Science Questions

A Better Balanced Bike

Sun, 26th Oct 2008

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Bunny, Portland, Oregon asked:

On a recent trip from the farmers' market with a heavy load of fruit in my bike basket I started to wonder if the placement of the load on the bike makes any difference in the efficiency of the work I do. Would it be easier for me to have baskets on the back where I would pull the load as opposed to on the front where I must push my load. Is it simply too small a system to make a difference?


We put this to Jos Darling, Mechanical Engineering, University of Bath:

Thatís a tricky one, really.  The answer is pretty much however you want to do it.  If youíre looking at just the rolling resistance the way tires work is that the more weight on the tire the more rolling resistance.  What I mean by rolling resistance is how difficult it is to push the wheel forward against the road.  As long as you keep the pressure in the tires at a nice high value youíll find they roll very easily.  Whether you put the weight on the front or on the back, given itís got to be shared by one or the other, itís not going to make that much odds.

Milk churns being carried on bicycles, Kolkata, India.Thatís the simple answer.

But of course, itís never quite that simple.  If you look at the finer detail youíve got more complicated things like aerodynamics.  If youíve got a basket on the front thatís probably not going to help a whole lot.  You might do better to put the luggage on the back where itís hidden behind the rider.  The other thing that you might find is that if youíve got all the weight over the front then the front gets a bit wobbly.  So rather than cycling in a nice, straight line, youíre going to be wiggling a bit and trying to balance the bike more carefully.  In a way thatís going to waste some of your energy like scrubbing off the speed if youíre trying to go round the bend quickly.

In a sense youíd better try and share the weight between the front and the back.  The answer is itís not going to make much difference at the end of the day.  Unless Bunny is whizzing down hills at enormous hills at speed in Portland I suspect the aerodynamics arenít going to be a big issue.  You neednít worry yourself about where you put the weight.


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When I was in college, I used to ride many miles back and forth to school carrying books, groceries and other odds and ends.  I actually still do a bit of my grocery shopping by bike, and am also an active racing cyclist and scientist, so I may have some experience to throw at this.   

The placement of your load either forwards or rearwards on a bicycle makes very little difference for the efficiency of your work.  The beauty of the bicycle is that it is capable of staying upright by it's ability to automatically steer itself underneath you and your load.  If you want to try this, hold your bike by the saddle and push it down the sidewalk.  If it's in good repair and you're pushing fast enough, it will stay upright for a while before falling over as it slows down.  Now, if you tie the handlebars into place so that the front wheel cannot turn and then do the same thing, it will quickly fall over.  This is because the front wheel can no longer keep the center of gravity balanced exactly between the wheels.

So, what happens as you change the placement of the load?

In the front, you place the load primarily over the front wheel.  However, a bicycle stays upright by its ability to automatically stay underneath a load with requiring only slight, automatic steering adjustments, so very little extra work is really needed.  If the load is placed in the back and primarily over the rear wheel, the front wheel will have to move slightly less to keep the center of gravity above the wheels.  However, the energy involved in this steering is minimal at best.

Other factors which would affect your energy output are: the height of the load above the ground; and the aerodynamics of the fruit-filled basket.

As you ride, you are constantly wobbling as your center of gravity changes position over the bike.  Your bike will then automatically wobble underneath you in order to keep you upright.  (This is what allows you to ride hands-free while pedaling, and even executing a turn hands-free)  A load that is higher above the ground has a greater side-to-side inertial moment as you wobble your center of gravity as you ride, when compared to a load lower to the ground.  (In the same way that an apple on the end of a 1 foot stick is easier to hold than an apple on the end of a 1 meter stick.)  Assuming that your body wobbles the same about when you are riding with your fruit as without it, this inertial moment will be counteracted by either increased steering of the bicycle or increased movement of your body.  The greater the side-to-side inertial moment, the greater the counteraction because a bicycle is constantly in a state of wobbling.  However, this side-to-side inertial moment is, in many ways, negated while you are riding by the forward momentum of the fruit itself, which helps keep the bicycle moving forward.

If you place your load low on your bike, you decrease this inertial moment, so it is easier to your bike to change the direction of its load and overcome the load's inertia.  However, this effect is minor while you are riding due to the bicycle's own ability to self-balance.  If you notice any difference between your fruit being high on your bike or low, it will probably be while you are starting from a stand still or while making a quick turn.

Aerodynamics will have the most significant role in your power output, and indeed, the aerodynamics of your body and bicycle while riding on flat, level ground is the most significant force that limits your speed.

In recommendation, I wouldn't worry about it.     But if you are really interested in improving your efficiency, you can try putting your basket behind you for the aerodynamic benefits.  If you really want to geek-out, you are probably best off buying a skinny-tire road bicycle and carrying your fruit in a small backpack or on a rack behind your seat. (Or even better, a low-to-the-ground racing recumbent bicycle, and carry your fruit in your lap).   

If you are curious, here are a few potential situations where the fore-or-aft load placement *could* affect your required energy output:
1) If one of your tires is poorly inflated, putting a load over that wheel may increase your rolling resistance a bit more than if you put the load over the properly inflated tire (never to mind the increased chance of a blow-out).
2) If you are moving your bicycle less than 1 full length, the front wheel is higher, and the front wheel is on a downward slope, placing a load over the front wheel will cause the bicycle to move forward with no "direct" energy input since the potential energy of the load is converted to kinetic energy as the bicycle moves forward.  (Not a realistic scenario, however that's the only situation that comes to mind).

Whew...  Hopefully that made some bit of sense... 

All the best,

Kris, Columbus, Ohio

submerged, Wed, 22nd Oct 2008

As with any means of transport of goods, the weight should be evenly distributed with a bias to the rear axle.

Overloading the front axle can have an adverse effect on the steering, making it too heavy. Overloading the rear axle can have the reverse effect, making the steering too light and therefore ineffective where poor road surface conditions prevail. Both these situations will also have an effect on the braking, but it is especially so in the case of overloading the rear axle giving insufficient road surface grip for the brakes to work efficiently.

Ideally, your load should be split into two. One third of the weight on the front and two thirds on the rear axle.

As for the matter of balance, the load should be evenly distributed across the axles. Any extra weight on one side can cause 'pull' in that direction. This would necessitate steering compensation leading to uneven ware on the tyres.

Another point to be borne in mind is that the lower the load, the less effect on the center of balance.

Ideally, the best place to carry the load would be beneath the crossbar as far to the rear as possible, but this would make pedaling difficult to say the least. Don_1, Thu, 23rd Oct 2008

I get my groceries delivered  DoctorBeaver, Thu, 23rd Oct 2008


Not you DB:)

The question I meant.
'How do you best balance a bike?' yor_on, Fri, 26th Dec 2008

Start pedaling. When you fall over you steer into the fall's direction until you fall the other way and (important!) before you actually have reached the ground. Then you repeat it the other direction.

The higher up the load the easier it is to balance. Try it with a stick balanced on your finger. Longer sticks are easier.

Karsten Karsten, Sun, 4th Jan 2009

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