Any aerodynamics experts in the house?

I was thinking of some kind of fairing for bikes on the roof of the car, to help with fuel consumption.

Then I thought that a fairing from the front up over the bikes would have too great an area.

Then I thought that something to cover the sides of the bikes would help – like disc wheels on time trials bikes do. But that’s no good in cross-winds.

Then I thought about some kind of deflector low down to just break up the airflow in advance..

Anyone got any ideas/experience?

Your concept seems based on the premise that say 3 bikes side-by-side is less aerodynamic than a car (which would be of similar width).

I’m not sure that’s true.

How much fuel do you hope to save?
How much will it cost to develop this thing?

I’d say a horizontal box with bike on it’s side and pedals sticking out vertically would probably offer the best solution.

From my limited knowledge i’d say that you want to delay the point at which linear airflow breaks down into turbulent flow such that it’s as far back or away from the car as possible.

perhaps book some air tunnel time.
or just enjoy riding your bike instead.

Maybe some kind of trailer to enclose the bikes?

Or a bigger car to put them in?

You see where I am going?

I think you’re taking the ‘molgrip approach’, which is to get a wee bit too concerned with the minutiae of a problem rather than taking a step back, hitting yourself across the back of the head with a wet fish and asking ‘What would crikey do?’

Crikey says; don’t put them on the top of the car if you are concerned about wind resistance. The best place is inside the car, the next best place is on the back of the car, the next best place is lying them flat on the car roof, the next is stood up on the roof.

If the only place you can transport them is on the roof, suck it up.

tailgate a lorry

Your concept seems based on the premise that say 3 bikes side-by-side is less aerodynamic than a car

What do you mean?

No idea how much fuel I’d save, and I hope to spend no money on it, bodging something from the contents of my garage.

A horizontal box would be nice but I posted earlier about roof boxes, I’m not sure any would be wide enough.

or just enjoy riding your bike instead

I already do that in my spare time, however now I am at work being bored. I can muse on technical problems, however sodding off riding has far more serious negative consequences for my career.

Crikey says; don’t put them on the top of the car if you are concerned about wind resistance. The best place is inside the car, the next best place is on the back of the car, the next best place is lying them flat on the car roof, the next is stood up on the roof.

If the only place you can transport them is on the roof, suck it up

Concise….check. Incisive…..check. Accurate….. check. Mildly condecending…..check.

All present and correct then – thread closed 😉

if you were uber concerned you would make sure it fitted inside the car somehow or 2nd best on the back

No direct experience in this case but in general in aero dynamics / corners = bad.

Frequently the trailing edge is the very important, a smooth sleek design on the rear reduces vortices occurring and is as importnat as the front profile. This occurs when boundary layer separation occurs.

Milne-Thomson theoretical aerodynamics is a good book to look into this stuff.
available on line too.

For your particular application I expect best results would come individual full cover bike fairings with the bars turned 90 deg to create a thin profile.

Something like this upside made of CF may just work.

Obvious init, sell all your mtbs and buy TT bikes.

Sorry, I’ve been on nights so I’m a bit out of sorts, I was aiming for ‘humorous’ rather than ‘condescending’, just need to recalibrate my thingometer.

I hope to spend no money on it, bodging something from the contents of my garage.

Please post all of your journeys in advance so that the rest of us can avoid the risk of flying debris. 😉

The head and cross wind forces on a structure big enough to steamline a bike, yet hollow enough to contain the bike would be massive.

molgrips – Member

Your concept seems based on the premise that say 3 bikes side-by-side is less aerodynamic than a car

What do you mean?

You want to put a fairing in front of a few bikes to make them aerodynamic…meaning you think a solid shape covering the entire frontal area would be more aerodynamic.

If I have uinderstood you correctly.

Ooookay.

My question was about air flow over bikes.

It was NOT asking the best way to transport bikes. Obviously in-car is better, but not when the car’s already full to capacity! If it is already a given that I must take bikes on the roof, then I want to see if there’s a way of minimising drag.

Simple idea, isn’t it?

Obvious init, sell all your mtbs and buy TT bikes

This is good, like it. Would be interesting to see what the difference is between TT bikes and MTBs 🙂

Thanks to TheBrick for an interesting answer – turning the bars around does sound like a good idea. That plus simple front fairing of some kind could be a good one.

meaning you think a solid shape covering the entire frontal area would be more aerodynamic

That was my firs thought (2 bikes not 3 tho) but then I realised the area would be so big it would not work. Thinking of some kind of diffuser or something.

I was aiming for ‘humorous’ rather than ‘condescending’, just need to recalibrate my thingometer

In every respect I think your original post was spot on! 😉

http://www.barris.com/gallery_cars/Novelty_ProductCars/novelty.html

Have a look here, safe for work, although one or two of the ladies are rather precariously restrained in the upper chest area.

I’m no expert here, but if you really wanted to try something cheap and not that dangerous, how about cling filming the bikes? Wrap them entirely, individually.

Anyone confirm this could work? Explode? Cause a massive pile up?

Elfinsafetys dad?

how about cling filming the bikes?

They would act like sails.

You could get a bike in a large roof box if you took off the wheels and seat post.

They would act like sails.

Perfect, you wouldn’t even need to start the engine!

You need a big tear-drop shaped box.

No no no, what you need is to take a razor blade and shave a few grams off the bikes cause we all know that would them go faster through the air ….

out of interest.. how are you driving this car and what is it?

its amazing how many people you see on busy roads ragging it and then braking really hard.. and repeatedly doing so..

the next best place is lying them flat on the car roof, the next is stood up on the roof.

why would flat be any different to standing up? The wind doesn’t know which way round they are.

meaning you think a solid shape covering the entire frontal area would be more aerodynamic

Actually the front is less problematic than the back, Putting a fairing in front is pretty useless if the flow just brakes up after passing over the fairing. Most of your drag is suction drag rather than pressure drag, you need to make sure the flow comes off the fairing smoothly too, which means it will probably have to cover the whole bike.

Now of course once you’ve achieved this, you have what amounts to a wing stuck on top of your car, which is fine if you are driving in a directly into or out of the wind. Anything else, and you are generating lift, huge amounts of lift. Enough to tear your fairing and anything else attached to it, off your car. Does this help?

Get a rack for the back of the car.

/thread dead

/thread dead

You’re closing the thread dead tag? Doe that mean that the thread is alive again?

why would flat be any different to standing up?

Putting them in a box would help, which was what I meant. Or stacking them close together would help I suspect, but I don’t really know why.

I drive economically already.

I have a rear mounted rack but it’s hard to use when towing a van.

Thanks for answering a totally different question to the one I asked – I appreciate it 😉

Putting them in a box would help, which was what I meant. Or stacking them close together would help I suspect, but I don’t really know why.

because reducing the surface area will give you less drag

there will also be an optimal driving velocity because drag increases with the square of velocity IIRC

What about one of them caravan towing spoiler things…
Dont you tow a caravan Molgrips…?

Stick them inside the caravan. I’ve towed ours with 4 bikes inside, safely tied down obviously; & with a mind on the effect on the MTPLM. We’ll be taking four bikes to France in August in this way.

Of course this assumes you happen to be caravanning. It might be a bit lavish for an afternoon at a trail centre.

Oh – a shed-dragger and you want to improve aerodynamics? Get rid of the shed. You really are farting about in the margins fretting over bikes on the roof of the car.

Yeah this is for the ‘van. We do put them inside now, but two things – on the way back from Munich the van will be heavily loaded (and there’s bugger all storage space in this old van apart from right at the back, which is a stupid place for it). Also when touring it’s really nice to be able to stop and make a cuppa/lunch, without having to unpack junk from the van itself. Plus it’s very difficult not to scratch the crap out of the interior which isn’t an issue now but will be at some point in the future 🙂

Will be interesting to see how much bikes will affect towing economy.

You really are farting about in the margins fretting over bikes on the roof of the car

Yeah? Got figures for that?

If it were say 28mpg vs 25mpg with bikes on the roof, that’d mean an extra 43 miles ish per tank in my car. I reckon that’s significant. Dragging the shed is a given, because tent camping is sh*t.

What is it without the shed? Enough to add to campsite fees and stay somewhere nice?

You’re planning to put bikes on the roof of a VAN?

*rolls eyes*

Why not just reduce the amount of carp you’re carrying around and put the bikes inside, safe from damage, theft and road dirt? Failing that, get a towbar mounted carrier and put them behind the van where they will be in the patch of dead air that you’re hauling along and won’t affect the aerodynamics. Some even report improved fuel consumption from this, maybe because the bikes break up the area of low pressure behind the vehicle.

Buy one of those little teardrop shaped caravans, but your bikes inside that, then mount it on the roof of your car.

HTH.

One day when I am really really bored I will calculate how many miles I need to drive with my bike(s) on a rear mounted rack to justify buying one over continuing to use our roof rack.

Get some of these for the bikes, there’s a whole site for this stuff on cars called ecomodding.

http://ecomodder.com/%5B/url%5D

there will also be an optimal driving velocity because drag increases with the square of velocity IIRC

Unfortunately, that optimal velocity will be 0.0 mph 😉
Or get lucky and drive in a gale that is always perfectly from behind 😉

C+ magazine has a review of bike carriers this month, complete with a delta MPG. No idea how they came up with that delta MPG figure but my guess is it’s totally unscientific and complete codswallop. Apart from the “put the bikes in the car” figure (which will be 0.0MPG difference, ignoring mass of the bikes etc. of course).