Can someone put an end to a little discussion a friend and I were having?

He was telling me that if I get larger rims (increasing the overall outer diameter of the tire) that my car would have faster acceleration, I said that it would decrease acceleration. We went back and forth with examples.

He said that when you do a pulley set, you get underdrive pulleys which are larger than stock and that's why sometimes your alternator will not spin fast enough to keep your battery charged.

I said that when you have a 10 speed bike, if you put the chain onto the smallest of the gears on the back tire, you can turn the back wheel much easier than when the chain is on the largest gear.

So now we're both confused as to who's example is right, and why the other's is wrong....

HELP!!!!
-Chris

 

i think a larger wheel = slower acceleration. i think people get bigger wheels, cuz they mod the engine to put out the power to spin the bigger wheels easier. but if you were to just get bigger wheels without doing anything else, then yeah, in theory it will be harder to roll. i mean if you had the choice to change a flat on an s2k or change a flat on an escalade with some dubs which one would you choose, not the big old heavy dubs.

 

I'm not too sure about either of your examples, but the correct answer is (more or less), the larger wheel will be slower ... usually.

If the outer diameter of the tire is larger, you have effectively used a taller gear ratio. The engine will rev slower for a given road speed. Thus less effective gearing. Consider that the wheel gets the same torque, but the force at the road is torque/radius. Larger radius means less force which means less accel. Again, this is if the tire OD is greater. If you just +1 or +2, the wheel gets bigger, but the wheel+tire OD may not.

Second effect is that a larger wheel is generally heavier. Heavier means it takes more of the power to accelerate it, leaving less to accelerate the car. But again, this depends on the whole tire/wheel pkg. If you go with larger wheel and smaller (lower profile) tire, you may not gain much, if any, weight.

Plus even if you do effectively lower the gear ratio, it means you stay in that gear a little longer, getting back a bit of the initial loss.

Confused yet? Anyway, the conventional wisdom, usually correct, is that smaller wheel gives better accel. But honestly, it isn't a big effect.

 

One has to look at it in terms of "mechanical advantage". This can be achieved either by {increasing the driving lever and decreasing the driven lever} or {decreasing the number of turns of the driving wheel compared to the number of turns of the driven wheel}.

In the case of rear driven tires, the larger the diameter, the harder it is for the driving force to turn them. IE, the larger the diameter of the rear wheel of a RWD car, the slower the acceleration that will result.

The case of the "underdrive" pulleys is different. The pulley on the accessory (eg, water pump) is NOT the final driven wheel. That pulley, in turn, drives the shaft of the water pump (which is connected to the impeller), which is the final driven wheel. By increasing the diameter of the pulley, you give it more mechanical advantage to turn the impeller, thus needing less power to turn it because the impeller turns slower.

This analogy would be correct if your friend used the impeller of the water pump to compare to the tires of the car. If you increased the diameter of the impeller of the water pump, it would be harder for the driving force to turn it, just as it is if you increased the diameter of the tires.

As for it being insignificant, let's put it this way: A decrease in tire diameter of about 2" from our stock diameter of 25", represents about an 8% increase in mechanical advantage. This corresponds to about the same advantage if you were to put in 4.44 final drive gears. Believe me, this is "significant".

 

when i put my 18s i significantly noticed it was slower than with 16 stock rims

 

Let's keep it simple. Assume two wheels are the same weight and physical layout, but one is an inch greater in diameter. The larger wheel will slightly degrade acceleration, as its polar moment of inertia is greater. It has more original sin.

In the real world, the larger wheel may overcome its "original sin" by better tire grip, less sidewall deflection with the reduced aspect ration, and other mechanical factors.

The effective diff between a 16 and a 17 is negligible. Often, the smaller wheeled car will outhandle a larger wheeled one - why? Because unless the mfr does it, the likelihood that a "tuner" will get the required suspension alignment settings correctly adjusted to handle the changes precipitated by the 17" wheel. The engineers at Honda have all sorts of telemetry and black boxes, and I am sure they have CAD software that makes most of the dialing in possible behind a desk.

Don't believe my premise? Pick up a stack of old C&D's or R&T's, and you often see worse slalom, braking, and skidpad p[erformance when a +1 or +2 fitment is made, for the reasons I stated. On paper they look good, but a mechanical device that moves (like a wheel) is only one element of a system. Without considering all the elements of that system, results are a crap shoot.

Beyond 17", the effect gets worse more quickly and to a greater extent. If you go to a +1 and certainly before a +2, examine the tuner's reputation and their suspension changes. If they say "Huh?", my advice would be buy it for looks, but watch your tail in ythe real world.

 

As if H2's and Escalades weren't scary enough while on the road. . . pop some 22" (or bigger) rims on there and