There's a word you wouldn't expect to see on a Doblo forum

Still. Out and about this morning, ploughing through really deep puddles in the road (love the diesel engine & the road clearance of the dobbin), the constant spray off the road meant I was flicking my rear wiper on quite a lot.
Is there any way to improve the shape to stop the spray wrapping around the back end & hitting the rear window?
Would a spoiler improve things? What about side spoilers?
Anyone any experience of this sort of problem?

Just stumbled across http://www.airtab.com/
It seems that you can improve fuel economy and keep the rear cleaner by sticking loads of these things on.
OK, so fuel economy is only a small percentage - but it all helps, and if it really does keep the back cleaner that'd do for me.

Is it all hooey or will these things perform?

Those airtabs look interesting, I'm all for improving fuel economy. Can't see if they'll post to the UK but most companies will.
It's never bothered me having a dirty rear window as I've driven buses and coaches for the last 8 or 9 years so I got used to only having side mirrors. I very rarely use the rear wiper, only to see if it still works.
The nice thing about Doblos are the miniature bus/lorry style side mirrors that give you a proper view down the road.

They have a UK rep so I've sent off an email...
I know what you mean about the back window. I've driven buses, lorries etc & mostly rely on the door mirrors but it would be nice to have the back looking as clean as the the rest of the car, at the moment, it gets very grubby, very quickly.

Let us know what the reply is. If you need a minimum order we could club together and split a load.

I've often wondered - golf balls with dimples travel twice a fast and much straighter then golf balls without dimples - and sharks skins aren't glassy smooth, they're pitted and feel rough to the touch - so why aren't cars covered in dimples too?

DoubleO said:

Let us know what the reply is. If you need a minimum order we could club together and split a load.

I've often wondered - golf balls with dimples travel twice a fast and much straighter then golf balls without dimples - and sharks skins aren't glassy smooth, they're pitted and feel rough to the touch - so why aren't cars covered in dimples too?

Click to expand...

Excellent point, hadn't even thought about that (shark skin is used as a sandpaper substitute as well).

I think those brushes are meant for trucks.
The Doblo is actually quite aerodynamic, and the flat back helps with this, ironically.
I hate not being able to see out of the back and just wish there was an intermittent setting on the rear wiper like some estates/vans have. This would sort it, and since the electronic box of tricks can offer rear wiper operation when the car is in reverse and all manner of other charming tricks (500) I can't see why the necessary programming can't be integrated into the Doblo - but perhaps recent Doblos have these features?

They're not brushes.
The airtabs website does mention trucks a lot but searching the internet for an alternative, I kept being taken back to these things on all sorts of forums - people with motorhomes, large trailers, vans, etc. and everyone who has them on reckons they see a slight improvement in fuel consumption as well as a major improvement in the muck reduction on the back. Some have even mentioned that trailers handle much better with these things on, the alternating eddies that set up the snaking effect are negated so trailers are more stable.
I agree with the sentiment regarding intermittent wipe on the back. As I was ploughing through every puddle I could find I was constantly having to switch the wiper on/off.
Maybe they have put this on the 07 or 08 models?

I once owned a 1978 Ford LTD station wagon, with obviously a fairly flat back on it. It was fitted with roof rails and at the back of them was a curved section that directed airflow down the back window, to clear rain and dirt when driving along as it had no rear wiper, worked really well it was a nice little feature. Its not too clear in this pic but here ya go:

that's a really good idea, why don't other cars have this? Maybe it could be copied if you were friendly with a good welder.
Love that Ford btw

DoubleO said:

Let us know what the reply is. If you need a minimum order we could club together and split a load.

I've often wondered - golf balls with dimples travel twice a fast and much straighter then golf balls without dimples - and sharks skins aren't glassy smooth, they're pitted and feel rough to the touch - so why aren't cars covered in dimples too?

Click to expand...

learn something new everyday

23:03. Email returned re ordering the airtabs.
Installed at 3 per foot, covering both sides and roof (with clear tabs on the side windows) I reckoned on 34 tabs.
Total cost inc. P&P and VAT...


£125.65

Although I was offered some which needed to be re-taped (double-sided tape) plus tape for £85 inc.

That'd take me forever to recoup as I simply don't do the mileage or the speed (I'm guessing that these things really start to work at higher speeds).

I'd be really interested if I was on the motorways all day.

Auto_blanco, I've seen that idea somewhere whilst looking at spoilers, deflectors etc. The one I saw looked like a curved piece of abs that was attached to the top of the tailgate. The clamps allowed the abs to stand proud of the roof, catching some of the air flowing over the roof & deflecting it down. I just thought it was a weird spoiler.

23:21, another email.
I could colour code the tabs on the windows as per the pic. The cost then would be £67.

Looking interesting.

DoubleO said:

I've often wondered - golf balls with dimples travel twice a fast and much straighter then golf balls without dimples...so why aren't cars covered in dimples too?

Click to expand...

Why does a golf ball have dimples? The answer to this question can be found by looking at the aerodynamic drag on a sphere. There are two types of drag experienced by a sphere. The first is the obvious drag due to friction. This only accounts for a small part of the drag experienced by a ball. The majority of the drag comes from the separation of the flow behind the ball and is known as pressure drag due to separation. For laminar flow past a sphere, the flow separates very early as shown in Figure 1.



However, for a turbulent flow, separation is delayed as can be seen in Figure 2.



Notice the difference in the size of the separation region behind the spheres. The separation region in the turbulent case is much smaller than in the laminar case. The larger separation region of the laminar case implies a larger pressure drag on the sphere. This is why the professor experienced a longer drive with the marked ball. The surface roughness caused the flow to transition from laminar to turbulent. The turbulent flow has more energy than the laminar flow and thus, the flow stays attached longer.

So, why dimples? Why not use another method to achieve the same affect? The critical Reynolds number, Recr, holds the answer to this question. As you recall, Recr is the Reynolds number at which the flow transitions from a laminar to a turbulent state. For a smooth sphere, Recr is much larger than the average Reynolds number experienced by a golf ball. For a sand roughened golf ball, the reduction in drag at Recr is greater than that of the dimpled golf ball. However, as the Reyn olds number continues to increase, the drag increases. The dimpled ball, on the other hand, has a lower Recr, and the drag is fairly constant for Reynolds numbers greater than Recr.

Therefore, the dimples cause Recr to decrease which implies that the flow becomes turbulent at a lower velocity than on a smooth sphere. This in turn causes the flow to remain attached longer on a dimpled golf ball which implies a reduction in drag. As the speed of the dimpled golf ball is increased, the drag doesn't change much. This is a good property in a sport like golf.

Although round dimples were accepted as the standard, a variety of other shapes were experimented with as well. Among these were squares, rectangles, and hexagons. The hexagons actually result in a lower drag than the round dimples. Calloway HX make a ball with hexagonal dimples.
BTW, there are normally an even number of dimples on a golf ball, between 300 - 450 although one company makes a ball with an odd number.

Will this now go faster?

Undoubtedly!

Well, I've had these things on for a year or so now - can't really say that I've noticed any increase in mpg but they certainly make the car stand out and they seem to help keep the back end clean.
There's still quite a bit of spray hits the rear window on wet days but inbetween times I have noticed less muck building up. Whereas I would normally clean the bugs off my screen etc on a regular basis AND wash the back window at the same time, I don't have to touch it between usual washes.
I do get a lot of people asking me about the airtabs as well - in fact, only one person actually knew what they were as he'd considered buying some.
The ones I bought were white so I bought some spray from Wilkos to blend them a little.

It's good if it works for you but I can't say I've had any problems with the rear window becoming too dirty.

The problem I've found is with the side windows collecting rain water in a heavy down-pour at motorway speeds and blocking the mirrors.
I don't think wind deflectors would solve it either as the water seems to come through the gap between the two posts that hold the mirror on and the deflectors don't go down that far.

Maybe one of those aero things you've got would fit between the two posts. That might work.

sludgeguts said:

Why does a golf ball have dimples? The answer to this question can be found by looking at the aerodynamic drag on a sphere. There are two types of drag experienced by a sphere. The first is the obvious drag due to friction. This only accounts for a small part of the drag experienced by a ball. The majority of the drag comes from the separation of the flow behind the ball and is known as pressure drag due to separation. For laminar flow past a sphere, the flow separates very early as shown in Figure 1.

However, for a turbulent flow, separation is delayed as can be seen in Figure 2.

Notice the difference in the size of the separation region behind the spheres. The separation region in the turbulent case is much smaller than in the laminar case. The larger separation region of the laminar case implies a larger pressure drag on the sphere. This is why the professor experienced a longer drive with the marked ball. The surface roughness caused the flow to transition from laminar to turbulent. The turbulent flow has more energy than the laminar flow and thus, the flow stays attached longer.

So, why dimples? Why not use another method to achieve the same affect? The critical Reynolds number, Recr, holds the answer to this question. As you recall, Recr is the Reynolds number at which the flow transitions from a laminar to a turbulent state. For a smooth sphere, Recr is much larger than the average Reynolds number experienced by a golf ball. For a sand roughened golf ball, the reduction in drag at Recr is greater than that of the dimpled golf ball. However, as the Reyn olds number continues to increase, the drag increases. The dimpled ball, on the other hand, has a lower Recr, and the drag is fairly constant for Reynolds numbers greater than Recr.

Therefore, the dimples cause Recr to decrease which implies that the flow becomes turbulent at a lower velocity than on a smooth sphere. This in turn causes the flow to remain attached longer on a dimpled golf ball which implies a reduction in drag. As the speed of the dimpled golf ball is increased, the drag doesn't change much. This is a good property in a sport like golf.

Although round dimples were accepted as the standard, a variety of other shapes were experimented with as well. Among these were squares, rectangles, and hexagons. The hexagons actually result in a lower drag than the round dimples. Calloway HX make a ball with hexagonal dimples.
BTW, there are normally an even number of dimples on a golf ball, between 300 - 450 although one company makes a ball with an odd number.

Click to expand...

I can expand on this a little bit

The flow pattern past a circular cylinder at high Reynolds number is shown in Figure 1(a), (b) and (c). The fluid becomes stationary at the leading windward edge of the cylinder; the pressure of the fluid here is the stagnation pressure. The fluid particles flow around the surface of the cylinder on either side of the stagnation point with an increase in velocity in the streamwise direction. This causes a drop in pressure. Figure 1(b) shows the flow with a laminar boundary layer; here separation of the boundary layer occurs upstream of the maximum thickness, which translates to a larger turbulent wake; this causes large pressure force in the streamwise direction. This condition occurs for Reynolds numbers typically less than 2.5x10^5. On the other hand at significantly higher Reynolds numbers the boundary layer is turbulent and is much less susceptible to the adverse pressure gradient in the flow, thus delaying the separation until further downstream from the point of maximum thickness as shown in Figure 1(c). The consequent wake is much smaller and the net pressure forces acting on the cylinder surface are much less as compared to the condition discussed earlier. Thus in low Reynolds number flow, forced transition to a turbulent boundary layer is used in several applications to reduce drag.



The best way to reduce the filth on the back of your Doblo would be to introduce a rear slant angle on the roof to reduce the size of the wake. Even a small slant like 10 degrees would work significantly. I would expect the Cd to reduce from about 0.6 to 0.5, of course it would mean chopping your Doblo

Those sticky tab things are not going to much for the price I'm afraid.

Cheers,

Dan

RichDoblo120 said:

It's good if it works for you but I can't say I've had any problems with the rear window becoming too dirty.

The problem I've found is with the side windows collecting rain water in a heavy down-pour at motorway speeds and blocking the mirrors.
I don't think wind deflectors would solve it either as the water seems to come through the gap between the two posts that hold the mirror on and the deflectors don't go down that far.

Maybe one of those aero things you've got would fit between the two posts. That might work.

Click to expand...

My rain water mostly runs up the wind deflectors which, by the way, go all the way on mine, stopping just level with the bottom post of the wing mirror.

(from an earlier posting)