coastdown hill

[MetroMPG]

Originally Posted by CO ZX2

Like sooo. . . . . ginormity???

That IS ginormous. I was thinking something like that, yes. Only in cardboard & duct tape in its testing form

Originally Posted by brucepick

Um, keep your pants on. Unfortunately removing the air dam would be a fair sized PITA.

That's too bad. Pants remain on. But if you do get a chance to compare it, go for it.

Originally Posted by Gary Palmer

MetroMPG: I found where your test was done with the hatch full on open. I looked at the link and searched and I couldn't locate any test that was done with the hatch unlatched, but not raised. Was their a test with the hatch just cracked open, like with a broom handle or something?

No, that was Peakster's test, not mine.

Today I duplicated his - the hatch was wide open. I don't believe anyone's done a test with the hatch only unlatched & cracked open a little bit.

[CoyoteX]

Originally Posted by MetroMPG

Pantless Sven, we want data!
There's one other hill literally minutes from the house that I'm sure would coax upper 30's mph on the descent, but there's a railway crossing with brutal bumps and a traffic light in the run-out zone.

How about starting at the same spot on the hill and letting it coast down the hill to a certain spot and see what the speed is? Pick a road sign or tree or something before the tracks and see if you can get a good variance coasting down to that spot. That way you don't have to deal with the crossing and stuff. Also maybe try coming at the downhill run at speed to see if you can find a terminal velocity. If it is a long hill that should not be hard as long as you don't have to hit it at something like double the speed limit or whatever

The thing I always look for is a very consistent spot that I can go to one day to the next that always gives me the same speed/distance. So anything that would be more involved than removing a few screws to change I can do over a few days. The only thing I have seen that changes my distances on tests from one day to the next is really big temperature changes so if the temp changes by more than maybe 20 degrees F a day then the numbers wont be quite as good. The best way I have found is to pick a spot or a few spots and keep doing the test anytime you happen to be around there. This helps let you know if that spot has good repeatability for doing tests over the course of several days. Once you have a spot you know exactly how your car does consistently on, testing like this becomes way easier.

[MetroMPG]

I don't think I'll be able to easily find terminal velocity on this hill - it's not long enough (and there isn't much of a "runway" before the top to build up speed either).

[Bill in Houston]

I wonder how repeatable your data is. Like if you ran three trials with the same settings, how close would your distances be? Hmm.

[MetroMPG]

That's the key, isn't it?

Next time I try it, I'll be collecting data from multiple runs to answer that question.

[MetroMPG]

Answer to that question:

[Bill in Houston]

Well, you could do a "control" run every couple of test runs, and then correct for any apparent improvement across runs. Know what I mean? Not ideal, but pretty defensible.

[MetroMPG]

Well, you know what? This "problem" could be an opportunity to do a test.

[lca13]

I played a little with terminal velocity tests on hills and am thinking that I might get a more accurate test to test result by taking the acceleration factor completely out of it... by seeing what terminal velocity is "reduced" to rather than "increased" to. ie. hit the top of the hill faster than your expected terminal velocity and let aero (and RR) forces decelerate you to terminal velocity...

On paper it shouldn't make a difference but in practice you can hit close to terminal velocity at the top of the hill and get the reading for a much longer duration (than hitting the top at say 50 mph and seeing how high you speed gets by the bottom).

I have not tried it yet though... just seems to make the experiment a little more controllable and eliminates all factors other than aero and RR.

[MetroMPG]

Removing variables is always good.

I need to make a chassis dynamometer to pre-warm my drivetrain.