Funny little trailers

[GeorgeTelford]

Hi Rik

Yes the length is against it being as good has the Tatra 77a

The main thrust of my post, was that you were incorrect in saying that the teardrop shape was only thought to be aerodynamic. it is actually pretty aero/low drag not the best and nowhere near the Tatra but better than many modern cars.

I always remembered a low drag program I watched, mentioned snails being low drag shape (no thats pretty pointless aint it!) when they let a computer incromently change the shape then test it it morphed the snail to dolphinesque shape.
regards

George

PS I already have the measure of Gage and his posts

PPS Asian that would not be to bad as long as it flew rubber side forwards. The reason appollo was pointy was for thermo dynamics, to push the speed induced heat to one side, not low drag

[R Keller]

Hi Rik

Yes the length is against it being as good has the Tatra 77a

The main thrust of my post, was that you were incorrect in saying that the teardrop shape was only thought to be aerodynamic. it is actually pretty aero/low drag not the best and nowhere near the Tatra but better than many modern cars.

George,

My point was only that the "teardrop" shape as executed in a "teardrop trailer" is nowhere near being a good aerodynamic teardrop shape. As you point out, the Tatra is a much better execution of a good aerodynamic teardrop shape.

It'd be interesting to see aerodynamic tests on a teardrop trailer. And then of, course, since it is a trailer, it'd be interesting to see what is the best shape for a trailer being pulled behind a car/truck that is already disrupting the airflow; the best distance behind the rear of the vehicle, etc.

Andrew: any thoughts on this? Andrew? Andrew!? Wake up Andrew! I don't care if it is the middle of the night right now over there!

Rik

[mikeschn]

Gage,

Stay on topic.

Mike...

[GeorgeTelford]

Hi all

I did have the figures for a properly set up trailer spoiler, it made enough difference, even a small percentage makes a huge monetary difference at Low MPG.

Rik the biggey with the trailer is that its screened a fair bit by the vehicle in front, which I guess will help a fair bit. Large rounded at front and tapered slope at rear follow the ideals, btw length of rear slope its a percentage of the whole not actual length that counts, ie if you made a slightly smaller exact replicar of the Tatra the Coeff of drag would be the same BUT the actual drag, would be lower due to frontal area.

I get the feeling that a weekender would not be that bad on fuel even with the angular front.

[R Keller]

OK Gage, thanks for the demonstration of your maturity level. I think you proved my point quite nicely. Can you please go and inflict yourself on another thread now that you've done your best to kill this one?

Now back to the topic:

That Cd figure doesn't sound plausible ... According to my Tatra book, the 77a did 150km/hr on 75bhp and the 87 did 160km/hr on 75bhp, so the 77a probably has a higher Cd than the 87's 0.36.

Using the data furnished by both Andrew and George, I thought I'd take shot at calculating what a realistic Cd for the Tatra 77a would be. Sure, everyone quotes the 0.212 Cd, but this could have been a mistaken piece of information that just got passed down over the years...

I used the following assumptions for the two cars (based on Andrew's statements and George's website references:

Tatra 77a (1935)
- maximum speed: 150 km/hr
- engine type: 3.4 litre V8
- maximum power: 74 bhp (75 bhp based on Andrew's and George's citation; 69-74bhp (51.4-55.2 kW at 3500rpm from the Czech website))
- weight: 1,800 kg (from the Czech website)

Tatra 87 (1936-?)
- calculated Cd: 0.31 (based on top speed)
- measured Cd: 0.36 (based on VW wind tunnel measurement and also quoted in the websites)
- maximum speed: 160 km/hr
- engine type: 2.97 litre V8
- maximum power: 74bhp (75 bhp based on Andrew's and George's citation) 74bhp (55.2 kW at 3500rpm based on the Czech website)
- weight: 1,370 kg

Assume standard air density, temperature, etc., and that the following are the same with the 77a and the 87:

- frontal area (A)
- coefficient of rolling friction (Crr) for the tires
- transmission/drivetrain loss %

I came up with the following numbers for the Cd of the 77a based on the given top speed of the 87 and 77a (and based on the assumption the max. speeds are controlled by max. power, not by gearing):

- Even if we use the lower Cd=0.31 figure for 87, the 77a would have a calculated Cd of 0.36
- If we use the 69bhp figure for the 77a (and keep the 87 at 74bhp) and use the Cd=0.31 figure for 87, the 77a would have a calculated Cd of 0.34

So, that's an estimated Cd range of 0.34 to 0.36 for the 77a. To put it another way, if the 77a actually had a Cd of 0.212, it would only need ~50bhp of max. power to reach a maximum speed of 150 km/hr. Or if we assume 74 bhp max. power, it would have a maximum speed of ~ 175 km/hr.

My conclusion is that the 0.212 number is wrong, or that the specs given for top speed and max. power are wrong.

Rik

[GeorgeTelford]

Hi

The Tatra 77a was on that program about aerodynamics, they compared the profile to the snail/dolphin profile, it was favourable

I cannot accept your calculation as proof, a main battle tank as a massive HP rating, but a mini as a higher top speed and both have terrible CD numbers, one would also have to assume that the correct gearing and final drive were employed, not all vehicles are geared for maximum terminal velocity. What I am trying to say is that CD based on number crunching or reverse engineering the top speed is a guess at best.


Surely someone would have corrected a 70 year old mistake by now

[february1966]

I am not an expert or even a novice in the area of aerodynamics!!!

But I have a question.... How could the trailer's shape effect mpg or speed at all? Provided the trailer fits inside the profile of the tow vehicle and is, therefore, drafting - only the rear of the trailer could effect the areodynamics the two vehicles as connected. That is to suggest the trailers effect on the areodynamics is limited to how it effects the air flow at the rear.

Am I right or am I lost??? IF i'm right then wouldn't the rounded and sloped rear of a traditional treardrop be very efficient?

[R Keller]

I cannot accept your calculation as proof, a main battle tank as a massive HP rating, but a mini as a higher top speed and both have terrible CD numbers, one would also have to assume that the correct gearing and final drive were employed, not all vehicles are geared for maximum terminal velocity. What I am trying to say is that CD based on number crunching or reverse engineering the top speed is a guess at best.

You don't have to accept it - I don't necessarily accept it either. Although it does seem a reasonable guess, given the other things we do know about the car. Your point about the correct gearing for maximum terminal velocity is one I made in my post too - does anyone have any guesses as to the gear ratios for the car? I don't know if they would have changed significantly or not the next year for the 87.

As far as the Mini and a battle tank, you've got to consider the rolling resistance, weight and frontal area too. For my comparison of the 77a and the 87, I use the given weights, and assumed the same rolling resistance and frontal area for both. I can send you the speadsheet if you'd like to look at the assumptions...

Reverse engineering the Cd might not be totally accurate, but it can get pretty close. As Andrew pointed out:

The Tatra 87 (very similar body shape) was claimed to have a Cd of 0.244 from a 1/5th scale wind tunnel model test, but the claimed top speed gave a calculated Cd of 0.31. In 1979 a real 87 from the Deutsches Museum was put in VW's wind tunnel and the result was a Cd of 0.36.

The reverse-engineered figure (from the claimed top speed) was a lot closer than the claimed figure from a 1/5th scale wind tunnel test. If that 0.36 number is correct (and a number of the websites use it), then the actual Cd for the Tatra 87 is 48% larger than the claimed 0.244 figure. If the 0.212 figure for the Tatra 77a is from a similar 1/5th scale test, and there was a similar magnitude of error, we would expect actual Cd of 0.31. That's pretty close to my calculations.

Surely someone would have corrected a 70 year old mistake by now

You'd think so, but who knows? Far bigger and more important factual errors than that have propagated themselves through history. And if there aren't any surviving cars to test (like there were for the 87), the claimed Cd number would just get passed on...

Rik

[Gage]

OK Gage, thanks for the demonstration of your maturity level. I think you proved my point quite nicely. Can you please go and inflict yourself on another thread now that you've done your best to kill this one?..........

Excuse me, but what part of my posting offends you?

If you can't stand in it, it's a teardrop. If it's shaped like a Grasshopper, it's just not a traditional teardrop but it's still considered a teardrop. Some people here on the left coast get real sticky about what a teardrop is and still let a grasshopper 'style' teardrop in Teardrop 'only' gatherings.

Enjoy what you build and have a good day.

Heck, even your Road Toad could be considered a Teardrop.

Now back to the topic:Rik

Sounds like a good idea to me. But what part of the current conversation is on topic? The original question was:

I've spent quite a bit of time plundering around this site. I have seen every shape and form of tiny trailers. Tears, square tears and everything from squidgets to midgets. Grasshoppers and such. Question is....
Are the non tear shaped such as grasshoppers...square tears ect. still considered teardrops? I am asking for a reason.....I kinda like the grasshopper body style and was wondering if I built one would I be ostracisied for being different?????????????????????

Boy Mike, you tell me to get back on topic. All I did was defend a statement I made without insults. At least wentzzee got an answer before all this tec stuff got thrown into the pot and went way off topic.

Have a good day you all. I'm off this thread.

[Steve_Cox]

I appreciate all the responses....................Wifey suggested do a Grasshopper and paint the silouette of a teardrop then the rest sky blue so it would blend in with the sky.. She said it would look kinda cosmic and given just a quick glance no one would notice.............

wentzzee, I like the way she thinks. BTW, I am building a "tear" shape so I've still got all those corners I cut off perfectly good rectangular plywood sheets, trying to find a use for them.

Steve in St Augustine

[Steve_Cox]

Geez, I had a senior moment and skipped pages 2 and 3.... Did I miss anything?

Steve In St Augustine

[mikeschn]

Geez, I had a senior moment and skipped pages 2 and 3.... Did I miss anything?

Steve In St Augustine

I don't think so...

and I think I'm about ready to kill this thread!

Mike...

[angib]

.... How could the trailer's shape effect mpg or speed at all? Provided the trailer fits inside the profile of the tow vehicle and is, therefore, drafting....

Feb,

You're dead right, although there are a few restrictions:
- The trailer should not be too 'awkward' a shape - remember that even the closed tailgate of a pickup affects drag and that's a long way inside the wake of the cab.
- The trailer shoud not stick out beyond the wake of the tow vehicle. Note this includes the underside of the tow vehicle - the trailer should ideally not go below that.
- The trailer should be as close to the tow vehicle as possible. The bigger the gap, the more the trailer becomes a separate vehicle.

An interesting idea is that it would be possible to perfectly match a trailer to a tow vehicle, such that it reduced the drag when towing. The problem is that it wouldn't work on any other tow vehicle and it would have to have a nearly zero gap between the tow vehicle and the trailer.

Perhaps it would look something like this:


Of course, either you can only go round gentle curves or you have to invent the remote-control extending tongue for manoeuvring but, hey, this idea's still under development......

Andrew

[Arne]

I recently chopped down my trailer. I also recently just came back from a trip where I used 2 tanks full of gas, about 600 miles. One of the reasons I cut 6" off the top was to get the top of the trailer below the top of the towing minivan.... I also attempted to create a better airflow by eliminating as much of the flat spot on the front of the trailer where the buttom curve transforms into the top curve.

Here are the results. I usually get 20 mpg consistently during my normal driving which is the usual combination of city/highway..... with the former more bulbous shape, my mileage would drop to 17 mpg, and my van would down shift almost immediatlely when encountering the slightest hill.

With the new shape, I averaged 19.1 mpg and the downshifting was noticeably less sensitive to hills.

I mentioned in another thread that there is a long hill nearby, where my van would pick up 10-15 mph on the decline. with the old tear contour, the speed stayed steady while descenging the hill.... with the new shape, I pick up 10 mph going down.

There are several other things I may do.... one is, I have an idea for a new front design that would be more aero than what I have now, but that would require building no. 2, and I'm not sure it is worth the effort and $$.... it would pull easier and be about 250 pounds lighter. right now, my tear grew up to a porky 1,200 pounds.

I would build it so the bottom was enclosed for aero purposes and the design would be aimed more for lightness. there are several areas where I just built without thinking about weight. with full foam, i think the inside and outside walls can be 1/8 using cpes and uniflex 255 as long as the foam provides support..... the hatch can be flimsy 1/8 as it is supported open and closed, no need for 1/4 as long as it is treated right.

the front top would be a longer slope, with a tight radius of 6" to the front bottom curve. I'll post a drawing some time down the road.....

it would have a full torsion axle (not stubs), with a triangulation of 2 1x3 members meeting 4 feet in front of the trailer to form the tongue. There would be no metal frame for the tear, so the bottom would be flat, the only interference to air flow would be the double tongue and that would be at such a slight angle that turbulence should be a low as i can get it.

I would sleep head to front and all heavy items would be in the rear... and the doors would be near the back so i, and Shirley and the dog, would enter at the rear. I'm trying to decide about windows. the doors could have windows or not, but there would be windows in the walls where my head would be while lying down.

It would probably not be particulary attractive as it would depart is several ways from a typical tear. Also, I've having problems keeping it under 11 feet long.....

Front to head of bed, 12 inches (because of front slope)
bed, 80 inches
dog area 18 inches
galley length, 24 inches.

so, i'm up to 11 feet and it's starting to look like an airplane wing. the back would have the a/c and galley.... tv and so on would be mounted over dog bed... and no cabinets, which I just don't think are worth the effort, complexity and weight......

Anyway, long note, but I may make up a jig with a spring and guage for holding out a car window at 55 mph to find the design with the least wind resistance.....

[Arne]

Andrew, they used to make a single wheeled trailer with a double hitch on the bumper, one hitch on each end..... you might be onto the world's first single wheel tear, with the wheel between you and your sig. other.. and you wouldn't be able to open the tow vehicle hatch.... and it might fall over when you disconnect it.... minor design problems to be overcome....

but i think it would cut down considerably on the wind resistance with just one wheel..