29 replies to this topic

[Xeo]

I did this awhile back, not sure how accurate it is though. Might be of some use. Xeo

Attached Files

•  CarData.zip   133.01KB   16 downloads

[Crispy]

Woah that's hardcore!!

[the_bollix]

I've dealt with students in their final year Automotive department at Loughborough university who are writing their thesis on this very topic. While the principles may be sound there are too many unknowns for this problem to be solved mathematically. They start out thinking they can do it but it is never long before the grim realisation that what they are undertaking is never going to be accurate hits them.

Coopa - I'm an Lufbra Auto grad, has my path ever crossed yours? I graduated last July. What's you connection to the auto dept?

On another note, in my old job we used an in house simulation program which we used to modelled vehicles to within 2% accuracey of physical test data. But we had accurate windtunnel data, tyre models, torque curves, fead losses, gearbox losses etc etc.

Shane.

[Tony_M]

Coopa You can't work this out from first principles it goes way beyond degree level physics. You could devote your PhD to this and still be way off the mark. Other than the car you have all the external forces to consider. The coeffeicent of friction or loss of traction due to poor road surface, oil, desiel etc... Wind resistance, especially when topless. Any undulations in the road which will effect the cars balance. ...and finally SCAMERA's which will drastically reduce the 0-100 time especially if you get pulled over.

[walkes]

I did this awhile back, not sure how accurate it is though.

Might be of some use.

Xeo

Now we are getting somewhere...

[Xeo]

Now I’ve gone back and had a look at that spreadsheet I’m starting to remember it’s short falls. The main ones are, 1) It assumed the start rpm torque figure is applied in full when you dump the clutch all the time until the cars going fast enough in first gear for the engine to run at that rpm. To be more accurate it would need to take into account engine inertias and the slipping clutch. 2) The gear changes are instant and the cars not slowing between them. I did it for the Elise S2 as I couldn’t find all the data for the VX, but I’m sure people here will know what’s needed. I did the sheet to compare the S2 to a friends chipped 150 TDI golf, which turns out to be quite a quick car. It’s attached if anybody’s interested. Xeo

[Xeo]

Golf Data

Attached Files

•  GolfData.zip   134.48KB   6 downloads

[coopa]

Coopa

You can't work this out from first principles it goes way beyond degree level physics.

You could devote your PhD to this and still be way off the mark.

Other than the car you have all the external forces to consider. The coeffeicent of friction or loss of traction due to poor road surface, oil, desiel etc...

Wind resistance, especially when topless.

Any undulations in the road which will effect the cars balance.

...and finally SCAMERA's which will drastically reduce the 0-100 time especially if you get pulled over.

I know!

I think that is what I was trying to say in earlier posts.

What I have used is widely accepted chassis dynamometer theory for road load simulation testing of vehicles. I've just reproduced the maths that a dyno control system uses, but where a computer would sample at a rate of ~ 1kHz I worked it out at 1Hz. The vehicle coefficients I used where from the American National Standards for emmissions testing. You just enter a vehicle mass and they give you back standard coeffs, not necessarily those for a vx!

Using these principles on a dyno gives excellent results so I saw no problem with applying it here (except I would never normally use an average engine power!

the_bollix - I wrote (with a couple of others) the control system for the LUT chassis dyno. My company moved the dyno across the LUT site about 3.5 years ago and we recommissioned it with a new control system shortly afterwards. We did a lot of work with a guy called Marting Passmoor there who worked on the algorithms for curve fitting using amoeba methods. I met a few students doing their final year projects using the dyno, (one guy named Matt Neild iirc) who was trying to determine engine performance by putting the car on a chassis dyno. There were always the old question : What are the tyres doing? Answer: You don't know.

Obviously the tyres and clutch are slipping to varying degrees and yes this is extremely difficult to model mathematically.... Wasn't that the first thing I said?

[Xeo]

Ok guys, here is my newest version of what I think is a fairly accurate ( well near enough) sheet for modelling car accelerations. It now includes an adjustable gear change delay and an adjustable initial clutch slip factor. Anybody got any improvements I could make? Xeo

[Xeo]

Sheet

Attached Files

•  Car_Data_iss_3.zip   138.74KB   5 downloads