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Foam roll-downs. (Or rolls-down if you prefer.)

Wednesday, May 20, 2009 8:47:00 AM Categories: Foam fairing Roll-down tests

Anxious to Compare and Contrast, I took the newly built, but not yet photographed (a mistake as will be seen) fairing to a certain hill I know and love, and did roll-down tests before the forecast rain that didn't happen. (The above is of the tailbox only, which information I include for those who imagine I pedal backwards. My tailboxes have been huge ever since a conversation with John Lafford, who (I'm summarising) said 'Make your tailboxes huge'.)

Tailbox only:
25.2 mph
25.1
25.3

Blue front fairing plus tailbox:
26.6
27.2
27.2

Blue front fairing only:
26.5
then I got my cleat stuck while stationary and fell over and snapped most of the wooden slats inside the fairing and cut the top off my little finger on an errant bit of sharp road
26.4
26.3

No fairings whatever:
25.6
25.3 with my RH on the handlebar extension but my LH over my chest
26.0 with my RH on the inner part of the handlebar and my RH over my chest

all of which took exactly one hour.

Then a strange man came wandering over from the adjacent field, stubbly (the man, not the field) and dribbling a little, with an elderly shotgun lodged in the crook of his arm. Whereupon I felt I ought to speak to him to explain what was going on.

I said 'I'm just testing different fairings on a recumbent bike because when each is changed, the hill gives me clear results' and the stubbly man said 'Clear results, that's right' dribbling a bit from the corner of his mouth. I said 'I roll down the hill to see how fast I get up to on my computer.' The man said 'My computer, that's right, mm,' and I thought this is strange, he is unable to compose and speak a sentence; all he can do is repeat the last two words I've said. And then I started to think that this is going to be either the most interesting or the most uninteresting conversation I've ever had in my whole life so just to see which, I continued: 'First I put the tail fairing on and roll with that, and then I add the front fairing, and then I see what difference it makes.' 'Difference it makes,' said Stubbly, dribble dribble. At this point I wanted to hug him for having survived for so long despite being completely and wholly stupid but then I began to think of those horror movies that used to come on Channel 4 on a Thursday night where a Perfectly Innocent American Person goes into the woods and meets a Strange American Person Who Already Lives There and all sorts of unpleasantnesses that I'd rather not remember ensue. So I hurriedly set off in a southerly direction and was swiftly followed by a volley of shots.

(Okay. Maybe some of this is untrue. Maybe I just wanted to add a bit of colour to an otherwise bland account. )

So we can conclude that my fairings are rubbish, if we are to be honest and objective. And I wonder if this is partly because of the large hole - well, the non-floor - which creates colossal drag. Neglecting the hideous knobbly shape. Pictures will have to follow when - if - it ever gets repaired. Anyway this provoked the following angst-ridden exchange:

Uh? It seems that the tail fairing on its own makes no difference, indeed possibly a minor decrease in speed. The front fairing does make an improvement. The front fairing + tailbox is even better. Do you concur? I'm impressed that you can take a hand off whilst riding. There is no way I can do that on the ratracer. It is *pissing* down here at the moment and I am sat sitting here on my stool in a medium sized puddle having got thoroughly drenched on the way to work 'smorning. I've just read the paper and it gleefully reports a big depression that will cause a lot of weather later this week and at the weekend. At least I'll be able to get some planking done on the fairing. Incidentally I threw a sheet over the frame last night to get an idea of the shape and I was surprised how large it is. It is 20" wide at the widest point and this looks enormous. - Bob

Whenever I've made fairings, front and rear, 's orlways been the same. The tail adds *at best* half a mile an hour, the front adds about a mile an hour, and both together add a further half mph. But my level-ground racing speed is too slow for the tailbox to collect separated flow and reattach it. My brother tells me that his recumbent motorbike tailbox is very significant. Mind he does have a very good front fairing too. Implication of Hucho (p311) is you need a big rear section and a fairly high Reynolds number before air re-attaches once it's separated. I forget what my Reynolds number is - I worked it out once - but it was significantly less than the lorries Hucho was describing.
The present tailbox in earlier roll-down tests gave me an extra 0.36 mph at 23/24 mph. This is a whisker less than I had expected, but even so I was surprised by the result showing a reduction in speed with the tailbox. To be statistically valid, I need a *lot* more trials but they take a huge amount of time to do, and as the morning wears on the wind gets up so you have to stop. One problem is that I notice myself tensing, even during a roll-down, and when I relax I go faster. This *might* be because I flop lower down in the seat when relaxed, but there may be other reasons that I haven't figured out yet. I do know the gain is small, and it's also highly variable between runs.
In Engerland, fair Engerland, there was a hill outside Barrow on Soar which after a gentle slope getting me to 20 mph and holding it there for a good steady 200 yards where I could read off my (realistic) road speed, then dropped away to a valley floor where I hit 30mph before slowing up on the other side of the valley. The impact of tail fairings was much bigger at the higher speed.
Example: Paudy crossroads to Walton, 13.12.97.
MWB touring machine with large foam tailbox: 21.5 mph, max at bottom of hill 30 mph
Same with tailbox removed: 21 mph, max at bottom of hill 28.5 mph. - R

 

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