myke pocock
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| posted on 31/12/08 at 04:19 PM |
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Completing SVA1 form
I am just in the process of filling in SVA1. I have a 1300 x flow fitted with standard gearbox and standard axle. I have put 'Maximum power
engine speed' as 5500RPM but what should I put for 'Maximum road speed' My Haynes manual is of no help. Also on Question 15, the
Carlisle tester said weigh the front and then the back and add 150kg to both weights for the Axle 1 and Axle 2 part of the question. Does that sound
OK to those who have already done it as I am now a bit mystified as I thought that 150 kg (for two passengers) should be added overall ?
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iiyama
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| posted on 31/12/08 at 05:23 PM |
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Best guess for road speed. Hows 115 sound? All they are interested in is the speed rating for the tyre, so as long as the tyres are good for 115 then
your good to go.
Whats the design weight for the car?
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The Baron
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| posted on 31/12/08 at 05:41 PM |
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The design speed, is the speed rating of your tyres.
Cheers,
B
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nstrug
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| posted on 31/12/08 at 10:10 PM |
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Your maximum speed is easily calculated:
Vmax =60*Tcirc*RPMmax/(Rtop*Rdiff)
where:
Vmax = Maximum speed in m/s
RPMmax = Maximum RPM
Rtop = ratio of top gear
Rdiff = differential ratio
Tcirc = tyre rolling circumference in m
To convert m/s into mph multiply by 2.2369.
You can calculate an approximate Tcirc from the rear tyre size.
Where the tyre size is: Twidth/TprofRWsize (e.g. 225/50R15):
Tcirc = (Wsize*25.4+2*Twidth*Tprof/100)/1000
I have a spreadsheet to do all this (and calculate in gear speeds, torque curves etc) if you want it.
Nick
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speedyxjs
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| posted on 1/1/09 at 07:23 AM |
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^^^ Surely aerodynamics can affect your top speed? If you had a modeo and a transit with exactly the same running gear and weight, the mondeo would
still be faster?
How long can i resist the temptation to drop a V8 in?
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idl1975
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| posted on 2/1/09 at 08:51 AM |
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quote:
Originally posted by speedyxjs
^^^ Surely aerodynamics can affect your top speed? If you had a modeo and a transit with exactly the same running gear and weight, the mondeo would
still be faster?
Yes, but you're not going to be able to calculate that without MIRA's wind tunnel.  The method above will give you an geared top
speed - whether the car can pull its gearing is another question. As pointed out above, the concern is apparently whether the tyre's speed
rating is appropriate, and unless the builder is using H rated tyres with a particularly tall gearing, I don't imagine there will be a problem.
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nstrug
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| posted on 2/1/09 at 10:56 AM |
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As Ian pointed out, the above calculation is your maximum gear speed, which I think is what they want on the SVA1 application.
It's possible to calculate CdA and hence aero drag without a wind tunnel. You need a long, straight, flat and empty road.
You need to run two tests, one at high speed, the other at low speed. Pick 2 speeds in each range, about 5mph apart:
e.g.
Vh2 = 70mph
Vh1 = 65mph
Vl2 = 15mph
Vl1 = 10mph
Accelerate to Vh2, then shift into neutral and measure the amount of time it takes in seconds for the car to decelerate to Vh1. This time is dTh
Then repeat the test for Vl2 to Vl1. This time is dTl
Then convert the speeds to m/s and plug into this formula:
CdA = 8m/rho * 1/(Vh2+Vh1)^2 * ((Vh2-vh1)/dTh - (Vl2-Vl1)/dTl)
Where m is the mass of the car in kg and rho is air density in kg/m^3 (use 1.2, which is sea level at 20degC)
I used the following approximations in deriving the formulae:
1. Aerodynamic drag is zero during the low speed test (its a test of rolling resistance only)
2. The speed intervals are small enough that aero drag can be considered a constant over the high speed test.
Once you have your CdA, its pretty straightforward to calculate your top aerodynamic speed using your engine power and the drag equation:
Fd = 1/2 * rho * CdA * Vmax^2
Where Fd is the drag force and v is velocity - as we can see drag goes up with the square of velocity. Clearly, our maximum aerodynamic speed is when
the drag force is equivalent to the wheel thrust, or, multiplying through by velocity:
rwp = 1/2 * rho * CdA * Vmax^3
So:
Vmax = ((2 * rwp)/(rho*CdA))^(1/3)
All units SI of course so you will need to convert rwp to Watts and Vmax will be in m/s.
Nick (who is bored)
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