Tire weight...front vs rear.
 

Imagine a street-ridden bike with a 1000 gram tire up front and a 1000 gram tire in the rear.

Replace the rear tire with a 300 gram tire. I imagine the rider would notice a boost in efficiency.

Simple enough.

Now, put the 1000 gram tire back on the rear and install the 300 gram tire up front.

Is the rider going to feel like the 300 gram tire being up front provides the same boost in efficiency as it did when the 300 gram tire was on the rear wheel?

PS: For the hypothetical scenarios above, the "all else equal" rule applies.

Depends. Is the rider doing wheelies? If so, front wheel wheelie or rear wheel wheelie?

If going in a straight line and not doing wheelies, reducing the weight of either tire by the same amount results in a corresponding reduction in rotational inertia, so it would feel the same.

won't effect "efficiency" (physiological)

No wheelies. Just riding in a straigh line.

I thought that maybe since there is more weight on the rear and the rear is "powered", that the weight reduction in the rear may provide a greater increase in efficiency than a tire-weight reduction in the front of an equal amount.

Am I thinking about this the wrong way?

You said "all else equal", so I had assumed that the front wheel and tire combo is the same size as the rear wheel and tire combo, with the only difference being a difference in the mass of the tire. If riding in a straight line then both wheels are spinning at the same rate.

F = ma, where F is force, m is mass, and a is acceleration. Regardless of which wheel is heavier (as a result of the heavier tire), the total mass (front wheel + front tire + rear wheel + rear tire) is the same, and thus it takes the same total amount of force to accelerate both wheels, unless you can pedal so hard that you cause the drive wheel and tire to slip vis-a-vis the ground. This latter scenario is highly unlikely.

I am not sure what you meant by "efficiency" so I tried to answer in terms of how much force it would require to accelerate the bike. It works out the same if you calculate the force required to change the momentum, the flip side of inertia.

I think the direct effects of mass would be similar.

However, the coefficient of rolling resistance matters more on the rear tire because that tire bears more weight. For a given tire size, all else will probably not be equal.

For example, with my 26x2.0 MTB tires, a 1000g tire will likely have a lot of tread rubber and a stiff casing, while the 300g tire will be an ultralight tire like a RH RTP. In that case, the lighter tire would seem a lot faster on the rear wheel, mostly because of rolling resistance.

Whether it’s the tire you want on the rear will also depend on considerations of road hazards and flats, which tend to happen more on rear tires.

Otto

One of the effects of rim/tire weight that's often ignored on BF is just how much it impacts handling and feel. Our spinning wheels act like wheels in a gyroscope - the heavier wheel and the faster the spin, the more it's going to resist changes in orientation (the next time you change a tire, spin your wheel while holding on to both ends of your QR or TA - try to tilt the tire and note the resistance. Then take off the tire and try it again with just the bare rim - big difference and that's at a relatively low rotational speed). If/when you get out of the saddle and rock the bike back and forth under you, whether you're climbing or sprinting, the difference between lighter and heavier wheels/tires can be readily felt. A difference in initial lean-in when cornering can be felt, too.

Why do you think it might make a difference, having the heavier tire on front or on back?

Ah, I read through and you mentioned that because the rear wheel is powered, you thought it might make a difference. Emphatic "no" - you have to power both wheels anyway whether connected by a chain or pushed forward at the axle.

No mention of disc brakes or calipers or direct mount, internally geared hub, sock height, chain lube or what color the bike is. Impossible to answer.

Most punctures occur on the back wheel so, if I were forced to make the choice, I'd put the heavier tire on the back.

While it sounds like it should feel quicker with a lighter tire in the rear, the amount of torque you have to apply to the rear wheel turn a heavier front tire washes out any “perceived” acceleration gains from a lighter rear tire.

I reality, I don’t think there is any difference where the extra weight needs to rotate. Your legs need to turn them simultaneously.

John

Come on... we didn’t even get into inflation pressure, internal rim widths, hooked vs hookless, sock height, lube aroma, bar wrap, sore prostates, clipless, steel, carbon, 1x or cream vs black. (That’s for coffee or sidewalks.)