Bike rack drain

[InventEd]

Cars drag is mainly due to frontal cross sectional area (consider the biggest cross sectional area facing forwards) and the ability to 'reattach' the air at the back. Your ideal car (look at one of the solar cars that they race across Aus) is more like a wing. Gentle taper to the back and nothing sticking out to upset the smooth airflow. Reality is cars of that shape are not the most practical (too long and low). So commercial cars trade off and there is turbulent air at the back. Notice how the back of your boot round the number plate gets dirty sooner then other areas. Thats the dirt spun round in the turbulent chaotic flow at the back.

So a bike on the back doesn't help either of these. It increased your cross sectional area and has no taper behind it. I cant put it into numbers without a lot of number crunching and brushing up on my fluid dynamic skills but it will be ugly!

Now speed is important. In a high speed race car like a Le Mans racers the reattachment is probably more important then frontal area as the turbulent air will cause a significant vacuum at the back. Even at more reasonable speeds the effect rises with a square of speed. So at 60mph drag is 2.25 as bad as at 40mph. 70mph 3x as bad as 40mph. So best to slow down and have a more chilled drive if you dont want to suck the range of your electric car down.

Actually thinking about it, its worse then that. Force is a square of speed, but power is a cube of speed!

 

[Cogarch]

Absolutely, but can adding a wedge early in the roof line cause a smoother (and therefore more efficient) flow around the bikes. Essentially creating a low pressure zone of chaotic turbulence for them to exist in, rather than them causing such a zone. If I get time I'll try and draw some pictures later...

You mean like this.?

 

[Avendit]

View attachment 507501
You mean like this.?

Yep, but less lycra. Busy running simple simulations using flowsquare because I've thought about this before and wondered.... Images incoming in a bit.

 

[Avendit]

OK, more than a bit. Rendering is innexplicably slow, despite the use of a beast of a laptop and having paid for the extra calc speed. Its obviously limited by something that I can't spot as its only using 15% CPU. And it doesn't seem to like running multiple copies in parallel. So it will be a few days to gather and process 3 cases of 20 seconds worth of simulations :/

currently generating a base case:

 

[LukeT]

Tow bar or boot mounted rack?

I wonder whether removing wheels and strapping them to the bikes more centrally would make a worthwhile difference. The extent to which they protrude left and right is disrupting otherwise uninterrupted air.

I have so far managed to keep bikes inside this car, for the same reason, but have experienced big drops in ICE fuel economy with bikes before. I always thought roof worse than rack but haven't data to back that up.

 

[Avendit]

Tow bar or boot mounted rack?

I wonder whether removing wheels and strapping them to the bikes more centrally would make a worthwhile difference. The extent to which they protrude left and right is disrupting otherwise uninterrupted air./QUOTE]

I would have thought anything you can do to tuck things in behind the car would help. Taking the wheels off and dropping the bikes lower on a tow mount rack will help, but then you need to find a way of mounting the bikes (if you have a wheel support rack).

 

[InventEd]

OK, more than a bit. Rendering is innexplicably slow, despite the use of a beast of a laptop and having paid for the extra calc speed. Its obviously limited by something that I can't spot as its only using 15% CPU. And it doesn't seem to like running multiple copies in parallel. So it will be a few days to gather and process 3 cases of 20 seconds worth of simulations :/

currently generating a base case:
View attachment 507571

Looks like non of the air reattaches much off the roof so maybe its not as bad as I thought. I wonder what would happen if you made a rear bike mount that had the bikes pointing forwards (and hence more compact). I guess it would stick out and fail some safety standard but should be better then sticking out the sides. (slight issues that you cant buy a rack like that).

I really look forward to seeing what you simulations show and which configuration is best!

What speed are you simulating by the way? ~40mph?

 

[Avendit]

It's about 45mph, but my first time doing any of this so I've no idea if I've got it all set right.

The image above didn't match the images from unplugged performance, and I realised I'd put the 3 in as too small. Increased its size a bit and re-ran everything, will see what I get this evening. It looked like flow was smoother down the back. It's all very pretty pictures tho!

 

[LukeT]

Looks like non of the air reattaches much off the roof so maybe its not as bad as I thought. I wonder what would happen if you made a rear bike mount that had the bikes pointing forwards (and hence more compact). I guess it would stick out and fail some safety standard but should be better then sticking out the sides. (slight issues that you cant buy a rack like that).

I really look forward to seeing what you simulations show and which configuration is best!

What speed are you simulating by the way? ~40mph?

The sucker type mounts can on some cars be used further back and therefore mount a forward facing bike further down and more into the turbulent air. Suck it and see, as it were...

 

[ckarner]

Working on this setup currently, not liking the possibility of damaging the glass.

 

[Avendit]

I saw someone in the US with an S with bikes just bought a luggage rack for the back and put the bikes in a pile on that. Apparently considerable improvements were had.

Like this: AmazonBasics Hitch Mount Cargo Carrier https://www.amazon.co.uk/dp/B074PBSSFV/ref=cm_sw_r_cp_apa_i_EpzoEb0SGTZ2A

 

[Avendit]

Well, attempt 1 done:

Its obviously hitting some kind of boundary condition a lot of the time. Will try again with slightly different software. Hopefully you can see my thoughts with a pre-spoiler or wedge. Will also try it with an actual top of roof spoiler once its up and running. if its any faster I'll aim for a little higher res too.

 

[LukeT]

Working on this setup currently, not liking the possibility of damaging the glass.
View attachment 507772

16" is an usual wheel size for a road bike - are you even shorter than me?

 

[MrBadger]

Hopefully you can see my thoughts with a pre-spoiler or wedge.

Have you thought about becoming a pizza delivery guy

 

[ckarner]

16" is an usual wheel size for a road bike - are you even shorter than me?

Just showing the idea, I will work on the correct scale moving forward.

 

[Cogarch]

Well, attempt 1 done:

Its obviously hitting some kind of boundary condition a lot of the time. Will try again with slightly different software. Hopefully you can see my thoughts with a pre-spoiler or wedge. Will also try it with an actual top of roof spoiler once its up and running. if its any faster I'll aim for a little higher res too.

This is nice. It would be even nicer if it could somehow emulate a single road bike, which is significantly permeable to airflow. However I have no idea how you would do it!

 

[Avendit]

A road bike isnt going to be nearly so bad. At least mine would tuck behind much better and is almost a foot shorter in terms of height. The blob I ended up using represents my 21" MTB, wifes's woman's MTB and a 20" and 14" kids bikes. It makes enough of a barrier that I accept it really does need the light plate on the bike mount.

 

[squarepeg77]

Sorry, a little off topic, but has anyone here tried to fit two road bikes *inside* a Model 3?

I guess with the front wheels removed and handlebars turned they can be ‘posted’ through the boot opening?

 

[mjopling]

I move bikes with them on the roof and have not experienced much effect on range.

View attachment 497028

What kind of suction mounted top rack do you have? Is it clear that the danger of roof damage is minimal?

I have a Kuat Sherpa 2.0 rear rack that I love, but might consider a roof rack for the MY if it can go in and out of our garage without dismounting the bike...my wife and I have both forgotten the bikes in the past and driven into the garage a few times...not pleasant. New house has taller opening so might work.

 

[markst1]

New to Tesla but learning lots from the posts here about aerodynamics - love it!

Does anyone have a recommendation for a good tow hitch bike carrier that can carry 4 family bikes (i.e. avg weight every day mountain bikes etc...)? I’m not too worried about drag as it’s mostly for local use - although I have a feeling as soon as I put it on the car and saw the drain I would feel compelled to improve that also.