18% cd reduction

[jtbo]

I did put small Kammback behind my wagon, also made grill blocking so that there are only around size of palm opening when counted all two openings together, one is for IC other for radiator.

I have done coast down test before and after mods, several of them at several days. At one day I got lot lower rolling resistance calculation, car seemed to coast lot longer even there was no wind, but it is only one day testing, with that test I got cd of 0.429, others were average of cd 0.364.

With mods I got better result, rolling resistance seems to be same, but I did get lot better result for cd, several test average is now cd 0.299.

So I calculated that it would be now 18% (my math is not the best so that can be off) reduction in cd, with grill block and 50cm long kamm back extension, I believe that it can be plausible.

At least wind noise is less and car coasts better at 50mph speeds, but I hope to improve even more.

I hope to get under cd 0.25 eventually, bellypan is then next task, need to cover that parachute like section at rear, rear wheel well covers then, maybe that should get me close.

But I guess under cd 0.3, if that really is true, is already quite a success with a wagon shaped car?

[drmiller100]

that really is good!

any pics of your kammback?

[jtbo]

There is only rather poor pic at the moment, it is still unfinished but seem to be working well even how it is:


It has more or less constant arc, sides are taken inwards bit more aggressively than top.

I did measure from top that in 50cm distance there is 5cm drop, sides are maybe around 8cm which might be bit too much, but I was not able to make them any lesser angle while maintaining top side angle.

It is made from 0.5mm metal and seem to be very solid without any supports, however I plan to add some bracing which might get it shaped bit better too.

I'm bit surprised that there is so much of improvement at this stage already, but despite my repeated attempts in measuring, I have not been able to measure any worse results, so it looks like that thing is working so well.

edit: Oh yes, there is one pic from behind when I was still planning it, decided against this shape in pic and made it to be more of an arc, but sizes and general angles are pretty much same:

[Sven7]

If you're seeing 18% aero improvement you should be seeing about 9% MPG improvement, or about 3-4mpg. Does this coincide?

I'm a bit concerned that the sides are too drastically tapered. Have you tuft tested the Kammback to know for sure that it's working?

Anyway congrats on the improvements

[jtbo]

Originally Posted by Sven7

If you're seeing 18% aero improvement you should be seeing about 9% MPG improvement, or about 3-4mpg. Does this coincide?

I'm a bit concerned that the sides are too drastically tapered. Have you tuft tested the Kammback to know for sure that it's working?

Anyway congrats on the improvements

Sides can be too large angle indeed, angle of those is around 15 degrees, what I could measure, top is around 10 degrees, +/- 2 degrees easily to both because inaccuracy with measuring.

Fuel consumption is bit hard to know as I drive so little nowdays, full tank might last rest of the year if nothing special will happen.

But when I did drove 104km and around 30km of those with Kamm, I got already 10% better mileage than excepted, but even I fill slowly and so that fuel stays at ring level in filler neck, that can be easily just error as amount is so small.

Earlier experience however shows that this new Kamm is better than old Kamm and then together with grill block, rear wheel well fairings and front air dam, I could easily get 4l/100km level consumption and even lower, where as my standard level without mods is 5l/100km for summer conditions, that is 20% reduction if I'm not mistaken with my percentages (does happen and often).

So at the moment, there is no knowing, my car has no electronic brain so can't even get any kind of fuel consumption computer, injectors etc all mechanical.

Tuft testing is bit impossible as there are no one to film tufts, so that is making it bit hard to really know how well it does work.

That pic before painting is not really giving correct picture from angles, here is new photo from rear, still came out quite horrible, somehow it is such shape that is difficult to take good photo. Lower edge of support 'wing' I did turned too much in, but rest is not quite so horrible angled I think? It is not quite uniform currently and there are lot of bends to many direction, I plan to cure those with braces that I will put there, but to get that done needs of course that main shape is working, which I need to test more to really verify that.


In case you wonder, sticker reads "Reading a tuning magazine turns you into a f****** idiot".

I wanted to get top to curve down to around 10 degrees smoothly, but it is not quite easy to bend metal sheet way I like to, not with my poor skills at least and without any proper tools for such, so it is bit straight, but with speeds of 50Mph or less it probably is not so critical. It is better than without Kamm, it is better than old version, but perhaps not best that it can be.

[ecomodded]

Looking good, your getting there. Now for the frame work, which should be strong ,lite and easy to work with. makes me think of aluminum.
Or If the kamback and frame were made of fiberglass you could bond it it to the roof and rear window with silicon or the black goop auto adhisive.

[jtbo]

Originally Posted by ecomodded

Looking good, your getting there. Now for the frame work, which should be strong ,lite and easy to work with. makes me think of aluminum.
Or If the kamback and frame were made of fiberglass you could bond it it to the roof and rear window with silicon or the black goop auto adhisive.

I did drill holes to the trunk lid and attached that with pop rivets, easy to attach and remove (but bit costly), which is important as it might be not quite 100% legal mod here, law is not perfectly clear from that as decoration parts should be legal without extensive testing if they don't pose risk or performance decrease, but one to judge is party that does the inspection, so it remains to be seen how it goes.

Trunk lid opening is such that there is 0.Xmm gap between body and lid top foremost 20mm when it is opening, which limits a bit amount of surface to attach this, but with pop rivets it is not an issue.

I have fiberglass, but I don't know if that would be optimal for reinforcing, aluminium as well as coroplast are out of my reach, not sold around here for sensible price, even sheetmetal was last sheet that I bought for 20€ and they are not going to order more of it.

Living at rural countryside has it's problems and perks, even without fully illegal Velomobile/EV build (something in future plans) it is very unlikely to face any consequences.

I have been thinking of cutting sheet metal and turning 90 degree angle to 'stripe' of metal, then driving that to edge of Kamm, with pop rivets, which should make it pretty strong and it should also made it very rigid, adding very little to weight too, another good thing would be that it would make edge to be less 'offending' to inspector persons.

Leading edge is something I'm planning to put some goo or plaster to make transition smooth, especially corners came out horribly and I might need to cut those a bit so they sit bit more flat against trunk lid, angles are bit of challenge to make without much experience

As you can observe from last photo, there is quite wet weather here, ice and snow is slowly melting which caused my driveway to become 4x4 mudpool, only problem is I don't have 4x4, so it will take weeks until I can actually drive the car again, maybe I get finished the build by then.