Aerodynamicists - roof boxes

I don't use roof boxes but in my experience anything on the roof adds a lot of drag and noise.  I can certainly hear when my roof bars are on even when there's nothing on them.  I reckon you'd lose 20% of your range at least.

My guess is that the front area of the box will be the biggest variable.

Mol - Yeah, general opinion is 15% - 20% loss which is just about managable for the odd time I would need to use it. Given I've not got a box at the moment, I'm just tying to establish which shape / position would minimise this.

I used a roof box for years when my kids were younger and it made almost no difference to MPG, it could sometimes be a little noisier but the biggest noise came from the roof bars themselves. I had a half width roof box and started with cheapo square roof bars and it sounded like I was flying a kite, I changed to Thule aero bars and it was hardly noticeable.

Ampthill - I 50% agree with you

marcus
Free Member

Mol – Yeah, general opinion is 15% – 20% loss which is just about managable for the odd time I would need to use it. Given I’ve not got a box at the moment, I’m just tying to establish which shape / position would minimise this.

Depends on how quick you are going, i'd say my one is probably higher than that at 70 mph.

If you've got a towball you can get a 'roofbox' that goes on there instead behind the car. Called an Ixtabox.

https://ixtabox.com/pages/ixtabox-vs-roofbox-1

Comically expensive but maybe somebody else makes a cheaper clone of it

This yuppie reckons he's better off with a roof basket and dry bags on his tesla:

The obvious worry would be "security" but I'm sure there's ways to stop bags getting stolen at the services...

I've used a roofbag (so probably crappier aerodynamics vs roofbox) on several holidays with the kids. It's made a couple of mph difference but nothing to be that worried about.

The extra diesel was a lot less than buying a bigger car (or even hiring one).

edit: I use some big (but budget) drybags inside the roofbag, everyone gets a drybag to themselves. Yes, security, etc. You pays your money.

Multi - I'm not surprised you're getting more than 20% loss above 70mph. - I'm happy to drop the speed a bit to reduce the loss. I like the look of those trunk carriers, but don't have a tow bar and don't really want to add an extra 1m on the back of the car, particularly when charging.

With an ICE you are simply paying a bit more; with an EV you are slowing your journey down because you are having to charge more often.  That said, as is always the case with EVs, you might actually get to your destination quicker by driving slower and this could be more likely the less aero your car.  It depends on your trip and your attitude I suppose. I mean, we tow a caravan which means we have to drive a fair bit slower; but within the UK it makes little difference in reality because it still takes an entire day to get to Scotland for example.  You can't really do anything with the extra 90 mins you save or whatever.

Drag is proportional to speed squared, so a little increase in speed can be a big increase in drag. It's only directly proportional to cross sectional area so you could argue no point in worrying about a big box if you're willing to drive slower

I have used one on a ICE car and I reckon the consumption was 5%-10% lower using the roof box, that said we have quite a small (narrow) roof box, so I would expect similar range reduction on an EV.

I would definitely go with Aero bars, the square bars are a lot noisier - Also fill the slots in the aero bars as this reduces the sound.  You can get very Aero looking roof boxes but then you typically can't get much in them as they are so low at the front.

rsl - So a long thin box (small frontage) which overhangs the front of the roof will create less drag (better efficiency) than a wider box set further back ? Makes sense that the air flow from the bonnet and windscreen would pass under the box. Broadly speaking.

Alternately you might find the aerobox hanging over the front causes air coming up the windscreen to 'bunch' under the box and create a large turbulent  wedge effect increasing drag.  I think it's impossible to estimate without some actual testing.  Long  and thin will be better than short and wide tho'

They aren't as bad as 4 trail bikes on the roof.  Drop the speed to 60-ish and the 'loss' isn't anything like as bad. I find fuel efficiency isn't too shocking if I take my time, but doing 70 with 4 bikes up there gobbles fuel.

Also being EV, slower speeds increases the range !  Rear trunk bags will add weight at the back, and this isn't what you want in a loaded car.

marcus
Free Member

rsl – So a long thin box (small frontage) which overhangs the front of the roof will create less drag (better efficiency) than a wider box set further back ? Makes sense that the air flow from the bonnet and windscreen would pass under the box. Broadly speaking.

If you look on the aerodynamics tab on that Ixta page i linked above, there is a pic showing why this is the case. Basically the roofbox creates a large low pressure area behind it.

edit- the image (courtesy of the Ixta site)

Being further forward i don't think makes much difference, but being wider makes this low pressure region larger.
Certainly my full width box seems worse than the half width one.

This yuppie reckons he’s better off with a roof basket and dry bags on his tesla

I'm with the yuppie. I had a roofbag and bike on the roof of my car and still managed an indicated 28 mpg down from Fife to Ard Rock last year, which given it only does about 28 mpg anyway I thought was a bit of a result

I was chatting with the Thule rep the other day. He reckons that after testing by Thule it was shown the rear mounted racks with boxes mounted on them actually increase the EV aerodynamics and thus the range marginally. Thule have put a lot of money into this as they are going all out for EV accessories (and their entire company car fleet is now EV) so I wouldn't doubt it.

I use one every year on holiday. Can't say I've noticed a drop off in mgp. Although I do tend to remove it as soon as we get to our destination, so all the driving on the way there is at about 60-70 mph where the car is more efficient anyway.

I've just downloaded the Master thesis for the guy that did the aerodynamics work for Ixta . - I shall report back when read and digested.

Can you 'borrow' a roof box ?  I only ask as they are a bloody pain to store. My son bought a big one and I was like, where the hell are we going to keep that ?  It ended up hung in the garage taking up a load of space for 50 weeks of the year. I've moved it out behind the shed now, as it's taking up too much useable space.

I had a roofbag and bike on the roof of my car and still managed an indicated 28 mpg down from Fife to Ard Rock last year, which given it only does about 28 mpg anyway I thought was a bit of a result

It's different with different engine types though - fuel consumption isn't always proportional to load in an ICE. Petrol cars gain a bit of efficiency when you use more engine power, to a point, because you open the throttle which reduces pumping losses in the engine - particularly big engines that have to be throttled a lot to stick to legal speeds.  So that could offset the extra drag of a roof box.  With EVs that is not the case, as I understand it - more power means more current which means more heat loss in the wiring, above crawling type speeds.  Then again, in winter more current might bring the battery temperature up which could release more charge, depending on how it's managed.

Happy to be proved wrong by a real automotive engineer, but if you are getting the same MPG with a bike and a box on the roof there is clearly something going on as you are definitely causing a lot more drag.  A bike on the roof of my Passat cost me about 18-20% at motorway speeds.

Got the really long think Dolphin 470l box on our Octavia estate which allows two bike racks alongside. Have to mount it quite far forward to allow the boot to open fully without hitting the box, so it overhangs the roof by a couple of feet. Similar fuel hits as others, probably go from 60 to 50mpg on long runs

Echo the point about borrowing or hiring, ours just lives on our car the whole time. Luckily we don't use the car much at all

Our old, blunt, wide roof box was awful for MPG.

I have a very odd effect with canoes at times....

1 canoe = knocks maybe 10-15% off MPG.
2 canoes 'piled' - so one flat and one leaning against it = 25-25% off MPG.
2 canoes, arranged in a very slight "v" with 'point' towards bonnet and alongside each other, particularly on the V70 or Galaxy = no MPG difference.

The canoes I am speaking of:
https://flic.kr/p/25dspZU

'V' shape:
https://flic.kr/p/yE1dud

fossy

Can you ‘borrow’ a roof box ? I only ask as they are a bloody pain to store. My son bought a big one and I was like, where the hell are we going to keep that ? It ended up hung in the garage taking up a load of space for 50 weeks of the year. I’ve moved it out behind the shed now, as it’s taking up too much useable space.

I wanna get a roofbox, but keep getting put off by this aspect of it. It's a lot of dead space to waste in the garage for 99% of the year.
I was hoping if we got one of those narrow long ones we could shove it in the loft, but as we have a low-pitch roof I don't think it would go through the hatch.
My mate has one of those Thule roofbags. I should probably just borrow that.

molgrips

A bike on the roof of my Passat cost me about 18-20% at motorway speeds.

That sounds like a big hit in economy with one bike.
I used to regularly put the bike on my old Ibiza (1.9 TDi). That took around 10% absolute max off the fuel economy, probably nearer 6-8% generally.
The worst I ever saw was going to Cornwall with the boot stuffed with 2 weeks of camping gear, 2 bikes on the roof and a heavy right foot because it was a long journey. That was about a 20% drop in economy.

I can appreciate the storage aspect is a hassle. However, I'm not sure it's more hassle than storing / driving a Galaxy / Sharan, etc.

molgrips

A bike on the roof of my Passat cost me about 18-20% at motorway speeds.

That sounds like a big hit in economy with one bike.

I would concur with molgrips - a bike or two on the roof has quite the impact.

However, I’m not sure it’s more hassle than storing / driving a Galaxy / Sharan, etc.

I've driven a Galaxy, it was great. Not a sports car obviously but a really nice drive and not the bus you might be expecting.

I wanna get a roofbox, but keep getting put off by this aspect of it. It’s a lot of dead space to waste in the garage for 99% of the year.

Roof bag.

Roof boxes are great, but it's the blooming storage. If you already have a bike rack/roof rack, then a bag may be better. I was so sick of the space it took up in my garage, I shoved it behind the shed - it's not mine anyway, and I need the space in the garage.

I've already got a roof rack and 4 bike carrier stored in the garage, the blooming box and roof bars (to fit son's car) were just too much space wise.

I have big 500L roof box because my classic XC skis are 207cm long. For 2.0 TDI there is measurable hit in efficiency with the box, fairly close to 2 MTBs on roof on Thule Freerides. 3 bikes is worse than roofbox. I have exact numbers stored somewhere.

The roofbox creates some noise from head wind and especially if there is anything loose inside, it also makes car more susceptile to wind from the side so the box comes off immediately after the the trip. It is not my favourite thing to wrestle on and off the roof - I wish there would be space to install box lift kit on the garage roof.

I store mine standing on its butt, on top of few old rugs and strapped to a wall stud. Not ideal solution but it has worked for 6 years now.

I use a big Thule one regularly on an ICE doing long trips.
Normal use just me most of the time my Alfa gets ~6.5 litres /100km. A big trip (several thousand kilometres) fully loaded with bikes on the back family and the roof box it’s closer to 7. Litres. So about an extra gallon every 60 miles.

About 3mpg.

Mol - I'll struggle to go back to a diesel bus from my EV.

I absolutely agree, electric is far better. I'm not suggesting you change it!

Not all roof bars are created equally:

One of these will be horrendous, as you will know if you have ever heard them driving past (its pretty obvious).

Yakim Whispbars, flush, I leave them on permanently though as any extra drag and noise is genuinely imperceptible.

Combined with a long, slim roofbox, minimal affect. Forgot they were there at about 150km/h on the autobahn last summer to be honest, had to rein myself back in.

The Thule rear towbar mounted storage boxes look a great solution too.

I've got some Thule wingbars which look similar to those Wispbars, and I agree are great. I like the idea of the rear carriers but would be interested to know if they hinder the ability to charge, particularly if the port is on the rear

Can you ‘borrow’ a roof box ?

Yes. My local bike shop is also a Thule dealer. They rent out roof boxes.

I wanna get a roofbox, but keep getting put off by this aspect of it. It’s a lot of dead space to waste in the garage for 99% of the year.
I was hoping if we got one of those narrow long

Baskets as noted above tend to be smaller and easier to hang on wall hooks, what you put in them is only really limited by the bags/tiedown options you choose.

MPG? Meh, it's an edge case, unless you need 200L+ of rooftop luggage space every day.

However, I’m not sure it’s more hassle than storing / driving a Galaxy / Sharan, etc.

Our Galaxy was the shizzles with family of 5. Comfy, huuuuuuuge space, reliable, drove a lot better than I expected, more efficient than I expected etc. It really was head and shoulders above the Touran we had before.
We only got rid because a)168k and a growing number of issues and b)kids learning to drive and having two cars...

Anyone want a set of Aero bars for a Passat?

Can you ‘borrow’ a roof box ?

You can rent them from commercial places, certainly, or you could find a mate / vague acquaintance from a popular online fourm who has one and borrow it

MPG? Meh, it’s an edge case

It's an EV so trashing his range might be a pain depending on what car it is.  20% of a 300 mile car is doable, not so much a 120 mile car.

Two kayaks on the top of our Model 3 on the way to the lakes took about 20% off at motorway speeds. Hardly any effect at 30-40mph when we got there. Two mountain bikes had a slightly lower impact. I’d expect a modern aero roof box on aero bars to be lower still below 70.

It also depends hugely on the vehicle the roof box is strapped too.

We have a long / narrow roof box. On the top of a SWB campervan we had previously (with barn doors), we has it set up so the nose of the box followed the same line up as the windscreen; this had no noticeable effect on mpg to the extent we left it on permanently, which was just as well no impact as it it was also a pain in the hoop to stick on the roof of the van when we wanted to go away for a few days.

However the same box on our Volvo with standard estate door means the roof box has to be projecting out infront of the windscreen and it has a noticeable effect on MPG.

We found that storage isn't a huge issue as it fits nicely under the canoe on the rack in the back garden; clearly the issue you all are facing is a lack of canoes, not the owning of roof boxes.

in my experiance it make little difference as long as you are not hammering it.

Our trip from Manchester to the wilds of northunberland is pretty much 200 miles.
Kia eNiro range will show circa 280 when we set off, car absolutely chocker with 4 people, a dog and all our crap for the week (roof box rammed). Will get there with about 50miles left and that with the ACC on 65-70 all the way.

In comparison our trips down to London (also 200 miles), 4 people, no dog and only packed for a couple of days so no rookbox. I'll have the ACC set yo 70-75 and we'll arrive with about the same 50mile left.

So only a little bit faster but a much lighter car with no box. very little difference.

cookeaa

Baskets as noted above tend to be smaller and easier to hang on wall hooks, what you put in them is only really limited by the bags/tiedown options you choose.

Yeah, I have to say I have never considered a basket. The security side of things would concern me a little & I would prefer it if there is no risk of everything getting a soaking on the journey - I suppose I could mitigate this with dry bags & a tarpaulin cover.

I would not use a tarp, it will flap and make a hell of a noise.  You could use dry bags in a basket, or just get bars and a folding roof bag rather than box. Bars are pretty easy to store - I have mine standing up against the garage wall and held in place by a bungee strap attached to hooks in the wall.

If anyone else is interested. https://pureportal.coventry.ac.uk/en/studentTheses/aerodynamic-designing-and-position-optimization-of-roof-box

Take less stuff?

I once found myself loading the roof box onto a Passat estate when the kids were small.
I stopped, paused for thought and realised it was ridiculous. Put it back in the garage and then sold it.

I'm 100% not saying this applies to the OP, but do stop to think before you take the student bin approach to packing.

The Thule rear towbar mounted storage boxes look a great solution too.

Agreed - it looks about a million times better than that Ixta coffin.

We found that storage isn’t a huge issue as it fits nicely under the canoe on the rack in the back garden; clearly the issue you all are facing is a lack of canoes, not the owning of roof boxes.

Never a truer word said.