Goblin Center of Gravity

[Desert Sasqwatch]

I'm not going to let this thread die. Any chance this was tried again and just not posted? I'm sure there may be some builders interested besides myself, but this is really critical to the completion of the new suspension I'm working on to get the anti-squat and roll center set up properly. Thanks!

 

[Desert Sasqwatch]

Anything? Buehler? Buehler?

 

[Desert Sasqwatch]

Anyone else on the forum have the corner scales that can try this? Again this is important information needed from a completed Goblin and measuring it would be doing me very big favor. Thank you.

 

[Ross]

Not sure how @Adam & @Lonny feels about this suggestion, but...
How about bringing DF's corner scales to GoblinFest, and we can weigh the cars that attend?

 

[Rttoys]

That would be awesome

 

[Desert Sasqwatch]

Keeping this thread alive. Guessing the measurements proposed for Goblinfest didn't happen?

Some conversation on this topic has also surfaced in another thread - Aerodynamics - and covers much of what has already been discussed here.

Still need the CG to finalize the mounting points for the new rear suspension I'm building, please.

 

[baustin]

One idea to add in case someone has the tools: you don't need to have a professional level corner scale to get the front weights. Without driver, my Goblin is in the ~300lb per front tire of weight, a bathroom scale can work for this range as long as it's not the cheaper type with a max of 250-300lb. One front wheel can be on a block of wood to bring it up to level with the side that's on the bathroom style scale. In fact.... my bathroom scale might have a high enough range to check this out.

 

[baustin]

Disclaimer: these numbers might be accurate enough for DS but they seem to have a little error which is expected with a bathroom grade scale vs accuracy or race corner scales. If anyone has race scales that have enough weight range to weigh the rear tires, the side to side tip is more significant weight shift and will yield a more accurate CG height.

I lifted up the rear of my Goblin and set it down on ramps/blocks of wood to get close to the 10" lift target. At the rear I measured from ground up to the bottom of my center hub: 20.875". At the front I measured the scale @ 1" tall and from the scale to the center hub: 10.875". Resulting weights are based on 9" lift at the rear axle vs the front.

All 4 wheels on the ground
Driver front: 295.2 lb
Passenger front: 297.8 lb

Rear wheels lifted 9"
Driver front: 304.2 lb
Passenger front: 302 lb

These weights are with repeated lift/lower onto the scale to confirm it's as accurate as the bathroom scale will be. It seems odd that the driver front was lighter when the car is flat but slightly heavier when the rear is lifted. Either that's due to the accuracy range of the scale I used or it's due to fluid movement like the gasoline in the tank. My tank is likely around 6 gallons full. The ramps on blocks aren't 100% exact height since my tires are wider than the ramp flat surface and the tires are sitting on the edge of the ramps, maybe the driver rear settled into the ramp and the passenger rear stayed on the edges?

 

[Ross]

Adam brought DF's scales, but they didn't get there until late in the event.... I never got to them.
I was looking at bariatric and postal scales. Found this one which could weigh the rear wheels, and my bathroom scale could weigh the front wheels. Would have to make shims for the other 2 wheels to keep the car level.

 

[Dale E]

I found a friend with scales and we are trying to get a time for James to get to his house so we can get his Goblin weighted! Stay tuned.. It will be generic, since not all Goblins will weight the same.

 

[Desert Sasqwatch]

Thanks a bunch!
Plugged in the numbers from @baustin for the front weights and the rear weights from @Chubbs (all of which are within 1%) to get the following:

https://robrobinette.com/cg_height_calc.htm

This calculation indicated 21.3 inches above ground level.

So from this information the CG appears to be between 1-1.5 inches above the fuel tank and 2-2.5 inches behind the b-pillar roll bar loop - with a 5.5 inch ground clearance to the bottom of the frame.

 

[baustin]

I think the CG height from that website needs a correction put in due to how I measured. I measured to an easy to read edge, at the bottom of my wheel hub covers. The wheel hub cover is 2.5" diameter, so any center of axle values need to include +1.25". I listed my total heights as relative to the ground but while on the scale, my 'ground' was 1" above the concrete.

Height from ground to center of axle: 10.875 + 1.25 = 12.125"
Center of wheel to ground when raised: 20.875 -1 (scale lift from concrete) + 1.25 = 21.125"

This still leaves a lift of 9" like I mentioned previously but the final CG height shouldn't be up at 21.3". The accuracy of the bathroom scale I used really messes with the results and could be related to my garage not being purely smooth and the 4 contact points on my scale getting an odd split of the weight.

I get 20.5" if I use my front weights and Chubbs rear weights. When I've measured my entire Goblin previously I was just around 1600-1610 lb I think so his numbers should be close:

However, since something seems odd about the driver being lighter when on the ground but heavier when the rear is lifted, I tested with Chubbs driver front weight of 300 (driver stays slightly heavier in this example):

With that 5 lb measuring difference and since I only could tip my car to 9" lifted on the rear, the resulting CG is 3" different. For most situations that would not matter but I think it might impact your design calculations. Of course a driver will impact things as well but you can easily add them into the calculations.

Without measuring I'd estimate the vertical CG point to be about the mid rail of the frame where the roll bar connects. This would also make sense from the original DF design work for strength on the extended/track frame. 17-18" on my goblin is exactly this height, 20+ is above and would negatively impact the frame strength if thinking about absolute max stress allowed before failure.

I'm sure that most of yall know this but just keep in mind that the vertical value for CG of 17.5-20.5" from my Goblin is based on 225/45R17 tires making the axle heights be 12.125" above the ground. I have my lower front control arms at parallel to the ground and my CV axles parallel to the ground. Any ride height or wheel/tire changes will carry through to the CG height changing.

 

[Desert Sasqwatch]

I know that if I'm sitting in the seat, the CG gets pulled down significantly! (squatch butt!)

@baustin thanks for correction to the height measurements. I was going off the raw values and didn't realize there was an extra inch in there I didn't correct for. This moves the CG basically to the top of the gas tank - which is better, but not a significant change. Suspension calculations will now be more accurate!

Thanks again for all your help!

 

[Davidljones88]

One idea to add in case someone has the tools: you don't need to have a professional level corner scale to get the front weights. Without driver, my Goblin is in the ~300lb per front tire of weight, a bathroom scale can work for this range as long as it's not the cheaper type with a max of 250-300lb. One front wheel can be on a block of wood to bring it up to level with the side that's on the bathroom style scale. In fact.... my bathroom scale might have a high enough range to check this out.

Some support 500lb

 

[VoodooMatt]

I have a shop that has corner scales and lifts that you can use for measurements if still needed. I have a goblin on the way but it will be another 6 months probably before I am able to complete it to get the weights and measurements on my own car. If there is anyone with a goblin in southern California that wants to come by, we can probably get whatever measurements you all need and get it with various driver weights as well. Just let me know if that would be helpful and what exactly you need and I can assist in any way I can.

 

[Desert Sasqwatch]

Thanks! Any additional measurements on other Goblins would add data points into the information set that has been shared. This data is helpful to anyone setting their Goblin up to go racing or for the serious street driven car. Appreciate your help with this!