C of G height measurement

[Silente]

Hi,

i was at one of Rouelle Seminar this year and he showed to us his way to find C of G height.

He is actually saying to tilt the car on one side reading on a digital inclinometer the angle at which it find an equilibrium.

I haven't done it yet but i would like to do it soon but i have some doubts.

Is it actually easy to understand when the car reach the equilibrium? or you have to pass this situation before you can really understand you reached it?

I am afraid to tilt the car upside down http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Silente]

Hi,

i was at one of Rouelle Seminar this year and he showed to us his way to find C of G height.

He is actually saying to tilt the car on one side reading on a digital inclinometer the angle at which it find an equilibrium.

I haven't done it yet but i would like to do it soon but i have some doubts.

Is it actually easy to understand when the car reach the equilibrium? or you have to pass this situation before you can really understand you reached it?

I am afraid to tilt the car upside down http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Bazanaius]

Luckily these cars don't weigh very much, and the situation you're testing isn't particularly frantic. I'd suggest ensuring the thing you're resting the lower pair of wheels on is fairly stable, and securing a strap or something to a bit of the car. Having a few blokes around (carefully instructed) who can slowly ease the car up and 'feel' it going would be useful.

Your other option is not to tilt but hang. We have a large winch used for moving large objects around the workshop. hanging the car from the hoop, and rear box and possibly part of the front box is a good way of locating your cg (I'd suggest not having the driver in it). Plus, it looks cool.

[Fred G]

Put a piece of angle on the tyre and pivot the car on that. You'll find as the car tends towards equilibrium, the car is easier to handle.

If I were a judge at a competition, I'd love to see cars get lifted up in this manner to see how low CoGs are http://fsae.com/groupee_common/emoticons/icon_wink.gif

I don't think UWA have tried this method with a driver in the car as of yet http://fsae.com/groupee_common/emoticons/icon_smile.gif

Fred

[Adambomb]

We built some pieces of "large angle" that was about 6" x 6" to make sure the car wasn't sitting on the sidewalls (load was transferred more evenly throughout the wheel/tire). One semi-obvious thing to note is that the measurements aren't particularly useful unless you have a driver in there with helmet, etc. (that is, unless you're interested in how the car handles without a driver http://fsae.com/groupee_common/emoticons/icon_wink.gif ).

Finding equilibrium is easy, that's simply when it's balanced. Just like the others said, with a couple people around to lift one side it isn't too hard. Safety strap is a good idea too. Most likely your driver will be just a little bit freaked out, with last year's car we got 68 deg.

[Adambomb]

Ooh, one other thing, in terms of helping to prevent tipping the car over: As the car reaches equilibrium, assuming you've got a couple people holding it up that point, there's a good chance one could be pulling it down while another is pushing it up. To actually get it balanced, one method (*disclaimer...not to be considered the best, just the best I came up with) is that once the car gets close to balanced, slowly have people start letting "loose" of the car until only one person is holding it up. In other words, they still hold onto the car (for safety's sake), but don't apply any force (much like spotting when weightlifting).

It's pretty easy for one person to find the balance point. Again, putting a safety strap from the top of the roll hoop to something solid with only the minimal slack necessary to get the car up is a really good idea (just like they do with the tilt table at comp.). Also make sure there's nothing in the area the car could roll onto if by chance it did roll over. Barring all that, as long as the driver is belted in tight, wearing arm restraints and a helmet, and your roll hoops are rules legal, theoretically the driver should be safe even if the thing did go over.

Getting beyond all that, if you don't have a lot of trust between your team members, it might be good to do one of those "trust falls" to get everybody warmed up! http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

[Adambomb]

OK, one final thing, (last one I promise). This was some of the best "bang for the buck" testing we've ever done. Took like 20 minutes to make the tire blocks, 10 minutes to gather the safety gear, angle finder, and team members, and maybe 2 minutes to do the actual testing.

So now let's see some results! Our measured CG height came out to 10.8 in, and our seatback angle for that car was reminiscent of a compact regular cab pickup...so I know there are some better ones out there! (and no fibbing either, use an actual competition driver with helmet, hands on the wheel and sitting straight-up as if they were driving, etc.)

[js10coastr]

Yeah, my Formula Hybrid came in at 10.09 in and the driver is sitting pretty darn upright... but it really helps when you have 100 lbs of batteries an inch off of the ground. http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Silente]

Thanks for all the advices and also for the numbers!

I hope we can try on our old car next days, also to see the differences with our new car when it will be ready!

[Mike Cook]

There are some problems with tilt tests like this, namely the way you load the edge of the tire. This can effect the results a lot. Also, the suspension should be locked from moving because when the car is tilted, there will be less normal load on the tires which will cause the chassis to lift. Also, the chassis will roll and this moves the cg laterally. Finally, the tires will deflect which moves the CG closer to the outer tires. These last three things won't effect a fsae car as much as they will vehicles that roll a lot with high CGs and soft tires (i.e. big trucks). For all these reasons the tilt test will generally predict a higher CG than actual.

With all that said, I think it would be awesome if during the tilt test all cars are tilted until they pick up both tires. And the numbers are posted on the nose.

[Pennyman]

http://photos-b.ak.fbcdn.net/hphotos-ak-snc1/hs106.snc1/4603_108805460241_507885241_2714249_2019213_n.jpg

We used a piece of angle iron wedged against the tires to keep a solid edge to tilt about.

I agree with Mike, I think it would be awesome to compare CG heights between cars at comp...

[AJS]

We have used an engine hoist in the past to lift our car for the tilt test. Works great when only three people are in the shop and one of them has to be in the car. We had one person operating the hoist and the other playing let's-test-our-roll-hoop-preventer

[Adambomb]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Mike Cook:
There are some problems with tilt tests like this, namely the way you load the edge of the tire. This can effect the results a lot.
</div></BLOCKQUOTE>

Yep...just to re-iterate, for ours we used blocks we made that were a good 6" tall...much taller than most angle iron you'll find. I imagine that helped some small amount, but not entirely as tire bulge still prevented the blocks from acting on a "rigid" surface.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Mike Cook:
Also, the suspension should be locked from moving because when the car is tilted, there will be less normal load on the tires which will cause the chassis to lift. Also, the chassis will roll and this moves the cg laterally. Finally, the tires will deflect which moves the CG closer to the outer tires. These last three things won't effect a fsae car as much as they will vehicles that roll a lot with high CGs and soft tires (i.e. big trucks). For all these reasons the tilt test will generally predict a higher CG than actual.
</div></BLOCKQUOTE>

That brings up a very important "detail" I forgot to mention: we did this with our "super firm" shocks...i.e. solid chunks of aluminum machined to the length of a shock at ride height. That was the main reason we did the testing when we did, figured we might as well since we already had the solid blocks in. Might be interesting to run testing again with regular shocks in it to see the difference.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Mike Cook:
With all that said, I think it would be awesome if during the tilt test all cars are tilted until they pick up both tires. And the numbers are posted on the nose. </div></BLOCKQUOTE>

+1
Wouldn't take long, and would make another nice stat to stick on your tech sticker that already has weight, L/R dist., etc.

[cjanota]

That looks much easier, we tipped ours up on the back wheels. You have to lift it much higher our way but you don't have to worry about sidewall deflection changing your numbers (if it is a noticeable effect.)