Why do some bike hold speed better than others?

[adub]

This question was an obvious troll

[fourflys]

Originally Posted by adub

This question was an obvious troll

do you mean the OP's question? Why would you say that?

I think it's a legit question that has many possible answers.. some bikes just mesh better with a particular rider.. maybe it's the "planning" effect Jan speaks of, maybe it's a certain set if wheels that just seem to hold speed better on a certain surface, etc.. the only true way to know if a particular frame holds speed better than another is to use the exact same wheels and tires cross the frames, IMHO.. Cade Media on Youtube has a two sets of "house wheels" they use when they test bikes for this very reason.. one set is disc and one is rim brake, but both are set up exactly the same otherwise..

even then you would have to decide why a certain frame holds the speed better.. aero advantages- either in the frame design or your position, or ???

It's a good question to ponder for sure..

[jimcav]

I was hoping the answer was going to be ceramic bearings, and then I'd have an excuse to upgrade every BB and hub.
At least no one suggested internal cable routing.

[fourflys]

Originally Posted by jimcav

At least no one suggested internal cable routing.

that's the aero part!

[unterhausen]

I suppose if we're talking vintage frames, there may be a ridiculously bad alignment issue. Unlikely though. Lots of poorly aligned frames out there that people are perfectly happy with.

I was at the lbs, and they were adjusting a recently purchased bike to match the rider's normal position. Rider was previously really unhappy with the way the bike rode. Apparently it was way off from their normal position, but they blamed the bike. I feel like this is pretty common

[vespasianus]

On my newer bikes, I hold my speed under the stem and inside the handlebars. Older bikes did not have that ability.

[marciero]

Originally Posted by vespasianus

On my newer bikes, I hold my speed under the stem and inside the handlebars. Older bikes did not have that ability.

Is that a joke? If so its sort of funny. Actually it reminds me of Gino Bartali during WW II, ferrying around secret paperwork of various sorts, eluding the Nazis.

[Spdntrxi]

I prefer to increase speed to be honest

[marciero]

Another thing regarding mass/inertia is that they-it- doesnt come into play when moving along at constant speed on a flat surface. In order to maintain speed, you have to overcome rolling and wind resistance, independent of mass. It only comes into play as resistance to changes in velocity, which would be less for the bike with greater mass. The more massive bike would slow down more slowly, for example if you stopped pedaling. If I understand MarkMcM's remarks, they tell me I can treat the wheels simply as more mass- the units, lbm, are just kg conversion- so is not separate consideration.

[vespasianus]

Originally Posted by marciero

Is that a joke? If so its sort of funny. Actually it reminds me of Gino Bartali during WW II, ferrying around secret paperwork of various sorts, eluding the Nazis.

He actually pulled a cart full of people and said it was for training. Legend.

[cmg]

Originally Posted by Mark McM

This effect is often exaggerated. It is true that effective inertia of mass at the periphery of the wheel is double the inertia of the non-rotating parts of the bike. But mass at the periphery of the wheel (rims and tires) is just a small portion of the total inertia. Take for example, a rider that weighs 170 lbm wearing clothing, shoes and helmet that weighed 6 lbm, and that the weight of their bike is 20 lbm, for a total mass 196 lbm. Further, the rims + tires on this bike weight 4 lbm, and since the effective inertia of rotating mass is double, the effective inertia of the bike is 200 lbm. Now let's increase the weight of each rim + tire by 1/2 lbm, or 1 lbm for both wheels, which increase the total effective inertia by 2 lbm, for a total effective inertia of 202 lbm. This is just 1% increase the total inertia. On any given day and on any given road, the rider's power output, or total drag (rolling resistance plus aero drag) may vary by more than that, so on a good day the rider may be able hold speed better on the lighter wheels, and on a bad day they won't be able to hold speed as well on the heavier wheels.

Or consider this: A full water bottle weighs about 2 lb. this adds the same inertia as adding 1 lb. to the rims + tires. When was the last time you heard someone remark, "boy, my bike sure does hold speed better when I've got a water bottle on the bike!", or, "I'm going to be riding rolling terrain today, I better make sure my water bottles are full so my bike can hold speed better!"

Does your calculations consider that it is the rider providing the torque to spin it all? He's the one that feels the effect not the person observing from the sidelines.

[fourflys]

Originally Posted by cmg

Does your calculations consider that it is the rider providing the torque to spin it all? He's the one that feels the effect not the person observing from the sidelines.

I often take what this guy says with a grain of salt.. anyone who have ever replaced a standard clutch flywheel with a lightened racing one can tell how much rotating inertia matters to keep that circular thing spinning longer.. all else the same, I'm certain a 2,000 gram wheelset will hold inertia better than a super lightweight climbing set at 1,000 grams.. and that light set will probably feel better going up a 12% climb..

[Mark McM]

Originally Posted by cmg

Does your calculations consider that it is the rider providing the torque to spin it all? He's the one that feels the effect not the person observing from the sidelines.

Yes. The entire reason that the mass at the periphery of the wheel has twice the effective inertia as non-rotating mass is because the drive forces/torques have to accelerate the wheel both translationally and rotationally. The reason that it is a 2:1 ratio is because if wheel rotates without slipping, then the circumferential speed & acceleration of rotation is equal to the speed oand acceleration of translation.

[54ny77]

Originally Posted by Ken Robb

Are you a marriage counselor?

[marciero]

Originally Posted by fourflys

.. anyone who have ever replaced a standard clutch flywheel with a lightened racing one can tell how much rotating inertia matters to keep that circular thing spinning longer..

Thats because it has a greater resistance to changes in velocity from an applied force. So yes it slows down more slowly. But it takes no more force to keep the more massive flywheel spinning at a given rate than it does it does the less massive one- you only need to counter the resistive force.