that number as close to 1500 RPM as possible will tend to maximize highway gas mileage. Just make sure you’ll be making more than 50 horsepower at that RPM, or else the gear may be too tall to use.
Remember it’s just not the outer diameter, it is the rotational inertia. It takes more energy to change the rate of change of a object. The heavier the object (generally) the more energy it takes, the further away the mass of the object is from the center also the more energy it takes. As cars became more powerful, and non-steal rims are pretty much standard equipment, the rotational inertia of the tires are much less an issue, so tires can grow.
I think the main reason he got better gas mileage is because the car has to strain less on takeoffs. That’s a lot more important for city driving. For highway driving, you want the engine to be at the RPM where it can run most efficiently while cruising. Typically, this is around 2,000 to 2,500 RPM’s. My Supercoupe has ridiculously tall gears and runs at 1,800 RPM’s at 70 MPH. It would do much better at 2,200 RPM’s or so. I bought some new gears for this reason, but haven’t installed them yet. But yeah, it all depends on that particular car. There are no numbers that work for all cars.
- The reason that newer cars use larger wheels is a “sportiness” trend to use lower-profile tires and improve the “road feel”.
- Your car does go farther per-tire-revolution if you put larger tires on. What effect this has partly depends on how the transmission responds, as noted earlier. What I do know is that years back I had a Dodge Ram Charger and swapped its 25" tires for 33’s, and had to wait a few weeks before I could afford to get the axle ratios changed out. That truck had a ~5L V8 and the normal (automatic) transmission, but with them 33’s on it, it KILLED the accelleration. I’d guess that most-any car of that era you could pick would have beaten my truck in a drag race.
Another difference with bigger wheels/lower profile tires is a change in unsprung mass. Most of the weight of a car is riding on the suspension. Unsprung mass is the stuff that isn’t - wheels, tires, brakes, parts of the suspension itself. Generally, in terms of handling the less unsprung mass the better. So a set of lightweight alloy wheels with smaller tires may give you a handling improvement, especially when compared to cheap factory cast wheels and large tires.
The force that is available to accelerate the car is the rear axle torque divided by the wheel radius. The force accelerating the car decreases as the wheel radius increases.
I was under the impression that, often, when you purchase larger wheels, your tyres get thinner–so the overall diameter doesn’t change much. Is there truth to this?
Often, yes, but it totally depends.
In a lot of passenger car applications, it CAN happen, or it can not.
Much cheaper to match Revolutions per Mile so as to not have to get your gear ratios or speedo adjusted to keep your speedometer accurate.
If yer a country boy with a BEEG pickup and you decide to get bigger wheel/tire combos for mud bogging, it’ll likely be seriously off, and you WILL need to get the speedo redone.