Mole Posted October 7, 2002 Share Posted October 7, 2002 I am going to borrow an accelerometer (like the one in the DT catalogue) just for the fun of it. Apparently one of the measurements possible is bhp - to achieve this it is necessary to input the weight of the car. I have two questions: 1. My best guesstimate is ~ 700kg. The car is a roadsport SV with standard 1.8 k-series lump. Sales info says standard roadsport weighs in at 575kg, SV is 25 kg heavier and I estimate about 100 kg for me and other bits such as emergency tools etc. Does this sound about right - any suggestions of a simple way to get a more accurate weight. 2. For those who understand such things - how sensitive is the bhp calculation to the estimated weight of the car. I assume the calculation is something like power = mass * accelerration, so if my estimate of mass is (say) out by 10kg - how much difference will this make to the power figure? SV 52 CAT - the Mole is flying Link to comment Share on other sites More sharing options...
fireblade Posted October 7, 2002 Share Posted October 7, 2002 Drive the car to a weighbridge. Try "WEIGHBRIDGE SERVICES" in yell.co.uk. My local tip (Richmond, Surrey) has one it uses for commercial vehicles, though not sure how accurate this is. Please post back the results - I'm planning on doing this in a couple of weeks :-) John Link to comment Share on other sites More sharing options...
scooby dooby doo Posted October 7, 2002 Share Posted October 7, 2002 force = mass times acceleration. your acceleration can be measured by the accelerometer, and it will vary over time. the mass is constant the force will vary for several reasons - force means resultant force so its force from the engine (after gearing etc) minus transmission losses, minus the drag from the air, which is roughly proportional to speed squared. you also need to be careful as the car will squat under acceleration so will record more acceleration than there actually is. so now you've got force provided by the engine against time. so we need to turn it into a power. to do this we use power is force times speed over which it acted. if you split it into segments, you can see the average power output over the average speed of your segment. peak power is the biggest of these. ah! just realised - does this accelerometer have a load of electroncs with it to do the above for you? hmm... it'll still ignore drag and transissions losses though... HOOPY Today I shall be mostly wanting to go for a blat R706KGU Link to comment Share on other sites More sharing options...
Mole Posted October 7, 2002 Author Share Posted October 7, 2002 As far as I can tell, you enter the weight of the car into the accelerometer and it does the calculation for you - I presume it can calculate the power required to produce the measured accelaration of a "known" mass. This I can cope with conceptually, but since my last contact with this kind of calculation was for physics O level thirty years ago, I have no chance of working out what the equation is. I imagine it must be along the lines of: Power(in Bhp) = K(mass x acceleration) (where K is some constant). I'm not that interested in the equation per se, but I am interested in knowing how sensitive the answer is to error in the estimation of the mass. SV 52 CAT - the Mole is flying Link to comment Share on other sites More sharing options...
JAG Posted October 8, 2002 Share Posted October 8, 2002 This piece of apparatus is probably calculating the rate of change of kinetic energy. Kinetic energy being the energy of an object by virtue of its mass and speed. K.E= 1/2 x Mass x Velocity^2 Velocity = acceleration x time Kinetic Energy is measured in Joules and Joules are a direct equivalent of Watts. Kilowatts (1000Watts = 1 Kilowatt) are used by our continental friends to specify engine power output instead of Horsepower. 1 Horsepower = 746 Watts = 0.746 Kilowatts Therefore the rate of change of Kinetic energy = Energy output by the engine. JAG Link to comment Share on other sites More sharing options...
scooby dooby doo Posted October 8, 2002 Share Posted October 8, 2002 hmm... thats a MUCH simpler way to think of it However, you have to remember that its: Therefore the rate of change of Kinetic energy = Energy output by the engine that is being used to accelrate the car at 100 mph the device will tell you that you're only putting out 5 bhp as you're accelerating so slowly. Can't stop myself with this one - A Joule is a unit of energy. A Watt is a unit of power. 1 Watt is 1 Joule per second. sorry HOOPY Today I shall be mostly wanting to go for a blat R706KGU Link to comment Share on other sites More sharing options...
Mole Posted October 8, 2002 Author Share Posted October 8, 2002 Thanks Hoopy and JAG I think I need to spend some time trying to digest what you've said . I'm following up the weighbridge route but I've just noticed that the MAC (copy) has a gross weight figure of 900kg - surely this can't be right - that's 300kg (i.e. 50%) more than the weight given in Caterham's sales blurb 😳? SV 52 CAT - the Mole is flying Link to comment Share on other sites More sharing options...
scooby dooby doo Posted October 8, 2002 Share Posted October 8, 2002 The MAC weight isn't the actual weight, i forget what it actually is - something like the maximum possible permitted weight or a wild guess or something... HOOPY Today I shall be mostly wanting to go for a blat R706KGU Link to comment Share on other sites More sharing options...
JAG Posted October 9, 2002 Share Posted October 9, 2002 Hoopy, you're right, Whilst writing it I felt there was an energy per second thing I was missing somewhere didn't think about it long enough I guess. You are also right that the device can only measure the energy being used to accelerate the car. Therefore I would suggest that you perform some 0-60 type acceleration tests without getting much above 60, that should limit the effect of aerodynamics on the final result. JAG Link to comment Share on other sites More sharing options...
scooby dooby doo Posted October 9, 2002 Share Posted October 9, 2002 you can calibrate the losses to some extent by doing runs between the same rev points but in different gears. If you assume drag is proportional to speed squared and transmission losses are proportional to engine speed squared you shouldn't be too far out. Then calibrate by plotting the curves and doing some jiggery pokery... slight complication is that losses in the diff will appear in the drag part of the calculation. ah! you can do better than that ❗ if you do decelleration runs with the car in neutral you can see the energy absorbed by the air as it slows you down. 😬 then we've only got transmission losses to look at. the speed squared ones are the diff and the engine speed ones the gearbox. this is still a bit crude but i'm not a gearbox expert ❗ HOOPY Today I shall be mostly wanting to go for a blat R706KGU Link to comment Share on other sites More sharing options...
V7 SLR Posted October 9, 2002 Share Posted October 9, 2002 The AC22 calculates power at the wheels and has a "squat" multiplier built in. Worcs L7 club joint AO.//Membership No. 4379//Azure Blue SLR No. 0077//Se7ens List Tours Link to comment Share on other sites More sharing options...
JAG Posted October 9, 2002 Share Posted October 9, 2002 Right now I can't see why "sqatting" under acceleration will lead to more acceleration being recorded, can someone explain why?? Could understand an argument for recording less acceleration as some of the engines energy is 'used up' in compressing the suspension but not more JAG Link to comment Share on other sites More sharing options...
DohNut Posted October 9, 2002 Share Posted October 9, 2002 If is is a cheap accelerometer it will record in one plane only. The internals will be along the lines of a weight attached to a spring and something measuring how far the weight has moved. If the weight moves a set amount the spring must be compressed a set amount and the accelerometer will "know" what force is being applied. If it is set up in a stationary car on flat ground and set level within the car, with the built in spirit level (usualy). It will then measure along the axis of the car. If you jacked the front of the car up a little to simulate the nose rising under acceleration then gravity would pull the weight in the accelerometer along a little bit and "measure" acceleration. OR the short answer is its just like one of those flight simulator fairground rides, the front tips up and you feel like it is accelerating when it is staying still. An expensive accelerometer would read in virtical horizontal and lateral planes and get a clearer picture of what was going on as the the combination of forces in all three planes should give you the total. Nick Link to comment Share on other sites More sharing options...
Paul Ranson Posted October 9, 2002 Share Posted October 9, 2002 The acceleration you want to measure is parallel to the ground. If you align your accelerometer pointing slightly upwards it will under-read the horizontal value, but will start to read gravity. So to correctly estimate actual acceleration at a given squat you need to subtract a small amount of 1g and then add a small amount of whatever is being measured. It's quite possible that if your car is nose up at an angle of A and the accelerometer is measuring 'a' then the actual acceleration is, actual = a/cosA - gsinA (I don't think that helped much. Sorry. I'm sure Peter will be along shortly to fix the arithmetic) Paul Link to comment Share on other sites More sharing options...
JAG Posted October 9, 2002 Share Posted October 9, 2002 Cheers Doh!Nut - Now I understand! In a car like a 7 with its extremely low centre of gravity the actual squat and hence inaccuracy should be very small then JAG Link to comment Share on other sites More sharing options...
allen Posted October 9, 2002 Share Posted October 9, 2002 For my sins I operate a weighbridge as part of my job. Mole - The weighbridge is located in Watchet and designed for HGVs, and is only accurate to within the nearest 20 KG. You could contact Westco at Bishops Lydeard (01278 433411) who manufacture this and more accurate equipment and ask if they could help, or tell you who would have a more accurate system locally. Should you wish to use the bridge I operate (I'm off shift fri 11th through to tues 29th Oct) call 01984 631 456, ask one of my collegues if you can bring your car in 'out of hours' Sat or Sunday afternoon is best, (management is mostly at home). Tell them you're a mate of mine and they will not expect a contribution to the Xmas fund. allen Link to comment Share on other sites More sharing options...
JP Posted October 9, 2002 Share Posted October 9, 2002 (From school physics) isn't it something like: power = work done per unit time. work = force x distance force = mass x acceleration assuming a standing start and uniform acceleration can this be resolved to: power = mass x acceleration squared x time ...or thereabouts *confused* *confused* Where's Peter C??? Edited by - jp on 9 Oct 2002 20:26:01 Edited by - jp on 9 Oct 2002 20:28:15 Link to comment Share on other sites More sharing options...
mav Posted October 9, 2002 Share Posted October 9, 2002 Mole, Mine is 600kg with a full tank of fuel if that helps Regards, Maritn Link to comment Share on other sites More sharing options...
scooby dooby doo Posted October 9, 2002 Share Posted October 9, 2002 only at any one instant. you'd have to integrate it to get it to work... HOOPY Today I shall be mostly wanting to go for a blat R706KGU Link to comment Share on other sites More sharing options...
Paul Ranson Posted October 9, 2002 Share Posted October 9, 2002 My standard Superlight with some fuel in it and stuff was too light for a weighbridge. I think the weighbridge was +/-10kg, minimum 500kg. They put a fork lift on as well and did a difference.... Paul Link to comment Share on other sites More sharing options...
Mole Posted October 10, 2002 Author Share Posted October 10, 2002 Thanks all *thumbup* allen - may well contact Watchet - interestingly, the SCC web site lists local weighbridges but the list doesn't include anything in Watchet Do they really work Sunday afternoons - poor bu**ers ! 🤔 mav - 600kg is spot on according to the sales blurb, so perhaps all I need to do is stand on the bathroom scales clutching my emergency toolkit, hood, car jack etc and add the result to 600kg 😬 😬 If most commercial kit is only accurate to +/- 10 to 20 kg then this is probably going to be as accurate. I've also contacted the weights and measures guys at the county council to see what level of accuracy they expect when they do their testing of commercial bridges. SV 52 CAT - the Mole is flying Link to comment Share on other sites More sharing options...
mav Posted October 10, 2002 Share Posted October 10, 2002 Mole, Sales blurb does not include the following changes though: 6spd, Appollo, Adj. Platforms, plumbed in Exting. FIA bar, leather seats & Tunnel top. Link to comment Share on other sites More sharing options...
Peter Carmichael Posted October 10, 2002 Share Posted October 10, 2002 All the maths looks pretty good to me except I think Paul's equation should be: actual = (a - gsinA)/cosA Link to comment Share on other sites More sharing options...
JAG Posted October 10, 2002 Share Posted October 10, 2002 That's so good to know, Thanks Pete! JAG Link to comment Share on other sites More sharing options...
Peter Carmichael Posted October 10, 2002 Share Posted October 10, 2002 Oh. By the way. Aerodynamic drag *power* is mostly related to the cube of road speed, not squared, as Hoopy suggested, but he is on the right track that you can work out an approximation for the road load equation by repeating experiments, between different road speeds and using different gears. Worth recalling that the accelerometer measures acceleration and attempts to correct it for effects like suspension squat. Any errors in that correction get added up into the integral that determines the road speed. i.e. if the box is a bit optimistic about acceleration, it then adds up to a bigger speed, it then multiplies the too-large speed by the too-large acceleration to give you a too-large^2 power figure. Just been looking again at JPs stuff and it is a bit of a roundabout derivation: work = force x distance - Yes, even if you didn't explicitly fill in the in between steps: power = force x (distance/time) power = force x speed force = mass x acceleration - Yes, therefore: power= mass x acceleration x speed - but speed is not directly measured, but is derived as an integral of the acceleration v = u + at - more schoolboy physics, yet our only reference is for u=0 speed = integral between time=0 to time=T of: acceleration wrt. time - which for constant 'a' is acceleration x T If the acceleration is constant, then indeed we end up with: power = mass x acceleration x acceleration x Time But it is not particularly useful because acceleration isn't constant, otherwise we wouldn't be measuring it. The correct answer is not to blur the difference between instantaneous readings for acceleration and the integrated acceleration: Speed(t) = [iNTEGRAL T=0 to t of: acceleration(T) x dT] Power(t) = mass x acceleration(t) x speed(t) Link to comment Share on other sites More sharing options...
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