[quote=twisty ]I was hoping that the aero benefit for a rear fairing can be very significant as a majority of drag is created by trailing area wakes and not leading area pressure zones but I am far from being expert in this area so would really appreciate some expert advice, I while ago I did try to set up an analysis in openFoam but it got a bit too tricky/time consuming to do as a little project. Surely a rear fairing would make an order of magnitude more difference than switching from normal lycra stuff to full race gear and aero helmet though?
I'm fairly sure you're right - a lot of aero benefit from a rear fairing, and certainly far more than from all the standard aero stuff on a bike. The question is how practical it is to get something like that.
http://www.m5-ligfietsen.nl/site/EN/Carbon_Specials/Tailbags/ suggests 28% drag reduction with a tail fairing on a recumbent - you can't necessarily translate the benefit to an upright bike, but my gut feeling is that if anything there ought to be more benefit as an upright bike punches a much bigger hole.
I'm fairly sure you're right - a lot of aero benefit from a rear fairing, and certainly far more than from all the standard aero stuff on a bike. The question is how practical it is to get something like that.
When I've got a bit of time I'll try to set up some kind of CFD before I start carving out shapes in polystyrene. Although punching a bigger hole than a recumbent it is also a messier hole to reattach which may affect practical gains.
Has anyone suggested training yet? If you're only riding at 17mph aero isn't going to make much difference.
Yes, they have and that is a long standing and generally underperforming project which is seperate to this engineering exercise. To those of you who are faster than me, congrats, but don't worry I am under no illusions that my performance is anything other than pretty average (although feel obliged to point out I'm generally cruising at 20+mph to get a 18mph moving average :wink:).
All I can think of is that ride the green wave belongs in the management speak thread.
It's lingo picked up from doing traffic engineering. It's called a green wave because the traffic signals turning green at each junction progresses like a wave travelling through the ocean so it is co-ordinated with the front of the traffic platoon. What am I supposed to call it in lay speak?
Get a team car to carry the laptop, clothes, spare water etc.
It's what the pros would do.
Managing a 35km commute in Singapore is impressive, It can't be possible to live much further away from work can it?
When I've got a bit of time I'll try to set up some kind of CFD before I start carving out shapes in polystyrene. Although punching a bigger hole than a recumbent it is also a messier hole to reattach which may affect practical gains.
It's an interesting project - but there are a lot of variables and unless you have a lot of data you'll struggle to validate any of the results. Reynolds number is quite low, but from the bars backwards things get messy with legs moving etc. So I would expect a lot of turbulence, the team Sky skinsuits that have dimples to initiate turbulent flow at the shoulders, which presumably helps due to the flow lower down being so messy. If the fairing gets too long you might just end up increasing skin drag. Also don't forget to look into loading from side winds - it's no good being 5mins faster on a good day if you're being blown off the road when the winds coming from the wrong direction.
Bafang/8 Fun 750w motor with a 13.6ah battery. Dont get caught
I have one of these (but 25 odd ah) on a 29er+. TBH I built it a few years agoand use it to pull a trailer with my lad who is in a wheelchair and got a bit heavy for his dad.
Obviously never used it on the road, but "I'd guess" if I had tried it once a couple of years ago, it would do about 30mph and still have power after 50 miles (obviously without my son)...
the team Sky skinsuits that have dimples to initiate turbulent flow at the shoulders, which presumably helps due to the flow lower down being so messy.
Apologies if I'm telling you something you already know, but the standard reason for creating a turbulent layer by tripping like that is that a turbulent layer will stay attached to the surface for longer than a laminar flow. When the flow detaches you get the area of negative pressure (what twisty is trying to eliminate with his fairing) which causes a lot of the drag, so delaying that helps a lot.
Try a handlebar bag - it may sound counterintuitive, but have a look here [url= https://janheine.wordpress.com/2012/05/02/aerodynamics-of-real-world-bicycles/ ]Jan Heine Aerodynamics of real-world bikes[/url]
When the flow detaches you get the area of negative pressure (what twisty is trying to eliminate with his fairing) which causes a lot of the drag
Yeah, but the OP is talking about long aspect ratio fairings (7:1) which work great in laminar situations, but the flow is unlikely to be laminar at the fairing. Something a lot blunter would maybe help. I'm thinking of the 'dip' in cylinder drag that happens around Re=10^5 where a stable narrow turbulent wake is developed.
I've barely ridden this year and even I could probably do 17.5mph for an hour.
In the OP's defence, that's a fairly similar average to what I get, and in the only 10 I've done I got a shade under 25 mins (no aero gear), I'm probably a little fitter than that now. Depending on the route etc, traffic and junctions slow you a fair bit in a real world commute, even compared to a similarly hilly route out of town. It's quite possible that he's cruising a bit above 20mph, but with all the traffic to get through his average is hit by that.
I'm impressed with your speed, this isn't a timetrial we're talking about it's a commute and should be done at a nice leisurely pace so you don't get too sweaty. I'm in awe of anyone that would get up that early and commit to riding 20+ miles, do a days work and ride home every day.
[quote=irelanst ]Yeah, but the OP is talking about long aspect ratio fairings (7:1) which work great in laminar situations, but the flow is unlikely to be laminar at the fairing. Something a lot blunter would maybe help. I'm thinking of the 'dip' in cylinder drag that happens around Re=10^5 where a stable narrow turbulent wake is developed.
Hmm, you're clearly more of an expert at this than me (or just more current, it's a long time since I've done Re calcs, so don't expect me to join in if you're getting into that level of technical discussion!), but I'm not sure the OP was talking about such a high aspect ratio fairing, just a fairing in general. I agree a blunter one would be more effective - not just for the reasons you mention, but also because the performance would be much better in a crosswind (both from a drag and handling perspective).
Speeder - Member
I'm impressed with your speed, this isn't a timetrial we're talking about it's a commute and should be done at a nice leisurely pace so you don't get too sweaty. I'm in awe of anyone that would get up that early and commit to riding 20+ miles, do a days work and ride home every day.
It depends - if you're doing a short commute, up to about 5 miles, definitely chill out, ride slow, don't have to change/shower at the end.
Bigger commutes, and the OPs is definitely in this category, going hell for leather is definitely a viable option as you'll be mucky and a bit sweaty at the end of it anyway in all probability (and the comfort of specialist clothing is worthwhile).
I'm impressed with your speed, this isn't a timetrial we're talking about it's a commute and should be done at a nice leisurely pace so you don't get too sweaty. I'm in awe of anyone that would get up that early and commit to riding 20+ miles, do a days work and ride home every day.
I do 20 miles each way every day, there's no way I'm doing that at a leisurely pace.
When I worked locally I used to commute 6 miles across town in a nice steady 25-30 mins. Ok that was including lots of light stops but it was a nice gentle pace that I didn't get sweaty at and could just wear my clothes like you do in Amsterdam or Copenhagen. 20 miles is a whole other kettle of fish and hats off to you all. NWIDTS 😀
It is good that a blunter fairing is likely to perform better as the 7:1 I suggested earlier would be rather cumbersome.
I should say that I am approaching this from virtually 0 knowledge of CFD/FD. I am wondering if I am better off trying to run a simulation in openFoam, bodging together a scale model and little wind tunnel, or just jumping into randomly trying out bits of polystyrene on my bike.
I used to do the gentle commute in work clothes thing in UK but in Singapore this it is essentially impossible to remain presentable after cycling due to the heat, humidity, and thunderstorms.
I do live and work on opposite ends of the island and haven't found a quicker route yet, well I can shorten the distance a bit if I take the insanely dangerous Bartley Road and then run 5km through the jungle carrying the bike, which ends up taking more time.
As another take on the laptop problem, have you considered something like an intel nuc with a screen and keyboard at both ends? Gets you exactly the same computer at both end of your commute but a much smaller device to carry.
Oldrich that article is great, thanks for posting!! 🙂
It's interesting that it says you save 38% in an aero tuck. I love putting my elbows on the bars as it feels soo much easier to go faster like that, but your hands are off the brakes and it's doesn't feel super safe on bumpy roads so can only do it for shortish periods..
Some of those bicycle quarterly publications are quality, there was another one on drivechain efficiency that I rather liked.
People seem to be addicted to re-organising my work tech paradigm 😯
I, for the most part, have no problem accessing files or RDCing into a server/ workstation from home if I need too (although I am yet to find a workaround for some rendering apps that refuse to open at in RDC).
However, I often need a laptop for meetings and for a decent proportion of these it is not practical to go via the office before/after the meeting, thus if I cannot take my laptop from/to the office on the day before/after the meeting on my bike then I lose out on yet another bike commute trip.
I do like those Intel NUC's though I've been contemplating them as an alternative to industrial PCs for some stuff.
well I can shorten the distance a bit if I take the insanely dangerous Bartley Road
Is that the 3 lane road that's always rammed with traffic? No way I'd go anywhere near that on a bike!
People seem to be addicted to re-organising my work tech paradigm
Only because I've been through the same thought processes myself. I drag a (not very big) laptop to and from my office only to plug it into a screen and keyboard at both ends, so have been wondering about other options.
Back vaguely on the original topic, I think I remember reading something from Graham Obree suggesting front fairings having a place on "utility" bikes.
Some of those bicycle quarterly publications are quality, there was another one on drivechain efficiency that I rather liked.
Pretty sure I read that! Yeah they are great 🙂
I has some problems getting CFD to work. However last week I chanced across some nice blocks of polystyrene, so I stuck them together with silicone, hacked away at them and this was the result.
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I only got to do one run last night on the only cycle lane in Singapore, and it is always windy there so not much data to go on. However I won't get the chance to try it again for over a month hence posting now. I suspect more data may show this is on par with my TT bike. However, I think I can improve the design quite a bit, I need to try again with the CFD to try out a few design options.
Perfect for carrying a slice of an enormous cake or pie.
Nice bit of brie.
Genius 🙂
You should carry on with this.
We had a guy commute round our way for years with a clear perspex arrowhead front fairing about 30cm Wide and with about 30cm above the bars and 15cm below.
Always seemed fast.
I think it should be pointed down more to suit how the low pressure area created by the torso seems to be drawn downwards into the wake left by the bike.
In terms of the benefit, anecdotally I'm thinking if the design is right it'd have 5x the have the effect an aero helmet given that the torso wake is about 5x the size of the head wake.
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Massive game of trivial pursuit
