Source for CG Height of a 2012 Car?

[MCoupeAtlanta]

Hi guys!

Well, I'm an autocrosser - with an open wheel purpose-built car - but this January my local region has an event that sounds like a GREAT time to bring out my new ride, a 2012 Nissan Juke, for a spin. And what class is it in? Well, ah, no class, apparently. Yet.

It seems to be (appears to be) a bit tall for its width. A little monster of a car - AWD, CVT, four cylinder turbo/intercooled/direct fuel injection, force vectored AWD (the rear axles each have an electro-mechanical clutch which under certain conditions moves most or even all rear HP to the outer rear tire (fronts kinda being left to fend for themselves. . .)). With (it appears) ABS BY THE WHEEL. Almost unheard of in a street car. A sweet, tight little AWD car. But. . .

It has not been classed yet. Though it is a 2012 version of a 2011 car. A BIT too tall for a bit too narrow car. Too low a "Static Stability Factor" is assumed. Too tall for too narrow a track. But there is no number available on the interweb. . . If I could prove a number I could likely autocross, I think. If I can't, it becomes opinion for local events, and illegal for higher events (which I am uninterested in, in this car).

So. The calculation requires the "CG Height" of the car. I could measure the weight of the front axle tires. Then the rear axle tires. Then jack up one axle at least 10 inches and measure the lower axle weight and jacked-up height (jacked up wheels supported on something). Which will provide a number. Below this, low chance of approval. Above this, certain approval.

Does anyone have access to CG Height values for this car? I could likely do this on truck scales in 30 minutes, but that must be kinda pricy. The NHTSA has no rating for the car, rollover-wise. The IIHS lists the car as a "top safety pick," but it notes a particularly strong roof and traction control, which may mitigate a tendency to roll over.

Chris

 

[GregLocock]

While I appreciate your enthusiasm, 4 channel ABS is a bit of a yawn feature in every first world market except the 'home of the automobile'. Modern ESC systems (which are rapidly becoming compulsory) wouldn't work very well with a 3 channel unit.

As to CG heights the only web source I know is the NHTSA one.





Cheers

Greg Locock


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[NormPeterson]

Somebody in your Region or your area in general ought to have a set of individual wheel scales for such purposes as cornerweighting or checking left side weight, which would be far more accurate than devices intended for vehicles weighing 20 or more times what your Juke does.

I think I paid $7 for a single weighing of a car on truck scales once, and got separate axle weights.


Norm

 

[MiketheEngineer]

Roll it on its side and weigh it!! Too extreme?? Call the MFG!!

Actually you could pull it from the side until the wheels came off one side. With a little math, this might get you close enough.

 

[MCoupeAtlanta]

Mike. . . I know that you're structural, but. . . That's a bit too structural for me! I have advertized in the local region for someone with scales. But here in small-town West Virginia am coming up with nada.

Now $7/weigh for three weighs (front tire, rear, front with rears up on ramps or some such) is in the neighborhood. But of course 20-30 pounds off might mean a bogus test, I agree. . . But then of course there's a 50% chance of an error in my favor!

I have emailed Nissan but expect no response.

This debate is spreading through both the Juke and autocrossing communities.

Chris

 

[CRG]

Contact an A&P mechanic in the area and he will tell you who is doing weight & balance work/verification. Not exactly a low budget solution, but it should be accurate for weight.

 

[dgallup]

Get some of those ridiculous wheel spacers that were so popular a few years back and increase your track 4 to 6 inches. (Just kidding)

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[MiketheEngineer]

Weights on the tires will givew the CG in "pitch" direction but you want the CG in the "roll" direction.

I think tipping it would give the best solution.

 

[ivymike]

maybe you can find an overhead crane to suspend it from, and measure the pendulum frequency? that and the vehicle weight might get you a reasonable estimate of the CG location

 

[NormPeterson]

Weights on the tires will givew the CG in "pitch" direction but you want the CG in the "roll" direction.

I think tipping it would give the best solution.

Either way, you do have to "tip" it, and if the car was set up to behave in true accordance with rigid body theory it shouldn't make any difference if the tipping were to be in the pitch or the roll direction.

In a tested-as-is scenario, suspensions have ride and roll stiffnesses and tires have width and are non-rigid, so their effects will vary depending on which way you choose to tip the car.


Norm

 

[patprimmer]

Would it be more accurate if you fitted rims with no tyres for the tip tests.

Regards
Pat
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[MiketheEngineer]

Don't think so - since the tires raise the height of the vehicle a few inches. Not much - but different.

 

[NormPeterson]

I suspect that "correcting" for the tire height would be simpler than dealing with the possibility that the load could shift from one wheel flange to the other as you tip - especially if you were to tip it in roll - with neither the inner nor the outer flanges really corresponding to the tire contact patch locations. On the other hand, I suspect that there could be a "grey-area" method of "gaming" the acceptance criteria to one's event-entry-acceptance advantage hiding somewhere within this . . .


Norm

 

[patprimmer]

If your prepared to trash a few wheels, the solution to multiple contact points is pretty easy to solve with a gas axe.

Norm, I think your correct re the reason for the OP. Hardly work related engineering I think.

Regards
Pat
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[MCoupeAtlanta]

OK, a bit more info, and the method I will use. I have to measure the weight of the two axles separately, then again separately (though am only interested in the "down" axle) with one axle's tires lifted at least 10 inches. I stopped at a truck stop that had CAT scales yesterday coming back from my parents' for Christmas. They informed me in no uncertain terms that the scales would not produce any weight numbers unless I had at least two of the three pads loaded. I argued nicely for a bit and gave up in frustration. "But I just want to weigh by axle. Two tires, that's it!"

Yes, yes, I'm in the future too, now. :-D

So, a call to CAT Scales just now and a VERY helpful gentleman gently asked if my weights could be taken with the front wheels on one pad and the rear wheels on the other. :~/

Sadly, my ramps only go up six inches, but I can use a floor jack to lift the rear of the car and slip something 10 inches tall under the wheels, and lower the car onto that. Since I will be in no way no how UNDER the car, I'm actually considering a couple of cinder blocks. . . But am not sure that I can lift 16 inches (must be more than 10) plus wheel droop, even with a spacer between the jack pad and the car lift point. Though perhaps with the two tires-to-be-lifted starting on the ramps. . . But that's a solvable problem! (with the tires-to-be lifted almost certainly the fronts, since the parking brake locks those, and I'm not sure how my CVT transmission "parks.")

Oh, and $9.50 for the first weigh, and since it will be within 24 hours, second weigh for just $1.00! This coming weekend!

Chris

 

[evelrod]

In West Virginia it shouldn't be too difficult to get a couple cheap bathroom scales from Wall*TargetMart or such. The ones that go to 440 lbs. A couple two by fours and a couple properly sized blocks and the use of that engineering degree for something practical. Worked for me about 40 years ago before I had any common sense...Surprisingly, most of those 'shade tree' solutions I'm so well noted for are quite accurate and simple to do at home with simple tools. I'm was so good at it that I could do almost anything with nearly nothing........well, sorta! Happy New Year...;o)

Rod

 

[berkshire]

MCoupeAtlanta
From your comments you are in west virginia. Here are some of the airports: Raleigh County Memorial Airport at Beckley, the Yeager Airport at Charleston, the Harrison-Marion Regional Airport at Clarksburg, the Tri-State Airport at Huntington, the Greenbrier Valley Airport at Lewisburg, the Morgantown Municipal Airport and the Mid-Ohio Valley Regional Airport at Parkersburg.
One of these has to be close to you. Go out and ask the aircraft maintenance guys, what they would charge you to do a weight and balance on your car. Most aircraft scales will do about 1500 lbs per wheel, some are bigger.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[pwildfire]

If you have an accelerometer that is relatively accurate, even an iphone is probably good enough, you can do a 'skidpad' in a parking lot with a specific radius, measure the acceleration and the body roll, and calculate the CG.

FWIW if you are in Morgantown, I can probably find someone local who has corner weighing scales for you. I have been meaning to build myself a set, but time is tight for me lately.

 

[pontiacjack]

Umm... wouldn't you also need to know a value for roll rate? (polar modulus of elasticity)

 

[AeroFC]

take it to a shop that has the ability to corner balance the car. theyll be able to tell you the individual weight of each corner. Curious as to why you would choose to auto cross a juke though.