Technical > General Technical

Drop bars, bad backs, aerodynamics, centre of aerodynamic pressure

(1/2) > >>

Andre Jute:
Some remarks about the interrelationship of drop bars, bad backs, aerodynamics and centre of aerodynamic pressure inspired by discussion in Calculating saddle & handlebar position, but a bit off-topic there.

***

What gives cyclists bad backs is contorting themselves to fit bicycles carelessly "designed" to follow an obsolete fashion. Drop bars might offer more efficient locomotion -- I'm not so sure about that, actually -- but they can't ever be good for your back (unless your "vertebrae have fused in the drop bar position" as a Melbourne physio used to say). Skinny tyres might be good for racing but they can't ever be good for your back. High pressure tyres may or may not look "fast" but generally speaking balloons actually have less rolling resistance and don't ruin your back like high-pressure tyres.

***

I'm totally unimpressed with the argument that a flat back on the drops somehow gives a cyclist greater speed by better aerodynamics. Aerodynamics work through CdA, the aerodynamic coefficient of the moving body Cd multiplied by its frontal area A -- and the human body is grossly aerodynamically inefficient, and so is a bicycle for that matter. But mostly the myth lives on because old motor racers rarely take up cycling, and old cyclists rarely take up motor racing. See, motor racers know, by comparing timed laps with different wings, that you have to be travelling around 100mph before aerodynamics make much of a difference. To state the obvious, cyclists rarely travel at 100mph.

Just for the sake of completion, sure, it is possible to make a bike aerodynamic, and to make the rider more aerodynamic with tight-fitting lycra and a helmet that will take his coach's eye out with its spiked point (which would do a lot better cut off short according to Professor Kamm's formula than remaining as the fashionable full length point), but you wouldn't be able to tour on a bike with disc wheels rather than spokes, and with the triangles filled in -- where I live, on an aerodynamic bike you'd come a cropper before fifty paces from my house when the wind coming all the way from the Urals up the river valley crosses your path at right angles, and that's before you even come to the footbridge across the river, one of my favourite crossing points.

There's something else old motor racers and car designers know that bicyclists and their associated designers apparently never heard of: A flat back on the drops moves the aerodynamic centre of pressure forward, adding to the instability of the bike. The rider must put more effort into keeping the bike running straight. The tyres work harder because the forward CoP (centre of pressure) makes unwanted steering inputs that the rider fights. That by itself might be enough to chew up the marginal advantage of drops in power delivery and perhaps another marginal aero-advantage in a flat back. The CoP works perceptibly from a much, much lower speed than aerodynamics do, and the better the CoP (meaning further back), the greater the aero advantage it brings with it because it brings because there is less space behind the rider for the newly unlaminated air flow to cause drag, that is, the most energy-sapping disturbed airflow is off the back of the bike sooner. Not that there's a lot of airflow clinging to a human: his body's various roundings are all hostile to laminar airflow in any direction except maybe -- I know it is counter-intuitive but so is a lot of proven aerodynamic theory and practice -- feet first.

Aerodynamic improvements on bikes can never be anything but marginal, and on touring bikes they're likely counterproductive. Thus there is very little reason to set up a non-racing bike for drops, and less to design it to take drops, except if the individual cyclist has ridden so long on drops that it would be too much of an effort to learn a new riding style, in which case the cyclist should of course be given what he wants  -- it's his money.

leftpoole:
One of my ailments is Spinal stenosis. I can only ride in one position using drop bars. I ride ‘on the hoods’. I’ve tried straight type bars but these result is severe pain.  One reason (1) I don’t go Rohloff is because of the lack of suitable handlebar.
Just my little bit of input today 😊
John

B cereus:
Andre I agree with much of what you say.  Especially that many modern bikes are designed for a  riding position that is unsuitable for a significant numbers of people that buy them.  I also agree that for most  of us the aerodynamics of the bicycle hardly matter, but I can't agree that the aerodynamics of the rider don't have consequences. They do, and become  increasingly significant as speed increases. Moreover the difference between motor cars and bicycles is that the former have much bigger engines and hence more power.

Drag increases with the square of the speed and even more significantly the power to overcome it increases with the cube of the speed.  Between 10 and 20mph drag will increase fourfold and the power to overcome it will increase eightfold. Even if you rarely travel faster than 10mph a moderate 10mph headwind will have the same effect of doubling your apparent speed.

In a worse case scenario (Cd) in your expression CdA will equal 1. The key to reducing rider drag is to reduce frontal area (A), getting low into an aerodynamic position using drop bars is but one way of achieving this. Even greater gains, and comfort, can be gained by freeing oneself from the tyranny of the UCI and buying a recumbent.

You can have fun playing with this online bike calculator,

http://bikecalculator.com/index.html

Investigate how different positions  affect the other parameters.

martinf:

--- Quote from: Andre Jute on January 15, 2019, 06:34:25 am ---Drop bars might offer more efficient locomotion -- I'm not so sure about that, actually -- but they can't ever be good for your back.
--- End quote ---

For long rides I prefer drop bars mounted fairly high up, so when riding the tops or the hoods I have a fairly similar back position to most of my other bikes with "straight" or "roadster" bars.

I'm fairly sure a crouched position as when using the lower position on drops is aerodynamically more efficient and that this makes a difference when riding at speeds over about 20 kph. It also gives at least a psychological boost when riding into a headwind for hours on end. Any aerodynamic gain is much less relevant when riding on the flat at fairly low speeds.

The main reason I use drops is for better hand comfort as it is easy to vary hand position. I spend most of the time on the hoods, hands parallel to the bikes axis. When I want a change I use the tops, hands at right angles to the bikes, or go down to the hooks, hands parallel to the bikes axis. I also use the hooks if I want to go fast or when riding into a head wind.

My Thorn utility bike now has "sit up and beg" bars, which allow a hand position vaguely similar to riding on the hoods of drop bars, hands parallel to the bikes axis. I have set this up to be slightly more uprght than my other bikes, so as to have a better view over the tops of cars. But I don't much like riding this bike for distances over about 50 kms as there is only one real hand position. And I feel my back more when riding more upright, the most comfortable back angle for me seems to be about 45° at age 62.

mickeg:
I used to use the drops maybe 5 percent of the time when I was really pushing into the wind, but could not do it very long.  Then I dropped about 15 percent of my body weight, much of it centered in the abdomen, I found it much easier to use the drops for longer periods of time once that extra girth was gone.  I think the change in girth really was the big difference.  Top of my drop bars is generally about the same as the top of the saddle, or maybe 5 to 10 mm below the saddle.

Most of the time I am riding on the hoods or if I want to sit up a bit, on the bar tops near the stem.  But when pushing into the wind I find I can usually keep a similar cadence and shift up one step on the cassette (eight speed 11/32, a step is generally 12 to 15 percent change in gearing) or one gear on the Rohloff (about 13 percent) if I switch from using the hoods to using the drops.  Thus, I am gaining somewhere between 10 and 15 percent in speed by using the drops.  There are some days in strong wind when I might use the drops for 30 to 50 percent of the ride for tens of miles.

With my bars as high as they are, when I am using the drops I am not leaning forward anywhere near as much as a racer.  So, no, I am not in a racing crouch with horizontal back.

Touring with panniers and stuff on top of the rear panniers, using the drops does not give me as much additional speed, but I still like to use the drops when pushing into the wind.  The extra stuff on the bike reduces the aerodynamic advantage of using the drop bars.

A friend of mine that is the same height as me but he weighs about 25 to 30 percent more than me (mostly abdominal girth) used to use drop bars but since he never used the drops he decided instead of trying to reduce weight, he would change his handlebars.  For the past six years he has toured on bullhorn bars instead, riding on the bullhorns is a posture that is very similar to riding on the hoods on drop bars.

I have had many different drop bar shapes over the years but the shape of the bars I have on my Nomad is the shape I like best.  A short reach so I do not have to lean too far forwards and a drop of roughly 13 cm.

First photo is the bars on my Nomad (hubbub adapter for my Rohloff shifter on the bar end), second photo is my friends bullhorn bars.

Navigation

[0] Message Index

[#] Next page