Wedgie, how aero?

Odd · Postby **Odd** » 18 Oct 2009 16:13

<blockquote id="quote">quote:<hr height="1" noshade id="quote"> Hay Odd, how fast can you tow that van. With a late 50s V8 Chrysler, I used to tow my
Brabham, in an enclosed trailer at 80 to 90 MPH regularly, back when that was legal.

I once got it up to just over the 100MPH, on the long downhill straights in Victoria,
but I had to slipstream a semi trailer pantechnicon to do it. Young & silly, you know. <hr height="1" noshade id="quote"></blockquote id="quote"> Can, or Dare?
What the combination can do - I don't know. I'd guess 150+, or so, kph. But with the Police hiding
in most every bush around here I'd be ruined if I go past 90-95-100 kph. Allowed speed while towing
a trailer in Sweden is 80 kph... So for financial reasons I don't dare go past ~95 kph.

And as I said above - I will chicken out long before the car reaches terminal velocity. I'm not young
and immortal any longer... [:D]
Theoretical top speed [@5500rpm in 5th] is something like 272 kph with my gearbox, rear axle and wheels...

My two 1980 Wedges...

roverman · Postby **roverman** » 24 Oct 2009 19:08

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by PeterTR7V8

How aero is the wedge? Not as much as you might think. There is some info here that might help. http://www.forum.triumphtr7.com/topic.asp?TOPIC_ID=9076

Never say die. At least not while you're still breathing.
<hr height="1" noshade id="quote"></blockquote id="quote">As I was taught in "Race Car Prep and Design" at Chaffey College,Ca., ohh soo long ago,One would multiply the Cf/d x the frontal area to calculate total drag? Anyone got total drag? Plan is to use Vortex generators over rear window to reduce neg. pressure/lift. Wind Tunnel not in the budget. Yours perhaps? Tapely pull? Anyone? Thanks, roverman.[?]

Art Gertz

PeterTR7V8 · Postby **PeterTR7V8** » 25 Oct 2009 10:43

The formula is Drag = 1/391 x Cd x A x Vsquared

391 is the air density at sea level apparently. I've guessed the Cd to be .35 using the published number of the Toyota MR2 Mk1 as a reference. Frontal area (A) of the wedge is 21 sq/f. V = mph.

http://insideracingtechnology.com/tech102drag.htm

Never say die. At least not while you're still breathing.

roverman · Postby **roverman** » 25 Oct 2009 16:18

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by PeterTR7V8

The formula is Drag = 1/391 x Cd x A x Vsquared

391 is the air density at sea level apparently. I've guessed the Cd to be .35 using the published number of the Toyota MR2 Mk1 as a reference. Frontal area (A) of the wedge is 21 sq/f. V = mph.

http://insideracingtechnology.com/tech102drag.htm

Never say die. At least not while you're still breathing.
<hr height="1" noshade id="quote"></blockquote id="quote">

Art Gertz

roverman · Postby **roverman** » 25 Oct 2009 16:38

Thank You one and all for the input. My thought using a Tapley Pull/ strain gage?, would be to fine tune the aero factor while towing the race car on road surface, smaller arrest than driving it on road.One could even somewhat measure frictional losses in trans/rear end, say in 5th gear-no motor? Fastest class I'm allowed to enter in Silver State is 110 mph.(first timer). Altitude runs from 5700 to 1900ft.I feel the car can stay in 5th including the "esses", this is a "speed rally". Race Car -no speedo.Don't expect to win class as .25 sec. accuracy over 90 miles of open road, not likely. To finish above last and no "drama" will be good enough. "All Possible Speed", roverman.[:)]

Art Gertz

PeterTR7V8 · Postby **PeterTR7V8** » 25 Oct 2009 19:14

What's the event? Good luck with it & can we get videos & pics please! [:)]

Never say die. At least not while you're still breathing.

roverman · Postby **roverman** » 29 Oct 2009 22:25

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by PeterTR7V8

What's the event? Good luck with it & can we get videos & pics please! [:)]

Never say die. At least not while you're still breathing.
<hr height="1" noshade id="quote"></blockquote id="quote"> Suggest, peeping their webbsite,"Silver State Classic", Nevada, USA. Runs twice a year on Hwy 318,(closed for the race).Cars from all over the world enter. 82 yr.young lady entered her Aston Marton and averaged 142 mph. I hope to run the "Huffaker" in smaller events prior to May.Thanks for the interest, roverman.

Art Gertz

Marko · Postby **Marko** » 30 Oct 2009 11:27

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by roverman

Thank You one and all for the input. My thought using a Tapley Pull/ strain gage?, would be to fine tune the aero factor while towing the race car on road surface, smaller arrest than driving it on road.One could even somewhat measure frictional losses in trans/rear end, say in 5th gear-no motor? [:)]

<hr height="1" noshade id="quote"></blockquote id="quote">

the thing is you would need a very long towing cable to take out the "dirty air" that the pulling vehicle generates.

but ist a great way of measuring rolling resistance

drive-train losses can be accurately measured on a chassis dyno, since the crankshaft HP is calculated by adding drive-train losses to wheel HP.

as i can see you are trying to calculate the max speed based on HP the engine has.

lets lay down some simple equations:
power of the engine at max speed is equal to aerodynamic losses, drive-train losses, and rolling resistance of the tires .

aerodynamic losses go up with the speed^3
drive train losses go up with the revs of the engine (around 10%)
and rolling resistance has a very low value , and it grows wit speed but at a very slow rate ( eg. 1.34HP at 18m/s grows to 13.4 HP at 162.0m/s)

to find out the C you can do some reverse engineering by calculating it from stock TR7 HP and max speed , then apply your modified HP to get the max speed at that HP

edit:

wait a sec , ill make you an excel spreadsheet

Marko · Postby **Marko** » 30 Oct 2009 13:02

[quote]Originally posted by Beans

I read somewhere that the CF/CW-value for a TR7 was 0.39.

<hr height="1" noshade id="quote"></blockquote id="quote">

on the spot! [8D]

0,4 worse than a renault clio

here is the spreadsheet , i used the data from the workshop manual on dimensions of the car as stock , stock tires, stock HP at stock revs.

http://rapidshare.com/files/300012936/tr7.xlsx.html

dont modify the red cells,
you can modify green but be aware of what data they rely on.

i calculated the surface area of the car minus the ground clearance so you don't have to modify that if your car is lowered, only thing that is taken in the account is the surface area of the chassis

so how the spreadsheet works?

by modifying the HP , max speed, revs, tire radius, you need to get the cell on the bottom of the page marked in red to 0.

since the excel is a crude tool, and the calculations are based on iterations i couldn't get it to work more smoothly. so closer to 0 you are the calculation is more accurate. the value of the number is the excess or insufficiency of kW (power) that you have for your max speed. if you get it under 1 you are inside "less than 1HP " range

could you try it out with your own data so i can make any modifications if necessary

Beans · Postby **Beans** » 30 Oct 2009 18:43

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Marko

on the spot! [8D] ...<hr height="1" noshade id="quote"></blockquote id="quote">
That value is from an old (Dutch) road test I have lying around somewhere, but haven't been able to unearth yet [;)]

<center>

1981 TR7 FHC Sprint (better known as 't Kreng)
1980 TR7 DHC (my first car currently being restored)
In parts a 1980 TR7 PI DHC, 1981 TR7 DHC, 1981 TR7 FHC
http://tr7beans.blogspot.com/</center>

PeterTR7V8 · Postby **PeterTR7V8** » 30 Oct 2009 20:59

My formula seems a bit more straight forward:

Max Speed = cube root ((HP @ Wheels x 150,000) / (Drag Coef x Frontal area in SqFt))

Using Beans' new figure of 0.39 for the drag coef & an effective 90HP for a non-smogged car the top speed of the 2L 7 is 118MPH.

118 = cube root of ((90 x 150,000) / (0.39 x 21))

The published top speed is 110MPH at 6000RPM so it is restricted by gearing presumably. Is it possible to pull 6000RPM in 5th gear in a standard TR7? It is in a TR7V8. [^]

Never say die. At least not while you're still breathing.

Workshop Help · Postby **Workshop Help** » 30 Oct 2009 23:24

You folks can put away your slide rules and abacus's now, I found it. It took a while but after some time in the library, I read in 'Autocar' magazine from January 25, 1975 in a technical article on the new TR7, a paragraph comparing the factory claimed drag coefficient of the "brick-like TR6" and the new TR7. The TR6 came in at 0.44 while the TR7 is 0.396 for the coupe. I can find nothing yet on the convertible. Beans memory was correct.

Naturally, the top side of the cars design was the major contributor to this low aero dynamic drag number. One then can't help but ponder if the factory had smoothed out the bottom of the car how things might have been. Particularly after first seeing the 1977 James Bond movie, 'The Spy Who Loved Me', when the very similar Lotus dove into the water turning into a car-sub.

What makes me remember these things?

Mildred Hargis

Hasbeen · Postby **Hasbeen** » 30 Oct 2009 23:41

I think you may have hit the point there Peter, with gearing. I
think most old things run out of breath quite suddenly. Back when
they used to publish power & torque curves, you could see the
sudden drop of both. How steep the hill is, can also help.

An example, with all bhp FIGURES GIVEN FOR 4500 RPM, AS WE USED TO
TUNE FOR THE REVS WE USED MOSTLY, NOT PEAK POWER.

My 62 Morgan +4, [TR3A engine], in 1963 was timed down Conrod
straight Bathurst at ..124,2 MPH, with 79 BHP at 4500 RPM [wheels]

1964 in same place,...124.6 MPH, with 115 BHP Everything relating to
speed was the same.

A mate built the ultimate 60s Morgan +4. Same as mine, even had my
aero screen. The TR4 engine was out to 2.4L & was extremly highly
tuned.

1969 in same place,..126.2 MPH, with 165 BHP at 4500 RPM.

He got only 2 MPH with over double the power of my 1963 effort,
which led us to believe that you could not overcome the basic
porting & combustion chamber shape, no matter how big you made it.

I doubt my 7, which is pretty good, [stockish] would get anywhere
near 118 MPH. It accelerates to about 90 MPH, but is only slowly
winding up, after that.

An example of wind resistance, I drove a Hillman Imp at Bathurst, in 1964.

It was not getting over 78 MPH, on its own down Conrod, & was not
fast enough to "get a tow" from faster runners.

A mate, in a Triumph 2000 actually slowed down to my speed, infront
of me, at the top of the straight, & then slowly accelerated, which
allowed me to "catch his tow". On that time down Conrod they timed
me at 107 MPH, that's more than 35% faster.

That was reported on the front page of the Sydney Morning Herald on
the monday. something like "Hillman Imp does 107 mph". I wonder how
many people rushed out & bought one, & were a little disappointed
in their speed performance?

Hasbeen

Cobber · Postby **Cobber** » 31 Oct 2009 01:23

G'day Hasbeen,
having been a nautical man you will be no doubt familiar with the concept of hull speed: The theoretical maximum speed of a design of displacement hull will be able to move through the water. This speed can be reached with relative ease, but to gain the slightest increase in speed above the hull speed requires a huge increase in power for very little gain, as the effort required to displace that same mount of water at the increased rate is enormous.

As air is really a fluid too you have to expect the principles of fluid dynamics to be similar to aerodynamics.

So looking at the speeds of the Morgans down conrod straight it would suggest the 79bhp engine in the 1963 effort had the old Moggy pretty well close to it's "hull speed", when you compare it with the following year's effort with an increase in power by about a 1/3 you see only a very small gain in speed. And we see in the 1969 effort that doubling the power still did not gain a great increase in speed.

Mind you if you were to re-body the moggy in something a bit more aerodynamic than a brick wall then I think you would find that the power increases would be put to much better use.
A bit like comparing a dingy to a racing kayak, you can paddle the both but the kayak is going to go faster for the same amount of effort.

Of course this is an over simplification and there are many more factors at play here but if you were to double the power again you sill wouldn't gain much more speed. Alas the laws of diminishing returns apply.

80'Triumph TR7, , 73'Land Rover (Ford 351. V8),
'89 Ford Fairlane
85'Alfa 90, 69'Ford F250.
76' Ford F100

roverman · Postby **roverman** » 17 Nov 2009 20:26

Suggestions of most efficient method for reducing 700lbs/lift at 150mph.? My plan is "Wheeler", vortex generators. Anyone? Thanks, Art.[8D]

Art Gertz

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Wedgie, how aero?

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