Bicycle Uphill

What have you been smoking?

Whatever it. is, I want some.

I need to fly up on my roof and get my kite back.

Only on the way down the other side. Keep pedalling.

Not exactly...and not at all. You will use any less energy by going faster.

Momentum is the product of a mass times its velocity. It is the tendency of an object to push against resistance (or an incline) due to its motion.

Your momentum will not help you use less energy because you are doing work against a conservative force (gravity) with losses due to friction and biological inneficiencies.

Your momentum will help you get past the deceleration between pumps on the pedals and might make it seem easier to pedal up hill, but you are still loosing speed between pumps.

Keep asking questions...and making hypothesis, being wrong is part of learning! Look at how many flying machines didn't work before the Wright brothers (barely) flew!

Drew K

No. It means that you are going downhill.

Possibly, if you can reach escape velocity....

It's all about power matching.
Accelerate hard into the start of the hill when it's easy to gather momentum, then use your body and gears for the most effective power transfer.
I've found it 'feels' easier to go at the hill fast, then stand on the pedals and keep the speed going until you eventually loose speed. Then drop into a low gear and take it easy.
I expect the seasoned serious cyclists would go slowly through the gears maximising the extra thrunge generated by standing on the pedals.

What's most effective for you? Legs whirling or a long toquey stoke from standing on the pedals? The length of hill is a big factor, a short incline can be taken with momentum and grunt... a longer climb needs a plan... or maybe a car?

Del

(It was all sounding so convincingly... until I said thrunge )

There will be a point...

When this happens...

There is no discrete point at which momentum starts to help or hurt. The instant at which anything moves it has momentum.

There is no point at which momentum is helping you if, as you said, you are accelerating uphill. (Thus, very light bikes with low momentum are faster and easier to pedal than heavy bikes, which have more momentum.) When you are accelerating on either the level or on a hill, momentum is hurting you (requiring you to expend more energy). Only when going down hill does momentum help.

The optimum cadence differs from rider to rider, but you pedal most efficiently if you keep your cadence constant, at its optimum, which means, for most people, dropping down several gears when going uphill. That, in turn, means a lower uphill speed than speed on the level.

Assuming a long even hill, a rider will use less energy going up more slowly, rather than faster. If it were not for air resistance, the energy consumed would be the same. But air resistance goes up with the square of speed. At a very low speed, the rider uses too much energy wobbling around trying to stay upright, so there is a very low speed limit at which this logic does no apply. But Lance Armstrong uses more energy going up hill at 20 mph than I do at 10 mph, assuming we weigh the same.

Ignore everything all these guys say. Momentum is going to help you the very instant you start moving. Otherwise you'd use the same amount of energy maintaining a given speed that you did accelerating to it! And this applies just as much going uphill and slowly accelerating as well!!

See? You're smarter than them.

Actually, you're smarter than me too. Now, if you can explain the bit about going fatser while using less energy I'm really interested.....

If you "ignore everything these guys say", then you are ignoring the laws of physics.

Can you show mathematically or by example how momentum helps you "the very instant you start moving"?

If you ride very far, then you do indeed use more energy maintaining your speed than you used getting up to that speed. It is difficult to show examples, unless a lot of factors are specified. On a level surface, ALL of the energy required to maintain a constant speed goes to overcoming friction forces. If you are riding on level soft sand, the friction is so great that you can't go very fast, so virtually none of the energy expended is associated with acceleration.

Even on a smooth hard level surface with a very light bicycle, it does not take long for the energy of maintaining speed to exceed the energy required to reach that speed.

The energy required to accelerate is simply the kinetic energy of the total mass at that speed: E_k=1/2 m*(V²). Let's say a 150 lb rider riding a 20 lb bicycle is to ride at 10 miles per hour. The total weight is 170 lb, so the total mass is 77.3 kg. 10 mph is 52800 ft/hr, or 14.7 ft/sec, which is 4.47 m/s. The energy required to accelerate is then 77.3/2*(4.47²)=38.7*(20.0)= 773 kJoules.

The energy required to maintain speed is E_m=F*D, where the F is the friction force to be overcome and D is the distance travelled. That force depends on many factors such as wind, tire inflation, lubrication, clothing, etc. Let's guess the conditions are such that the rider exerts 1/4 of his weight on his pedals. ignoring the fine details of the varying angle of the pedal crank etc., roughly half of that force actually pushes the bike forward. So F=1/8(150)=18.8 lb, or 4.45 Newtons.

Solving the energy equation for distance, we get D=E_m/F = 773kJ/4.45N=173 meters, or 568 feet; just over a tenth of a mile.

This is ignoring the friction forces & energy during the acceleration period. So if this person rides just one mile at 10mph, he/she will have expended roughly 10 times as much energy maintaining his/her speed as he/she did accelerating to that speed.

If the rider is on a hill, the speed will be lower, and the gravitational potential energy must be added, to the friction energy, so the distance where the maintain speed energy is equivalent to the acceleration energy will be much smaller!

I'm afraid that part was a bit tongue in cheek as the saying goes...

There are a couple of points you make that I would say don't relate to my points as posted. While you are certainly correct that it is easy to use more energy over a given distance than in the initial acceleration, that is not what I posted. Take a snapshot of a rider accelerating to a given speed under a given set of conditions and compare that to a snapshot of the same rider maintaining that speed under the same conditions. The momentary energy expenditure will be more in the acceleration phase than in the maintenance phase- due to the very laws of physics you refer to.

There is no such thing as momentary energy expenditure! There can be momentary power, but there is no energy until a time/distance is involved.

A pair of snapshots could illustrate that greater force/torque is required to accelerate than to only maintain speed, but that has nothing to do with "momentum helping you the instant you start moving".

My bad- power is the correct word.

I would say it depends on the hill! There are some around here that start out as as gentle rises that you could increase your speed on, but then turn into 15% grades near the top...you tend to slow down there!

Well, I've been commuting 10km to work for 18 years...

I have yet to work out how "When you ride a bicycle up a hill, and you keep increase your speed slowly"?????

ALL I have EVER managed is to DECREASE my speed RAPIDLY when riding up hills!!!

Good Luck with that.

OK, maybe I asked the wrong question.I will try a different approach.This is something I noticed when riding:If there is a downhill followed by a uphill, theres a few ways to approach this, but I am interested in just the two listed below.

1.You coast down the hill then pedal up the hill.

2.You pedal fast down the hill then pedal up the hill.

There are no gear changes or brakes being applied on both situations.What I have noticed No.2 is much faster and easier, a lot less exhausting.I was just curious behind the science.

Alright, I think that I might have an explanation for your experience. I think that what you are experiencing is a biomechanical effect and not a physics effect. While you're pedaling faster and faster down hill you are not actually providing more energy into your motion. Gravity through the release of potential energy is doing that, not your legs. However, the more rapid muscle movements in your legs are increasing the blood flow through your leg muscles that are now barely doing any work. This pumps more nutrient and oxygen rich fresh blood through your muscles while also purging the natural toxins (lactic acid) out of the muscles for your liver to cleanse. When you start to climb uphill on the other side you have fresher muscles than you did at the start of your descent.

Remember one of the banned bicycling doping techniques is blood doping. In the simplest case, a bicyclist would draw and store several pints of their own blood several days before a race. Their body would replace the lost blood naturally before race day. On race day they would add a pint of their own blood to their body. With more blood in their bodies they would have a better blood flow.

Indeed that is a totally different question!

Here the momentum and kinetic energy you acquire on the downhill in mode 2 will definitely reduce the effort required to go up the following hill, at least for short hills.

Well thats just ducky!

In that case if this hypoth were true then I could add a 200lb weight to the wheels and just use all that stored "free" energy to go up the hill without pedaling at all right?

I love getting something for nothing.

However, the only something I ever got for nothing was fired! strange they actually expected me to do work?

I ride my bike two miles uphill to the grocery store. Then I load the basket full of groceries. I go downhill faster and hardly have to pedal at all. On one hill I gained 3 miles per hour top speed with a full load of groceries. (43 vs 46 mph)

If you really want to have fun ride a tandem bicycle through rolling hills. You have twice the mass and it carries you farther up the next hill. You just keep pedaling and jamming the gears and seems like you fly over the hills and you're going along. Not mention tandems are just faster in general. Sometimes we like to let the couple on the tandem break wind for us single bike riders as they pull us along faster than we could ever pedal alone..

I don't think I'd want to ride behind a couple of people breaking wind.... !