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Recumbent What IS that thing?! Recumbents may be odd looking, but they have many advantages over a "wedgie" bicycle. Discuss the in's and out's recumbent lifestyle in the recumbent forum.

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Old 08-25-07, 10:42 PM   #1
lowracer1
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reprint...recumbent bikes are not faster

Recumbent Bikes Are Not Faster
8/24/07 - Warren Beauchamp

Recumbent bikes are not faster than upright bikes.
There, I said it. Now before you get all uppity and start writing the nasty-grams, let me explain. Most recumbent bikes are built to be comfortable rather than fast. Those comfort recumbents generally have more upright seat angles, wider tires and lower bottom brackets. That combination makes a recumbent about the same speed-wise as your average upright road bike, not faster. There are over 260 models of recumbent bikes currently manufactured, and only about 50 of them are constructed in a way that would make them faster than a road bike.

What makes those fast recumbent bikes fast?

Frontal area
The less wind that you block with your body, the faster the bike will be. In terms of bottom bracket height, your knees should come up to just below the height of your shoulders while pedaling for best aerodynamic efficiency. The further you lean the seat back, the less air you will block. There is of course a practical limit to how far you can lean the seat back, and extremely laid-back bikes require headrests. Usually 20 degrees is the practical lower limit. At angles lower than that, you cannot see over your knees, and handling is compromised. Extremely laid-back bikes are more difficult to control than their upright seat brethren, but some people feel it is worthwhile to go faster. Reducing the seat angle from 30 to 20 degrees will result in a .5 MPH speed gain at 20MPH, or a full MPH speed gain at 25MPH. Lowracers and Highracers generally have the smallest frontal area of all recumbent bikes.

Weight
Light is good. Because the speeds going up a hill are much slower than those going back down, on any ride with a moderate amount of hills you will be spending most of your time going uphill. Anything that can be done to help the uphill speeds will greatly increase your average speed. The average recumbent bike weighs about 33lbs. Just reducing that weight to 25 lbs results in a .5 MPH speed gain on a 5% grade. Generally a recumbent bike under 25 lbs is very well optimized for weight. Under 20 lbs is very difficult to accomplish and is usually ridiculously expensive. Highracers and upright bikes can generally be built lighter than lowracers or other recumbent types due to frame simplicity.

Drivetrain
Drivetrain efficiency is most important for high power riding such as sprinting or cranking up hills, but not as important when just cruising. To build an efficient drivetrain, reduce the number of idlers and remove the chain tubes. The drive side idler should be a hard plastic or a chain cog, and should be have as large a diameter as possible. The chain line should be kept as straight as possible, but not at the expense of frontal area or weight. Internally geared hubs are less efficient than derailleur systems. Some long wheelbase bikes such as Tour Easy can be built without any power side idlers at all, and as such can be quite efficient up hills.

Vehicle height
Most recumbent bikes are lower than upright bikes. A lowracer is faster than a higher bike for many reasons. The most obvious is that the rider, wheels and frame are all at the same height, so it will have a smaller frontal area and poke a smaller hole in the wind. Less obvious is that because the wind speed is lower close to the ground, on windy days lowracers will be less affected by the wind.

Wheels
While much debate has occurred over whether small or large wheels are faster, the truth is “it depends”. Small wheels have less rotational mass so they will accelerate faster. Also small wheels have less frontal area, so are more aerodynamically efficient. Large wheels are more efficient on rough roads, and offer better cushion to the rider. Actual difference in smooth surface rolling resistance so small it is not noticeable.

Coda
Most recumbent bikes make many compromises for price, comfort, rideability, and perceived safety. Fast recumbent bikes are still comfortable, but they take more practice to master, and are generally more expensive to obtain. Actual difference in speeds between “slow” and “fast” recumbent bikes can be amazing. The difference in cruising speeds between my “practical” commuter bike and my lowracer are about 5 MPH. That’s a huge speed difference when you are talking about human power.
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Old 08-25-07, 11:01 PM   #2
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I have no experience with 'bents but as an engineer I agree with everything there. In fact, you've pretty much covered the criteria I'm using as a basis for selecting my first 'bent.

The two other areas that you haven't mentioned are the steering layout (uder seat, over seat, tiller) which I believe has a measurable impact on your air resistance, and of course the use of a (well designed) tail fairing where you have to balance the additional weight against the aero benefit. I'd be interested in any more info on these two areas.
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Old 08-26-07, 12:14 AM   #3
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hey lowracer...........i see your chain goes past your front wheel..........how does that work out...........looks alot easier to do than 3 or 4 different cogs.........im buildin a new bent with RWD and FBB.........i guess i can do it that way first and change it later

thx again for that post
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Old 08-26-07, 07:55 AM   #4
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I can give personal testimony: Uprights are faster...I get tired faster, I get sore faster, and I want to quit riding sooner on an upright.
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Old 08-26-07, 03:45 PM   #5
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Quote:
Originally Posted by lowracer1 View Post
Wheels
... Small wheels have less rotational mass so they will accelerate faster.
I've seen this quoted often, but I think it's a wash. Do they 'spin up' faster? Yes, BUT... due to their smaller circumference, they need to be spinning one third faster than a 700C wheel for a given speed. Do they spin enough better to make up the difference in rpm? IOW, do they make it to 133 rpm before the 700C wheels make it to 100 rpm?

Last edited by BlazingPedals; 09-03-07 at 08:08 PM. Reason: correct spelling
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Old 08-26-07, 04:13 PM   #6
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I think the title of this thread is totally misleading.

There are conditions when recumbent bikes are faster than upright bicycles.

There are cetain recumbent bikes that weight less than certain upright bikes.
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Old 08-26-07, 09:08 PM   #7
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When the comparison is made, it's usually bents vs road bikes. But bents are a big category. Often, the bent in question would more accurately be compared to a hybrid.
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Old 09-02-07, 02:40 PM   #8
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I think the title of this thread is totally misleading.

There are conditions when recumbent bikes are faster than upright bicycles.
Yes, it is misleading. It should be "Reprint ... Not all recumbent bikes are faster"

The OP is quite aware of this, as they wrote, "There are over 260 models of recumbent bikes currently manufactured, and only about 50 of them are constructed in a way that would make them faster than a road bike."

My bent is slower than a road bike, but I didn't buy it for performance.

I would be surprised if only 50 out of the 260 being manufactured were faster than a road bike. For when I shopped for comfort, I didn't find very many that were oriented for comfort over speed. That is, I didn't feel that I had many choices available.
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