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Aerodynamics of Gravel Bikes

renehersecycles.com

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Re: Aerodynamics of Gravel Bikes

#201

Post by wiredfool »

Melatonic wrote: post #174
ActorNightly wrote: post #148

Earlier quoted context omitted.

Rotational inertia matters extremely little for bike feel compared to the overall weight of rider+bike. The torque required to accelerate a wheel by itself without any resistance is minimal.

You have any sources for this? Not trying to call you out but I am generally curious. We know on a car that rotational mass makes a huge difference but a car is continually accelerating and has a constant type of locomotion that is very different than a person riding a bike. In addition the rotational mass of a bicycle wheel is much, much larger diameter comparatively to the total weight of the vehicle (compared to a…

It's first year kinematics. The (first order) math is pretty easy to work out. 1/2 I w^2 is the rotational potential energy, Assuming all the mass of the wheel is at the rim (worst case) I = mr^2, w=v/r, so that works out to 1/2 mv^2, which is the same as the translational kinetic energy. So the 'energy penalty' of rotating mass is 2x. So, a 10kg bike with 2kg of wheels and 100kg of rider is approximately equal a non-rotating mass of 112kg. However, that rotating mass gives you 2% more kinetic energy to trade for wind drag, gravitational potential, or other friction. So, not much.

A bike's does not require gyroscopic effects to stay up -- there was an experiment with counter rotating wheels (to cancel the gyroscopic moment) that was ridable. The actual stability depends on the geometric and pneumatic trail, flop, weight distribution on the steering axis, and a few other things.

There is a middle ground that people have converged to -- Somewhere in the range of around 600mm bead seat diameter and an inch or two of tire. (26"->700c, 25->50mm width covers most of the biking world.)

Re: Aerodynamics of Gravel Bikes

#202

Post by dvzk »

prmoustache wrote: post #48
Steltek wrote: post #19

Earlier quoted context omitted.

Gravel bikes are good commuters, if you like drop bars. Many commuters prefer flat bars for the more upright riding position, which allows for a better field of view for misbehaving drivers. You still also have the classic problem that higher end bikes lose all the commuter friendly features. Carbon forks and frames often lose mounting points for fenders+rack. The marketing vision of a gravel bike is someone expectin…

Drop bars can be set as high as you want. There are even off road drop bar bikes built with stems looking like goose necks to put the bar high enough (see example in the link). It is more about the position in which you want to put your hands/wrist that matters. Gravel bikes are sold in many different kinds, and different materials. Steel, alu, titanium, carbon, you name it. They are also build with different use in…

To clarify for some readers, you really only need super tall stems for major downhill sections and drops. Standard gravel bike geometries are just fine for most singletrack, even on gnarly terrain that most riders would balk at. Brake levers that are angled inward and spaced properly matters much more for instant responsiveness and precise turning control on hazardous technical terrain.

Re: Aerodynamics of Gravel Bikes

#203

Post by vbarrielle »

phlipski wrote: post #190

The article mentions the upper back fairing of a motoGP racer but then blows off the idea for a cyclist because they might get too hot? I've always wondered why road cyclists don't ditch their frame mounted (and drag inducing) water bottles for a camelback. The original camelback was slim and I can't imagine it added any drag seeing as it sits on the back of the cyclist. Plus it has the advantage of allowing the cycl…

In pro cycling it's actually forbidden to have aero fairings on your bike. During the Armstrong years there has been a controversy about the placement of his radio on his back during time trials that would incur fairing.

Re: Aerodynamics of Gravel Bikes

#204

Post by Lio »

Melatonic wrote: post #171
Lio wrote: post #20

Earlier quoted context omitted.

A skinsuit or roadsuit will definitely make you faster on the bike. It's pretty easy to measure the effect with a power meter. Weirdly "aero socks" are both a thing and make a big difference to the point that they are heavily regulated by the UCI. Any cylindrical part of the bike or rider is going to perform badly so if you can encourage the airflow to stay attached around it you reduce the amount of effort required…

Always made me wonder if taking the swimmers route instead would provide all the same benefits without the cost and look. Shave your whole upper body and go shirtless and you would be pretty damn aero no? Would a skinsuit really improve upon that? In addition after switching to a Brooks saddle (I tried literally 10s of saddles using the "saddle swap" forum feature on some forums) I now no longer ever want to use padd…

It's actually been tested and modern fabrics are better than bare skin for drag.

In Michael Huchinson's Faster[1] it mentions that they tested a rider naked in a wind tunnel to find out!

If you look at a modern TT skinsuit you can see that they try to cover as much of the arms and legs as possible. They usually stop above the elbows and knees because it's hard to cover the joints without wrinkles.

Likewise, socks are now as long as permissible (without breaking the UCI sock rules!), which I think is about halfway up the calf. It would be faster to cover all the way up to the knee joint.

It all sounds silly if you don't race but if you've put some effort into getting fit then after awhile it's hard to gain even 5 or 10 more watts. If you can reduce your drag by more than that just by wearing a better pair of socks... well why would you?

BTW I have a Brookes on my commuter Brompton too. :D

1. http://www.michaelhutchinson.co.uk/faster.html

Re: Aerodynamics of Gravel Bikes

#205

Post by antihero »

ActorNightly wrote: post #141
matsemann wrote: post #5

A gravel bike is the perfect commuter bike! At least if you're not going electric. Almost as light and fast as a road bike. But a bit more upright riding so it's more comfortable. Wider and more treaded tires means tram lines, pot holes, curbs etc. are less of a problem and can handle rough surfaces and not just asphalt. Mounting points and wider forks allow for mudguards / fenders so rain is no problem. The wider sp…

From a money perspective, there is no reason to buy an analog bike for commuting anymore given the current selection of ebikes, and even more so, aftermarket ebike kits like Bafang. You can buy a single speed beach cruiser for $200, add $1000 BBSHD kit that comes with a battery, and basically get everywhere faster than a top of the line aero road bike.

Well other than that with $1,200 commuter/cx bike you are getting much better quality kit, and are still getting a load of exercise.

Re: Aerodynamics of Gravel Bikes

#206

Post by heywherelogingo »

qwertox wrote: post #58

I started biking around three years ago. Lots of biking, like 5000-8000+ km a year. As long as I don't weigh my ideal weight and aren't planning on becoming a machine myself, I have zero interest in spending anything in optimizations which will make my bike a better one. I don't care if it could go faster or be more lightweight, spending 600€ for the bike was enough. The cheapest 30€ Schwalbe tires are good enough; I…

Lots of biking? My commute alone: ~10 000km. A 600€ was crap twenty years ago. Contributions are great when the person making them has something to back them up.

Re: Aerodynamics of Gravel Bikes

#207

Post by ActorNightly »

Melatonic wrote: post #174
ActorNightly wrote: post #148

Earlier quoted context omitted.

Rotational inertia matters extremely little for bike feel compared to the overall weight of rider+bike. The torque required to accelerate a wheel by itself without any resistance is minimal.

You have any sources for this? Not trying to call you out but I am generally curious. We know on a car that rotational mass makes a huge difference but a car is continually accelerating and has a constant type of locomotion that is very different than a person riding a bike. In addition the rotational mass of a bicycle wheel is much, much larger diameter comparatively to the total weight of the vehicle (compared to a…

You can test this yourself. Flip your bike upside down, shift it into a gear that you use to accelerate, take a torque wrench with hex socket and put it in the left pedal socket on the backside of the crank like you would when installing it. Then test the torque required to get the wheel moving. You would find that these numbers are minimal.

Also gyroscopic effect has very little to do with the bike staying upright. The reason a bike stays upright is because of the trail interaction and camber thrust. When the bike leans left, camber thrust of the tire makes it turn left, which moves bottom of the bike to the left, correcting the initial lean, while the geometric trail prevents the tire from turning left completely.

Re: Aerodynamics of Gravel Bikes

#208

Post by ActorNightly »

antihero wrote: post #205
ActorNightly wrote: post #141

Earlier quoted context omitted.

From a money perspective, there is no reason to buy an analog bike for commuting anymore given the current selection of ebikes, and even more so, aftermarket ebike kits like Bafang. You can buy a single speed beach cruiser for $200, add $1000 BBSHD kit that comes with a battery, and basically get everywhere faster than a top of the line aero road bike.

Well other than that with $1,200 commuter/cx bike you are getting much better quality kit, and are still getting a load of exercise.

Most ebike motors have different power modes. My wife has a beach cruiser bike set up with BBSHD with 5 power modes (custom programmed) depending on what she wants. Lowest power mode makes it feel like a regular bike because the added power compensates for the extra weight, mid power modes provide varying level of assist, while the top power mode gives you 30+ mph on throttle alone.

With this bike, she can either get exercise, or commute without pedaling at all.

Kit doesn't really matter when you have that kind of power, weight of the frame, wheels, e.t.c doesn't really matter, and you don't need gears - single speed setup is more reliable, with a thicker single speed chain that can handle the additional power. The only upgrade that I did on hers is just bigger 200 rotors front and back, which with mechanical brakes work totally fine for the weight.

Re: Aerodynamics of Gravel Bikes

#209

Post by Melatonic »

wiredfool wrote: post #201
Melatonic wrote: post #174

Earlier quoted context omitted.

You have any sources for this? Not trying to call you out but I am generally curious. We know on a car that rotational mass makes a huge difference but a car is continually accelerating and has a constant type of locomotion that is very different than a person riding a bike. In addition the rotational mass of a bicycle wheel is much, much larger diameter comparatively to the total weight of the vehicle (compared to a…

It's first year kinematics. The (first order) math is pretty easy to work out. 1/2 I w^2 is the rotational potential energy, Assuming all the mass of the wheel is at the rim (worst case) I = mr^2, w=v/r, so that works out to 1/2 mv^2, which is the same as the translational kinetic energy. So the 'energy penalty' of rotating mass is 2x. So, a 10kg bike with 2kg of wheels and 100kg of rider is approximately equal a non…

Sounds like those crazy 650B people then have been right all along!

Re: Aerodynamics of Gravel Bikes

#210

Post by Melatonic »

ActorNightly wrote: post #207
Melatonic wrote: post #174

Earlier quoted context omitted.

You have any sources for this? Not trying to call you out but I am generally curious. We know on a car that rotational mass makes a huge difference but a car is continually accelerating and has a constant type of locomotion that is very different than a person riding a bike. In addition the rotational mass of a bicycle wheel is much, much larger diameter comparatively to the total weight of the vehicle (compared to a…

You can test this yourself. Flip your bike upside down, shift it into a gear that you use to accelerate, take a torque wrench with hex socket and put it in the left pedal socket on the backside of the crank like you would when installing it. Then test the torque required to get the wheel moving. You would find that these numbers are minimal. Also gyroscopic effect has very little to do with the bike staying upright.…

Interesting!
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