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Discussion Starter #1
Hello. First time post. I was wondering if any of you have an idea of what the range would be on the following kit car project:

Motor: Tesla front drive unit with 243 lb ft of torque
Battery packs: total of 53 kWh
Weight of vehicle: 1800 lbs
Driving style: Mostly highway cruising with occasional squirts to humble big block Vettes, etc.
Tires: 205/55-16 with 6.7" width

Any feedback is welcome. It is a small vehicle so space is at a premium and trying to hold down to no more than ten 5.3 kWh Tesla battery modules. If I can get 250 miles per charge, I’d be delirious.

Thanks for your ideas.

John
 

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The single largest determining factor for range will be your vehicles aerodynamics. you said kit car... which one?

A Tesla model 3 can do 300 miles at 60mph on 70kwh, or ~ 240wh/mi. Your goal is to do 250 miles at 60mph on 53kwh, or 212 wh/mi. If your vehicle is significantly more aerodynamic than a model 3 (smaller frontal area, lower Cd) then 250 mi range should be reasonable.
 

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Discussion Starter #3
Thx for the reply. Cd on the car is around .35. The car is a 550 Spyder with very low frontal area.
 

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Depending on everything i would say that 300 wh/mi should be a good starting point. so 180 miles.

You will not be using all your battery, depending on how your drive unit is setup it wont be as efficient as a tesla model 3.

your car will not weigh 1800 lbs 816kg.

53 kwh ->
Tesla Small Drive unit - 90kg
53 kwh battery - 320 Kg +/-
 

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Hi
A 550 Spyder is an open car with very old fashioned bodywork - your drag is going to be much HIGHER than a Tesla type 3
It will NOT have a CD of 0.35 - 0.60 would be closer - it's the open cockpit that kills your aerodynamics

It will not be as bad as my car - but it will be BAD
I use about 280 watt hours per km at 100 kph
So 450 watt hours per mile

Your Spyder will probably use 400 watt hours per mile at highway speeds

53 kWh - so 42 kWh usable (80%) will give a range of just over 100 miles
 

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While I wouldn't have guessed at a number, I agree with Duncan, that a Cd of 0.35 seems quite optimistic.

A published list of Porsche drag values does show 0.36 for the 550... but that's for the Coupe. The Spyder is listed at 0.45 (comparable to a pickup truck or older SUV), although that tiny frontal area is only 1.03 m2 (which seems implausible to me... how wide is a 550?).
Porsche's drag coefficient: from 550 to 924 Carrera GT
The coefficient and area product is then 0.46 m2, which is lower than any of the production vehicles given as examples in the relevant Wikipedia article.
 

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although that tiny frontal area is only 1.03 m2 (which seems implausible to me... how wide is a 550?).
Wikiiiiii says?

Width 1610 mm (63.4 in)
Height 980 mm (38.6 in)

The width will be the widest point, that's decently reliable, trim some for the rounded profile.

The height won't account for the ground clearance, which, aside from tires, you can probably retract.

Presuming a rectangle, 1/1.6 = 0.62m high. .62 != .98. It doesn't have a 14" of ground clearance.

But maybe once you take away the parts of the rectangle that are rounded off the numbers add up?
 

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Wikiiiiii says?

Width 1610 mm (63.4 in)
Height 980 mm (38.6 in)

The width will be the widest point, that's decently reliable, trim some for the rounded profile.

The height won't account for the ground clearance, which, aside from tires, you can probably retract.

Presuming a rectangle, 1/1.6 = 0.62m high. .62 != .98. It doesn't have a 14" of ground clearance.

But maybe once you take away the parts of the rectangle that are rounded off the numbers add up?
Matt you MULTIPLY width by height to get frontal area - so 1.6 m2
 

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Wikiiiiii says?

Width 1610 mm (63.4 in)
Height 980 mm (38.6 in)

The width will be the widest point, that's decently reliable, trim some for the rounded profile.

The height won't account for the ground clearance, which, aside from tires, you can probably retract.

Presuming a rectangle, 1/1.6 = 0.62m high. .62 != .98. It doesn't have a 14" of ground clearance.
Matt you MULTIPLY width by height to get frontal area - so 1.6 m2
I'm sure Matt realizes that, so he calculated the height which a 1.6 metre wide car would need to be to have only 1 square metre of frontal area... and assumed that we would understand that's what he was doing. Some of us did. 😉
 

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I'm sure Matt realizes that[...]... and assumed that we would understand that's what he was doing. Some of us did.
:p

Yep.

I paused and thought "should I explain this, or is it obvious?". You win some, you lose some.

The width and height are obviously 60% too large, so, I was trying to justify how they might get down to 1m^2.

The width is fairly reliable, so I checked to see just how much ground clearance you might, optimistically subtract to make the math work. It came out to 14", which was silly.

But, maybe 5" ground clearance, and then subtract the rounded portions off the max width... could you get within 20%? Ehn, I dunno.
 

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Discussion Starter #14
First of all, thanks for all of the replies on this interesting topic. The Cd referenced for the 550 coupe of .36 and the Spyder at .45 I have no reason to doubt but I will check further. I am a longtime PCA member and have lots of folks that I can double check this with and will do so and post later.


The other factor that I should mention is that we will likely place a tonneau on the drivers side which I think will improve the Cd as well so I would guess that we could end up with a number somewhere between .36 and .45.


The frontal area on the car is tiny but I can get some measurements to add to the equations.


Lastly, on the vehicle weight, my calculations are as follows:


  • OE 550 spyder is 1,347 lbs dry. (Aluminum not fiberglass body but don't have weight of a glass kit car. Interested if anybody does.)
  • Various websites give the 1600cc pushrod VW motor a weight of 220 lbs.
  • Same websites give the VW gearbox a weight of 65 lbs and the starter a weight of 9.5 lbs.
  • I am assuming Tesla 18650 modules weigh 55 lbs each. X 10 equals 550 lbs.
  • The Tesla front drive unit weighs 210 lbs.
  • Using the numbers above you get a net weight of 1877. So I might have been a little optimistic at 1800.
Anyone who has stats to help fine tune Cd, frontal area, or overall weight, I welcome a chance to hear from you.
 

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Take a look in user "tropes" dragster thread, it has lots of calculatus.
https://www.diyelectriccar.com/forums/showthread.php?t=190610

The attachment has a run from the ecomodder online tool using 2000 lbs, .45 Cd, 1.6 m^2, .01 rolling friction.

So here is a back of the envelope for you:

To cruise down the road at 60mph will need power of about 11 kW. Assuming you have a controller for the motor to operate using this pack with a voltage swing from 180 to 240 VDC, you will draw 50 to 60 Amps. With 80% of your 53kWh capacity you can run for about 3 hours, so my quick guess for your range is 180 miles. (~300 WHr/mile)

This does not include power consumption for acceleration runs against vettes.
 

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First of all, thanks for all of the replies on this interesting topic. The Cd referenced for the 550 coupe of .36 and the Spyder at .45 I have no reason to doubt but I will check further.
My conclusion from the discussion is that the frontal area is plausible (although surprising at first), and the Cd is also plausible... as long as you use the 0.45 of the Spyder rather than the 0.36 of the coupe.
 

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The other factor that I should mention is that we will likely place a tonneau on the drivers side which I think will improve the Cd as well so I would guess that we could end up with a number somewhere between .36 and .45.
I think the tonneau is a good idea (especially a rigid panel rather than fabric), but don't expect much. In pickup trucks, a tonneau cover over the box may not help at all. I doubt it will get drag anywhere close to the coupe level.
 

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Don't forget about the weight of all the stuff which you haven't listed. For instance, the battery modules can't just be stacked on the floor - they need a supporting structure. For safety, they also need an enclosure. Then there are cables, junction boxes, etc... everything that is always omitted from the publicity photos of EV chassis.
 

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You missed the obvious
The frontal area is NOT 1m2 - but is actually over 1.6 m2
Nope. Stop and reset, you're thinking I'm doing something I'm not.

Starting over...

1 - There's a document that says the 550's frontal area is 1.03m^2.

2 - Brian is suspicious that that's possible because it's so low. He wonders what the measurements are.

3 - I dig up the measurements for width and height. You're correct, they result in a 1.6m^2 frontal area. Not 1.03m^2.

4 - I suggest that the max width and max height of the vehicle (which creates a rectangle) do not actually result in the true amount of cross-sectional area, only the max. The actual shape of the vehicle's cross-sectional area is much smaller.

5 - I suggest that Porsche's document about the frontal area might be possible, and attempt to reconcile that with the measurements of the width and height.

6 - I suppose that one of the biggest ways that the vehicle height (i.e. a bar it could drive under) does not reflect the frontal area, is the airspace under the vehicle.

7 - To just ramrod a ballpark of how much ground clearance we would have to subtract to end up with Porsche's numbers, I presume 100% of the discrepancy to be ground clearance and force the numbers to match. This makes 14" of ground clearance. That's silly, but, maybe 1/3 to 1/2 of it can be explained by that.

8 - I suggest that, as the shape of the vehicle is somewhat ovular as viewed from the front, the rest of the discrepancy might be made up by subtracting all the parts of rectangle that would be cut away if replaced by an oval of the same max height and width.

9 - I conclude that it's not as far fetched as it first seemed, it might be possible that the total frontal area is in fact only 1.03m^2 as Porsche claimed.
 
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