6 May 2015

5th May 2015. All bags are a drag!

Tuesday 5th 58-70F, 15-21C, mild, gales, overcast with rain. A warm low is passing over with "busy" weather fronts. The forecast is morning rain turning to thundery showers later. Followed by thundery rain possibly with cloudbursts. As I write this at 11am the sun has just come out! Meanwhile the wind is expected to turn from east to west. It is supposed to calm and clear tonight so I could try and find my lights. Rest day.
With all this rest time on my hands I might convert my 'Goth' bag to a real saddlebag with a dowel and short straps to hold the bag to the Trykit rack's, top crossbar. Getting rid of the massive, layered leather handle and associated "ironmongery" should be good for a half pound saving, at the very least.

Bag without handle for a sleeker and lighter look.  The Head Gardener did not agree! Said it spoilt the original, antique, "Doctor's bag" look. While I still think the handle made it look like a toolbag being used as a saddlebag as an afterthought. Any resemblance to a doctor's bag harked back to the medieval period and I'm trying my best to avoid any blood letting!

It might would be rather silly to hang the new bag from a B17's bag loops. If the bag started swaying back and forth to my pedalling cadence, as the Junior did the other day despite being strapped to the seat post, I might actually start bunny hopping! Or even pulling involuntary wheelies! And that would never do. I'll keep you posted. Me obsessed? Surely not? ;ø]

It stayed warm, dry and windy for a couple of hours. Allowing me to work outside. See attached images for the work involved in adapting the toolbag to a 'proper' saddlebag. Unlike Carradice I do actually own a tape measure [6 of them actually] and steel rule [several] to make the bag central on the rack. [Or saddle loops if desired.]

The Carradice 'hanging' straps are the wrong colour but will be replaced if I can find some black ones. 

The Trykit rack now properly supports the bag all over. With the slightly oversized dowel helping to stiffen the bag and hold it tightly against the face of the rack for stability and to maintain the boxy shape. The alloy, handle-fixing rivets have yet to be reset in their original holes. I ought to find some stainless steel or alloy washers to avoid them being pulled out again. Note how the "hanging" straps buckles are hidden inside the bag for security, weather protection, neatness and 'minimalist' appearance.

The stand-off, rack crossbar spacers were specially made by Trykit's Geoff Booker to allow more leg clearance. This is not normally a problem when pedalling but I like to straighten my legs to stretch my calf muscles occasionally. I find it helps to loosen up tired muscles on longer rides.

The air drag/turbulence caused by such a large rear bag will depend to some extent on the angle of incidence of the solid body to the airflow. Unfortunately the airflow passing over the riders legs and between them is highly disturbed. Not to mention the vastly greater complexity afforded by rapid leg movements from pedalling with both legs 180 degrees out of phase. It is no wonder the top cycling teams and bike manufacturers spend so  much time in wind tunnels!

I have [roughly] drawn the major angles and dimensions of the bag and its slopes relative to the nominally horizontal airflow. The bag probably behaves like a crude, very thick and very short aerofoil but with much greater turbulence due to the squareness of all its edges. While the Carradice Camper is much more rounded but has pockets sticking out into the airflow on either side of the rider. It is almost certain that the airflow separation behind both bags is rather dire. Greatly increasing air drag compared with a "clean" tricycle frame with rider. Testing bare bicycles or tricycles alone in a wind tunnel, without the rider aboard, is likely to be rather pointless.The rider so modifies the airflow that a naked bike is meaningless in the real world.

Only lifting such a "squarish" bag, as most saddlebags are, to rest against the rider's lower back, as a continuation without any air gap, is likely to help to reduce drag. Then, probably only a little depending on the rider's forward lean. Not to mention the difficulty of actually "throwing a leg over" such a raised bag. It might make much more sense to have a long [sports type] bag running along a rear rack instead of the wide, traditional saddlebag. Panniers seem to fly in the face of drag reduction.

Chopped-off tails were once much in vogue and are still seen in the latest high performance cars. The fully formed, 'streamlined' tear drop shape is no longer considered the perfect form for low air drag. Many recumbent racing bicycles sport a rear fairing. Though exactly how well they perform  in practice is probably unknown outside a wind tunnel test. Adding the inevitable weight of a rear fairing to a road going machine may not pay the dividends which were hoped for. For steady racing speeds on a flat "oval" circuit improved lap times may give a better clue. A chopped rear fairing might improve on a typically long tail. I can't imagine that adding anything, except tri-bars, to an upright trike would offer any improvement.

This simple YT wind tunnel video is fascinating. Note how both riders resent lifting their chins to make the most of the excessively long, aero helmets.  Even so the smoke still seems to pass smoothly down the rider's backs without obvious disturbance. Both riders might still gain from using a short helmet if only for comfort. The vertical sections of both their legs and arms add lots of turbulence. As do the open front wheels. The "smoke" can even be seen being dragged around and briefly filling the rim.

Note how the smoke is forced downwards between the riders legs to end up in strong turbulence behind the rider's bum as the air struggles to detach cleanly. The conventional saddle pin and saddle look as if they could be causing far more serious problems than expected simply because of the greatly increased wind speed just here. An aerofoil section saddle pin and "clean" channel-formed saddle could be well worth trying. A normal round saddle pin and typical saddle, with closed off nose section, is very likely to add lots of turbulence. When ideally the wind wants to rejoin the relatively clean air streaming over the rider's back. The first rider's more humped and higher position is actually cleaner than the lower and flatter second. Just trying to get very low, for its own sake, is not always the best choice.

An interesting image borrowed from: VeloViet: Preparing For The Next Epic Ride   [Felt Bicycles] 

Highest pressure areas are red. Note the faster airflow over the hips and channelled under the rider's crutch fail to meet at the rear. The lower air pressure here will try to pull the rider backwards into the partial vacuum like a big suction cup. A slightly simplistic response might be to add an air scoop well below the saddle to push the high velocity air upwards into this low pressure area at the rear. A wide, inverted MTB mudguard, starting somewhere near the lowered top tube and curving sharply upwards, could be used for experiments. Though such a device is very unlikely to be approved for competition. It might be worthless anyway if it introduces more drag through turbulence. Anyone with a wind tunnel could test such a device in seconds at almost zero fabrication cost. If we start seeing upward tilted plates starting around the seat pin clamping area on Triathlon/TT machines remember where you read about it first. Though I'm not holding my breath!

Click on any image for an enlargement.

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