TimothyH

OK, so I'm not alone. Thanks.

Seriously, I wondered about this for years and every year test it on downhills, watch other riders and sure enough, I roll faster downhill. Someone said something about it today after the ride so I decided to ask about it here.

I like the descending muscles and valve cap theories quite a bit but am going with good tires and possibly more aero as the most plausible explanations.

I have always used good tires - Michelin Pro3/4, Vittoria Rubino Pro III and now Rubino G+ Speed with Conti Race Light tubes. A pair of Corsa G+ are on deck. Clothes are always close fitting - club cut is OK but I don't like things baggy.

Y'all think tires/tubes could make that much difference? You guys who coast faster, what tires are you using?


-Tim-

canklecat

I just double-checked my helmet video cam from yesterday's group ride, using slo-mo to see what I missed. Everybody I passed was riding their brakes.

Mystery solved.

rm -rf

At downhill speeds, it's essentially all aero.

See this chart of power to pedal at different speeds on the flats. Wind resistance in gold, other drivetrain resistance in purple.

At 10 mph, it's about evenly split between aero power and drivetrain/tire power. At 30 mph, it's way more aero power.



Coasting downhill, the "power" comes from gravity, and is proportional to weight. And the frontal area of a larger, heavier rider doesn't increase as fast as their weight increases. (Essentially, weight is proportional to changes in width*height*depth, but frontal area is only width*height, and perhaps even less when in a bike position.) That's why strong, big riders can sprint, but get dropped on hill climbs.

But different riders at the same weight can have different body shapes or can sit in a more aero position on the bike.

So: minor improvements in tire rolling resistance, slight improvements in wheel and bike aero, but mostly in rider aero. (or affected by timid/prudent braking, of course.)

atwl77

I'm a featherweight yet I coast downhill a lot faster than riders a lot heavier than me. I attribute good hubs (I use White Industry T11) to it.

Kimmo

Versus weight. Terminal velocity is a function of weight divided by drag.

bigsky109

Coasting? Hmmmm.....Not sure if I want to try that. I tried last year drafting and coasting down every hill another cyclist was riding....I got tired of coasting because I like to pedal down the hills, usually as fast as I can.

Sy Reene

Of course you're not alone.. on BF, everyone coasts faster than everyone else (who obviously don't post on BF)

KraneXL

This. Sounds like its all about being aero.

fietsbob

Physics; Bend over lower, so present less air resistance? skinnier but heavier? gain flatulence-thrust?

Or just humble bragging..

woodcraft

I'll be the guy that you all coast faster than.

You're welcome.

kbarch

It's the shape of your prominent nose - cuts through the air better, resulting in less turbulence.

Litespud

Ack - you're right - didn't think it through. Gallileo showed that acceleration under gravity is weight-independent, but, as you say, the heavier object will accelerate to a higher speed before opposing forces (drag, rolling resistance) balance out at terminal velocity

elcraft

How recently have your wheel bearings either been serviced or replaced,if sealed? What type of lubricant/grease do you use? Maybe the choice of grease is a factor. If one "overloads" the wheel bearings from a really tight Quick Release lever(or one doesn't plan to leave the bearings a tad looser to compensate for the end loading that the quick release will apply),'then there might be more drag on the wheel bearings. These factors should be considered.

rossiny

Some have mentioned that hubs and bearings may make a difference. I kind of tend to agree, I often wondered if better hubs and bearings on cars could get more gas mileage also?

FrenchFit 

Me too, always. I think the answer might be simple, if you are smart about distributing with your weight over your wheels, you will beat riders that are improperly balanced. Try it; move your weight fore or aft when you are coasting and watch your glide taper off.

I suspect such an advantage disappears with pro level riders who are much more disciplined about knowing where their center of gravity is on the bike, I suspect they intuitively go right to the sweet spot.

bigsky109

Just fart a few times to gain speed. LOL

one4smoke

It's not that you're faster ...but rather they're slower.

TimothyH

This is a great post.

I had not though of this.

It is certainly plausible and worthy of testing.


-Tim-

prathmann

Ok, I've tried it and discovered many years ago that if I move way back and sit on my rear rack instead of the saddle I can coast substantially faster. Of course it makes my weight distribution worse, but the aerodynamics are improved by stretching out my body with my chest flat on the saddle and arms out straight in front. A true prone position is even better aerodynamically and gives still faster coasting speeds.

At typical downhill coasting speeds the rider's weight and aerodynamics are the dominant factors. The tire rolling resistance also plays a significant role with other things like bearing resistance of much less importance (barring some serious mechanical problem).

DiabloScott

Terminal velocity is when ma=f(drag).

You can affect your drag by rebalancing your weight and getting into different positions, but in general, people with more mass are always going to have the downhill advantage.

carlton

Can't coast down hill where I live. The wind will blow you uphill backwards.

Nachoman

Seriously. All these fast internet descenders should try riding with some real-life fast descenders.

therhodeo

All the "fast descenders" I know are usually needing that to try and chase back on after climbs.

SHBR

Switch bikes with other riders, then you'll have your answer. Its probably not the bike IME.

One of my riding partners always out coasted me, even when we switched bikes.

Its mostly weight distribution and aerodynamics.

TimothyH

I said coasting, not descending.


-Tim-