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Center of Gravity Height Again http://motorsportsvillage.com/forum/viewtopic.php?f=8&t=5782 |
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Author: | rick360 [ Sun Apr 19, 2009 10:29 am ] |

Post subject: | Re: Center of Gravity Again |

I did some comparison calculating between my spreadsheet and Billy Shope's online calculator. When the front and rear tire diameters are equal and you calculate the slope correctly, Shope's and mine have the same outcome. The problem is the slope is NOT what most might think. Most would think the slope is the amount you lift one end divided by the wheelbase. or Lift/Wheelbase. While this is almost true it still creates a small error when calculated this way. The problem is the horizontal distance between the wheels changes as you lift one end. Example: A 110" wb car with same size tires front-rear will be 108.5" horizontal distance when lifting 18". So the slope isn't 18/110 = 0.1636, but rather 18/108.5".1659. This error increases as you lift it more if you don't calculate the slope correctly for his calculator. For even tire sizes you can calculate slope for the Billy Shope calc by using this formula ... slope=lift/sqrt(wb^2 - lift^2). This is the basis for the increased error when front-rear tire size is different since you begin with SLOPE between front and rear wheel centers and which way you lift it affects what the lifted wheelbase becomes. I don't see any way with the Billy Shope calculator to compensate for different tire sizes. When you make one calc error, the other decisions and calculations that build on that will lead you down the wrong path. Comments? Rick |

Author: | rick360 [ Fri Apr 24, 2009 10:44 pm ] |

Post subject: | Re: Center of Gravity Height Again |

Well, it seems most don't care about this. One request for the spreadsheet ... oh well, I made it so I'd calculate it right, others can do as they wish. I won't bother posting any drawings. Maybe there's not enough people with scales for this to matter. hmmm Rick |

Author: | BillyShope [ Sat Apr 25, 2009 8:20 am ] |

Post subject: | Re: Center of Gravity Again |

rick360 wrote: I did some comparison calculating between my spreadsheet and Billy Shope's online calculator. When the front and rear tire diameters are equal and you calculate the slope correctly, Shope's and mine have the same outcome. The problem is the slope is NOT what most might think. Most would think the slope is the amount you lift one end divided by the wheelbase. or Lift/Wheelbase. While this is almost true it still creates a small error when calculated this way. The problem is the horizontal distance between the wheels changes as you lift one end. Example: A 110" wb car with same size tires front-rear will be 108.5" horizontal distance when lifting 18". So the slope isn't 18/110 = 0.1636, but rather 18/108.5".1659. This error increases as you lift it more if you don't calculate the slope correctly for his calculator. For even tire sizes you can calculate slope for the Billy Shope calc by using this formula ... slope=lift/sqrt(wb^2 - lift^2). This is the basis for the increased error when front-rear tire size is different since you begin with SLOPE between front and rear wheel centers and which way you lift it affects what the lifted wheelbase becomes. I don't see any way with the Billy Shope calculator to compensate for different tire sizes. When you make one calc error, the other decisions and calculations that build on that will lead you down the wrong path. Comments? Rick, I didn't look at this until AFTER I sent you the PM. This matter of the slope difference cannot be the cause of differences in calculated results because I do not calculate the tangent in my spreadsheet. The user inputs the tangent, so, if there is a tangent error, it must be the user's fault. Again, I hope that my PM adequately explained why only the front tire radius is significant. http://home.earthlink.net/~whshope over 140,000 page views Rick[/quote] |

Author: | BillyShope [ Sat Apr 25, 2009 10:21 am ] |

Post subject: | Re: Center of Gravity Height Again |

Rick, you're absolutely correct when you say that the answer changes when the front and rear tire sizes are not the same. As I said in my PM, I didn't even recall including that spreadsheet at my site and, when I looked at it a bit closer, I realize I must have assumed equal size front and rear tires. Check out my spreadsheet again, for I've made the change. I made it very quickly, so I hope I got the plus and minus in the right place. http://home.earthlink.net/~whshope over 140,000 page views |

Author: | Elkyman [ Sun Apr 26, 2009 6:49 am ] |

Post subject: | Re: Center of Gravity Height Again |

Billy and Rick, I am reading and understanding what your both saying, but I have another question towards this, isn't it important that the suspension be locked in place with the exact tire size that we would use to race on heigth wise, and, the angle of the chassis at the raised point of distance required to get the most accurate data????? I see that no one has mentioned this, and I think it is important also, am I wrong???? Suspension will change during this raising process giving deflected numbers to calculate with to reach the final info!!!! Thoughts on this part of it??? anyone??? John |

Author: | Elkyman [ Sun Apr 26, 2009 7:15 am ] |

Post subject: | Re: Center of Gravity Height Again |

rick360 wrote: Well, it seems most don't care about this. One request for the spreadsheet ... oh well, I made it so I'd calculate it right, others can do as they wish. I won't bother posting any drawings. Maybe there's not enough people with scales for this to matter. hmmm Rick Rick, I find it hard also to see not as many racers reading and getting involved in doing this measurment as it is very important to fing each C/G so the power and weight balance can be corrected for the best weight transfer and quickest reaction off the line!!!! I know scales are expensive for most racers to justify buying, but if they get more involved as to what is actually going on with a chassis suspension, and can see where there setup may be off somewhat maybe they could get into this area even deeper than what they have. I wish I had scales, and one of these days, I will bite the bullet when the time is right and I will get a set of scales. Many racers can't or should I say, don't want to spend the cash to work with a good chassis shop to find this POINT of interest on there hotrod!!!! The other difficult area when setting up the chassis to do this measurement correctly is, most guys don't want to spend extra money to go find or buy a set of tires & rims of equal heigth and fill with air as to have minimal deflection of the sidewalls, and I understand this, but this step is important also correct???? John |

Author: | rick360 [ Sun Apr 26, 2009 8:22 am ] |

Post subject: | Re: Center of Gravity Height Again |

BillyShope wrote: Rick, you're absolutely correct when you say that the answer changes when the front and rear tire sizes are not the same. As I said in my PM, I didn't even recall including that spreadsheet at my site and, when I looked at it a bit closer, I realize I must have assumed equal size front and rear tires. Check out my spreadsheet again, for I've made the change. I made it very quickly, so I hope I got the plus and minus in the right place. Our numbers are pretty close now until the difference in tire diameter gets large. I'm wondering if we are defining wheelbase the same. My definition of wheelbase is if you drop a straight line to the ground from each wheel ctr and measure the distance between thee points. This means the ctr-ctr distance between the wheels is not the same as the wheelbase if the tires are different sizes. My definition of wheelbase may be wrong after reading wikipedia etc, most definitions indicate distance between wheel centers, not horizontal distance between wheel centers. Someone calculating the slope will need to do this properly to get good numbers from your calc since the horiz distance between wheel ctrs changes as the car is lifted and makes the slope calc more difficult for non-math type people. Rick |

Author: | rick360 [ Sun Apr 26, 2009 8:38 am ] |

Post subject: | Re: Center of Gravity Height Again |

Elkyman, Yes, blocking the shocks to prevent movement would be the best way to do the tilting/weighing as well as stiff tires. Finding exact matching stiff tires will be difficult and may create more potential errors. They should put the wheel center in the original place with normal tire pressure in the normal tire, not the same diameter. These things seem secondary to calculating it properly. Get the formula right and then refine the procedure as needed to get the required accuracy. Tilting my roadster 17" gave a change in weight of ~ 15lbs. I don't think that is enough to have a significant effect on suspension compression/extension. It may make a bigger difference in some cars. I may take some measurements (shock travel) next time I do this on a car to see how much effect tilting rally has on this. My front suspension has almost no travel, so that isn't an issue. I think Shrinker has a good point (from "Suspension Dynamics" thread) about weighing the rear tires and rearend (all unsprung weight) and excluding them from the calcs, since they would have no effect on the suspension and you really want to know the C/G (all 3 dimensions) of the suspended chassis. Possibly the same for the front un-sprung components as well. Rick |

Author: | Bubstr [ Sun Apr 26, 2009 10:54 am ] |

Post subject: | Re: Center of Gravity Height Again |

While I can see that unsprung weight has little to do with compressing springs in weight transfer, it is still part of the mass of the car and should be included to get true center of gravity height. If you didn't include front unsprung weight, when tires clear the ground where did that weight go? If we was just counting weight that transferred, wouldn't we have to eliminate all rear weight behind the rear wheels? This weight don't need to be transferred, it's already there. Seems there is a flaw in this idea. I believe we have to include all of the mass of the car to find true COG height. Now if you wish to discount some of it when calculating suspension loading, fine. If you transferred 100% of a 2,000lb car to the rear, go ahead and deduct the rear unsprung weight to find real suspension loading. |

Author: | BillyShope [ Sun Apr 26, 2009 4:16 pm ] |

Post subject: | Re: Center of Gravity Height Again |

rick360 wrote: I think Shrinker has a good point (from "Suspension Dynamics" thread) about weighing the rear tires and rearend (all unsprung weight) and excluding them from the calcs, since they would have no effect on the suspension and you really want to know the C/G (all 3 dimensions) of the suspended chassis. Possibly the same for the front un-sprung components as well. Rick No, the free body with which you work to determine weight transfer, component locations, etc. includes both sprung and unsprung weights. The unsprung weight at the rear does have an effect on the EXACT location of a given percent antisquat line, but this effect is small and can usually be ignored. I seriously question the value of extended effort to establish an accurate CG height. This is particularly true when considering the discrete holes available for adjustment. I would encourage you to try different CG height estimates in any of the 4link spreadsheets. I've found that a considerable difference in CG height keeps you within the range of a single set of adjustment holes. This is particularly true when you consider the change in front hole positions as the front of the car rises on launch. Rick, I would define the wheelbase exactly as you would. I verified my equation set by using a 14 inch radius tire at the front and a zero radius tire at the rear. That's about as different as you can get. Perhaps I'll play around with it some more with my CAD software. http://home.earthlink.net/~whshope over 140,000 page views |

Author: | BillyShope [ Sun Apr 26, 2009 6:37 pm ] |

Post subject: | Re: Center of Gravity Height Again |

Rick, I just went through a little exercise with the CAD software, using a 108 wheelbase, 3500 pounds 50 inches back from the front tire centerline, 22 inch CG height, and a slope of 0.6 inch per inch. I used the software to accurately measure the horizontal distance between tire patches and horizontal distance back from front tire patch to CG. Using these numbers, I was then able to calculate the scale readings in the tilted position. I tried 3 rear tires sizes: zero inch radius, 14 inch radius, and 28 inch radius. The front tire radius was always 14 inches. In each case, my spreadsheet calculated the CG height to be 22 inches. Rick, I don't make any effort to hide the source codes for any of my spreadsheets, so, if you like, you could view it and see how I calculated the CG height. Don't hesitate to call, email, or PM. http://home.earthlink.net/~whshope over 140,000 page views |

Author: | ky mustang [ Wed Sep 30, 2009 7:20 pm ] |

Post subject: | Re: Center of Gravity Height Again |

I have been thinking about this a couple of days since talking to Billy on the phone. Ok If I understand right and I admit when I put my cog into Billy's a/s spreadsheet it was the cam height , not calculated correctly like you guys are discussing. My car done a huge wheel stand at 73% a/s with the cog at the cam height , I then moved it to 51% a/s (cog at cam height) and it calmed it down with very little suspension movement . I guess I have to look at the fact that the a/s setting might be closer to the real cog height than my guess I figured with. I will calculate it correctly when I get time just to verify the results to my self . Could this be where all the disagreements come from with chassis tuners they just happen to get real close to the true 100% a/s line without knowing ? I can now see (I think) how there has got to be a neutral line in every chassis and the suspension acts on this balance of weight. I do see how extreme power could still cause a wheel stand problem at 100% a/s and as Billy and I discussed on the phone its just a matter of the chassis being improperly balanced when it was built. Am I thinking correctly ? |

Author: | Bubstr [ Wed Sep 30, 2009 11:31 pm ] |

Post subject: | Re: Center of Gravity Height Again |

You are thinking correctly. Take any suspension out of it completely and you always end up with a basic formula for a good working race car. There are three parts to this. Power, Traction and center of gravity placement. Traction is best at a little slip above dead hook. Then higher powered cars like lower and forward COG. Lower powered cars like higher and rearward COGs. This is not usually achieved on your first build. It's something that you learn and juggle as you go. If you have the COG right for your car, your suspension has very little to do. This is the secret why some cars work and some don't. |

Author: | BillyShope [ Thu Oct 01, 2009 6:41 am ] |

Post subject: | Re: Center of Gravity Height Again |

Bubstr wrote: This is not usually achieved on your first build. But, it should be! Or, at least, you should know, before heading for the strip, that your car is...or is not...balanced for drag racing. This requires, of course, that you first scale your car. Then, using weights that include the driver, divide the product of front end weight and wheelbase by the product of total weight and center of gravity height. This number should be at least equal to 2 if you don't expect to be running better than a 1.4 sixty and should be 3 or larger if you plan on being around a 1.1 sixty. The "2" and "3" reflect the tire performance, with a good drag slick being able to pull 3 "g"s and get you those 1.1 sixties (average of 3.08 "g"s). If your calculator gives you smaller numbers, you're heading for wheelstand problems. http://www.racetec.cc/shope |

Author: | Beretta [ Thu Oct 01, 2009 3:54 pm ] |

Post subject: | Re: Center of Gravity Height Again |

I have been reading this all along and I just need some pictures along with it to help me see what you guys are talking about "lifting up the back" ....That spread sheet is not showing me anything...... Oh and I have a set of scales..... |

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