USAC Racing Forum

General Chat => Point .25 => Topic started by: racemom2000 on July 15, 2009, 09:26:06 PM

Title: CGH Formula?
Post by: racemom2000 on July 15, 2009, 09:26:06 PM
Okay, so I'm trying to calculate weight transfer for determining our baseline handling:

Transfer = WxCGHxG divided by TW

Lateral Weight Transfer = Car's Weight x Center of Gravity Height x Lateral Force Coefficient, divided by Track Width.

But how the heck do I figure out Center of Gravity Height & the Lateral Force Coefficient for our .25? I've searched online, but can't find formulas pertaining to quarter midgets.  :P Can someone explain how to figure this out? Heaps & truck loads of thanks......

Title: Re: CGH Formula?
Post by: Swartz on July 16, 2009, 09:04:22 AM
Finding the CG for all cars is the same no matter what the size. This might help:

BUT, unless you are building a car from scratch, you are limited to the adjustments provided by the construction of the car and what the rules allow. The box is kind of small.

Title: Re: CGH Formula?
Post by: racemom2000 on July 16, 2009, 09:38:12 AM
You're brilliant - thanks!  ;D

Title: Re: CGH Formula?
Post by: BQwkR on July 16, 2009, 09:44:15 AM
The Williams ( I forget his first name)book does a great job in describing the center of gravity height issue, roll rate, weight distribution, etc. This book sells for about $25.00 to $35.00. Google >Williams>quarter midget. (not .25--this is a USAC designation). The big book stores say the carry it or they can get it, but I had to buy it on line from a vendors website. It's a good book--easy understandable reading.

Title: Re: CGH Formula?
Post by: BQwkR on July 16, 2009, 10:00:59 AM
I'm sorry--The Steve Smith book.

Title: Re: CGH Formula?
Post by: racemom2000 on July 16, 2009, 10:40:36 AM
I'm reading the Quarter Midget Chassis Technology book, but got hung up on p7 because Smith doesn't explain how to find the CGH for the Lateral Weight Transfer formula. I printed off the pages that Swartz directed me to & that helped me understand.  :-X

Title: Re: CGH Formula?
Post by: dmmc on July 16, 2009, 01:00:34 PM
To find cgh, scale the car and driver like you normally would.  Record the total weight and the front weight.  Then, elevate the rear scales 1-2 feet and re-scale the car.  Record the new front wheel weight, wheelbase and the angle you tilted the car.  (this should be done with the driver in the seat and with as little tire/spring deflection as possible--more psi, stiffer springs-)  The formula for cgh from axle centerline is (wheelbase x cotangent of the tilted angle x the difference in weight on the front scales) divided by total weight.  Add that to the tire radius to get cgh from ground.  Example:  wb 48",  total weight 265#,  rear scales elevated 12",  front weight gained 15#,  tire radius (avg) 10"

48" x cot of 14.5 deg x 15# / 265# = 3.5" + 5" tire rad. = 8.5" cgh 

We tried this on a hawk a few months ago and those nubers are similar to our results.

Lateral g's can be calculated using the formula 1.22 X track radius / lap time squared.

Example:  1/20th mile circle has circ. of 274' and rad. of approx. 44'. 
1.22 x 44 / 6 second lap squared = 1.49 g
If you assume that the corners are a little tighter that puts the g numbers in the 2-3 range. 

Assuming no suspension travel, and calculating weight transfer as w x cgh x g / tw,  our qm's start to show the belly pan at about 2.75 g's ( that's why my 95lb light 160 class driver leans to the left to everyone's dismay )

Title: Re: CGH Formula?
Post by: Doug Adams on July 16, 2009, 09:34:44 PM
How about factoring in the difference of 5" wheels on the left, and 6 inch wheels on the right.  Furthermore, the radius of all 4 tires tend to be different on QMs.

Also, some cars may run axle lead which impacts wheelbase calculations.

You must also find the horizontal COG so you can determine the true X, Y and Z coordinate of the VCOG.

After you find it, then the fun begins trying to find a way to move it.

Title: Re: CGH Formula?
Post by: racemom2000 on July 17, 2009, 11:54:33 AM
dmmc & doug,
Thank you very much for your insight. We have an NC Grizz. I'm going to try to figure out the CGH at our next track time. ;)

Title: Re: CGH Formula?
Post by: sfreitas20 on July 17, 2009, 03:00:08 PM
This is some great information, thanks for posting the formulas!

My question for those that have setup QMs for far longer than me is; Is it worth the time and effort it will take to find this information, or do most people just start wrenching on the car and trying different things and see how they impact lap times?  Either way, I see myself going through the exercise on our car just so I know how to figure it all out later on, but I am curious to hear what some of the more experienced handlers might say.

Title: Re: CGH Formula?
Post by: RBurns17 on July 17, 2009, 03:13:36 PM
I think it just depends on how much test time you can get. Being club owned, we can test anytime we want, so we have pretty much figured out a set-up for every variable that there is. To me it always felt like plugging the numbers to get a set up never worked. But then for some people, it's the best way to go and always helps them perform. I think it's just a preference thing. If you have the time, it's worth a shot to read up and figure it out and see if it benefits you.

Title: Re: CGH Formula?
Post by: racemom2000 on July 18, 2009, 10:11:58 AM
I can only speak for myself, but since I'm coming from the point of view of not knowing "anything" about QM's, much less tools and what to use them for - to me, I feel it's important to run all the formulas, so I can understand the make-up of the car & what makes it tick.

People who are more experienced can probably get away with not running all the setup variables 'cause they probably already have a basic idea (in their head) what will happen when they tinker with X or adjust Y.

But for me, it's important to know all the baseline setup variables up front. That way, when I'm a bit more experienced, I'll know how to adjust those variables to acclimate to different track conditions. Especially when Al is working & can't attend a practice or a race. I'd hate for Noah to miss out on those races or practices because I didn't know the basics. But that's just my personal situation. Others may see it differently, & not feel a need to run all the formulas.....