Computer simulation of vehicle performance

[Pumaracing]

Nice website with Javascript calculators for wheel, tyre and engine inertia.

http://hpwizard.com/rotational-inertia.html

[Pumaracing]

I tried a little experiment with my own car, 2.0 litre Ford Focus 130 bhp, this morning. Blipping the throttle and timing the rpm increase with a stopwatch. As near as I could measure 1000-6000 rpm was 1.1s and 2000-6000 was 0.78s.

I know the power curve pretty exactly both from manufacturer figures and also rolling road tests.

I exactly match the above times with a total inertia of 85 lbs at 3.5" radius but that's far too high. If I put that number in a simulation the car is about 1 second too slow in the 1/4 and correspondingly everywhere else. It matches road test data with the 25 lb inertia figure I settled on many years ago after many simulations of different cars.

So I'm back to square one. Is there something about revving the engine when the car is stationary that prevents the rpm from rising as fast as just inertia would dictate? Maybe the rate at which inlet maniflow airflow can keep up with throttle demand or summat?

[Pumaracing]

Stan Weiss said:

Dave,
How would those time change if you removed the accessory drive belt from the engine?Stan

Dunno Stan. That would involve, oh what are those things called?, ah, "tools" and getting my hands dirty. Not my thing at all.

However if the accessory drive belt is taking a load equivalent to 60 lbs of inertia at 3.5" radius then it would snap like an elastic band.

[Pumaracing]

Stan, here is the flywheel power curve from my car off a Dastek chassis dyno. Rated power is 130 bhp so spot on. I had to guesstimate the rpms below 1500.

500...8.5
1000..18.0
1500..29.0
2000..44.3
2500..58.4
3000..68.9
3500..84.4
4000..97.2
4500..106.7
5000..122.9
5500..129.0
6000..126.0

I match the 2000 rpm to 6000 rpm time of 0.78s with 87 lbs at 3.5" inertia.

So about 60 lbs more than I usually use for a 4 pot engine of 25 lbs. No way it's really as high as 87 lbs.

[Pumaracing]

I just had a good play with the inertia calculation website I linked above.

I weighed the wheels and tyres off my Focus. 205/50/16 tyres with a rolling radius of 0.963 feet.

Tyre weighed 8.8 kg. Total was 17.3 kg so 8.5 kg for the wheel.

The website gave me an equivalent total inertia mass (including the base weight) of 16.9 kg for the tyre and 11.6 kg for the wheel so a total of 28.5 kg. That's 65% higher than the base weight of 17.3 kg and as I posted somewhere above my own rough calculations many years ago had come up with about 3/4 of the base weight as being the add on for inertia effects. So pretty close.

So I've been adding about 125 lbs for wheel/tyre inertia to my simulations and this more accurate calculation would indicate 17.3 x 4 x 65% = 45 kg or 99 lbs for those sized tyres. Maybe a tad more once brake disc, hub, driveshafts are added in.

From now on I'll stick to 100 lbs for the average family car though and increase or reduce that for different vehicles. However a few lbs one way or the other on a 3000 lb vehicle are neither here nor there so I've been within 1% all along.

[Pumaracing]

Stan Weiss said:

Dave
Notice how the acceleration rate varies and follows the torque curve.

It would be deeply worrying if it didn't! It would mean Eddies in the space time continuum again and he's been told about that once already.

[Pumaracing]

Some more tyre weights from my collection of crap in the garage. This should cover a fair chunk of the car market for inertia calculation purposes.

195/50/15 - 7.4 kg
205/50/16 - 8.8 kg
225/60/16 - 11.2 kg
225/45/17 - 11.2 kg

Interestingly there's an almost perfect cube law relationship between the section width and the weight. Scaling the 100 lbs add-on calculated above for the 8.8 kg 205 tyre would give 84 lbs and 127 lbs for the other two weights.

[Pumaracing]

It's possible but highly sensitive to even small gradients and any head or tail winds.