(coastdown-related) So I remembered why I paused dev work on MPGuino for 3 years...

[t vago]

Turns out that coastdown testing calculation is not easy!

Right now I am looking at using this Instructable as a start, but with using a starting speed of 115 kph and an ending speed of 15 kph instead.

I remember the discussion about there actually existing a V term that corresponded to drivetrain-related viscous drag, so there's a complication there. Not really sure how I'd calculate a C(d) either, since people have pointed out that cars have a negative lift component at speed which is going to throw off the V^2 term calculation. Hm.. at least C(rr) could still be reliably calculated... perhaps.

I think the best to be hoped for is to just do a sort of stopwatch function that starts when the vehicle slows down and passes 115 KPH, and stops when the vehicle reaches 15 kph. One could do an average of several passes in that fashion, and store the resultant elapsed time for A-B-A comparisons.

[freebeard]

Turns out that coastdown testing calculation is not easy!

Uphill, downhill or level course? Uphill would bias toward weight and friction, but downhill would unload the drivetrain and bring aero drag to the fore.

I decended the Siskiyou Pass in neutral from a rolling start. The corners got hairy. Or is it harry?

hairy
hâr′ē
adjective
Covered with hair or hairlike projections.
Consisting of or resembling hair.
Fraught with difficulties; hazardous.

It's hairy.

[j-c-c]

Only if you believe in understatements

[freebeard]

It makes you believe in understeer. In the Beetle you can steer with the throttle.

[j-c-c]

And why racing down Pikes Peak would be so much more exciting.

[Piotrsko]

Racing down hill would only qualify you for a Darwin award

[Hersbird]

Originally Posted by freebeard

It makes you believe in understeer. In the Beetle you can steer with the throttle.

I learned about the Beetle's understeer in 1980 on highway 199 surrounded by old growth redwoods. My step mom was showing us how fun her old 68 was, I think she learned a little that day too. Luckily just 540 degrees of rotation and we were even still heading the right way!

[freebeard]

That sounds like oversteer; -- almost as bad. Understeer is when it dosn't go where you point it. Oversteer enthusiastically over-interprets your input.

Porsches (Porchea?) are notorious for snap oversteer on freeway onramps. It's bit me twice now with the Superbeetle. I think it's the mis-matched tire brands front to back.

With the Notchback I cold reduce the turning circle by stepping the back end out in first. It ran 145/165 Klebers.

[t vago]

Originally Posted by freebeard

Uphill, downhill or level course? Uphill would bias toward weight and friction, but downhill would unload the drivetrain and bring aero drag to the fore.

I decended the Siskiyou Pass in neutral from a rolling start. The corners got hairy.

Unfortunately, inclines will skew the input data if we concentrate only on going uphill or downhill.

Been thinking to rewrite the coastdown equation to make the constant term a function of lift, but it may make simultaneously solving for C(d) and C(rr) impossible. Right now i simply just do not know.

Heh, maybe i could divert myself with measuring oversteer in a Suzuki Swift or something...

[freebeard]

I wanted to get the Metro aligned but they want to sell me an inner tie rod, so there's a decision point there. I don't know if front wheel drive responds to rear alignment or not. The Notchback showed that rear toe in/out affects how it responds through corners.

Maybe one could skew the data so as to amplify the signal over noise. Raise the contrast so to speak.