Center of Gravity Location

[RMP]

Was wondering where the CoG is located i.e. optimal spot to place an accelerometer?

[Captain Snooze]

*raises hand*
Why does it have to be "optimal spot"? What are you trying to achieve?

[Shankenstein]

Don't know the answer, but it's pretty straightforward experiment, if you've got access to scales.

http://www.longacreracing.com/techni...spx?item=42586

Weight, lift the car, put blocks under 2 scales, lower the car, then weigh again.

If you only need an approximation, you can probably do some redneck engineering... by measuring the wheel gaps (or fender height) instead of weight and get in the ballpark.

@ Captain Snooze - It's "optimal" if you've matched your static roll centers to the CofG. Theoretically, it minimizes roll for the vehicle. It's a nice goal, but rarely achievable.

[DarkSunrise]

Here's a visual representation from the side profile.

http://media.caranddriver.com/files/...fr-s-specs.pdf

[fika84]

Originally Posted by Shankenstein

@ Captain Snooze - It's "optimal" if you've matched your static roll centers to the CofG. Theoretically, it minimizes roll for the vehicle. It's a nice goal, but rarely achievable.

Why would you want to achieve that? Seems to me that you wouldn't be using your suspension to it's full potential if you're just reducing elastic weight transfer and increasing geometric weight transfer. You are minimizing roll, but not minimizing weight transfer, right?

[Shankenstein]

Originally Posted by fika84

Why would you want to achieve that? Seems to me that you wouldn't be using your suspension to it's full potential if you're just reducing elastic weight transfer and increasing geometric weight transfer. You are minimizing roll, but not minimizing weight transfer, right?

I'm probably taking a too simplistic approach to this (not an expert).

My understanding was that you want to maximize the grip available from all tires. Disregarding camber gain, you do this by keeping an even weight distribution. You can do this through sway bars or through the geometry (as you said elastic vs inelastic force management).

This is why double wishbone suspensions are the balls... they send alot of the lateral force through the arms (rather than the spring). That allows the spring to manage bumps and road inputs. Another perk is that wishbone length can be used to control camber gain in near isolation if there's not much KPI.

Roll angle is just the difference ride height between two wheels. Sway bars suck the inside wheel down to keep the car flat. By using inelastic weight transfer, you resolve the forces without involving the spring or resorting to sway bars. If the car's chassis and bushings are stiff enough, there's really not much penalty (as long as you don't mind the car riding like a go-kart).

[fika84]

Originally Posted by Shankenstein

I'm probably taking a too simplistic approach to this (not an expert).

My understanding was that you want to maximize the grip available from all tires. Disregarding camber gain, you do this by keeping an even weight distribution. You can do this through sway bars or through the geometry (as you said elastic vs inelastic force management).

This is why double wishbone suspensions are the balls... they send alot of the lateral force through the arms (rather than the spring). That allows the spring to manage bumps and road inputs. Another perk is that wishbone length can be used to control camber gain in near isolation if there's not much KPI.

Roll angle is just the difference ride height between two wheels. Sway bars suck the inside wheel down to keep the car flat. By using inelastic weight transfer, you resolve the forces without involving the spring or resorting to sway bars. If the car's chassis and bushings are stiff enough, there's really not much penalty (as long as you don't mind the car riding like a go-kart).

I do agree with you. You do want to maximize grip through all 4 tires, but doing that like a go-kart I think is a bit on the simplistic side (as you said). The issue you would run into is having the weight transfer happen to quickly and breaking traction loose. Which is why go-karts are so much fun!! The elastic weight transfer portion allows you to tune the vehicle suspension to handle both small and large radius corners.

[cdrazic93]

Originally Posted by Shankenstein

This is why double wishbone suspensions are the balls...

I like this description

[Shankenstein]

Originally Posted by cdrazic93

I like this description

I mean... they typically have atleast 2:

[RMP]

Originally Posted by Captain Snooze

*raises hand*
Why does it have to be "optimal spot"? What are you trying to achieve?

I'm building a data acquisition system and the accelerometer is able to measure g forces most accurately when placed at or near the COG.

[RMP]

Originally Posted by DarkSunrise

Here's a visual representation from the side profile.

http://media.caranddriver.com/files/...fr-s-specs.pdf

Exactly what I was looking for

[fika84]

Originally Posted by RMP

I'm building a data acquisition system and the accelerometer is able to measure g forces most accurately when placed at or near the COG.

Please tell me you're building this with an Arduino?!?? I started collecting parts to do this... accelerometers as cheap as shit these days!! 6 axis w gyro and temp built in

You should let us know how this progresses!

[cdrazic93]

Ive been in a few tesla P85's with accelerometers on the headrests...

My assumption would be putting the car in the center of mass reference frame so you can find the point to place the sensor.

If i only had the computing power to model this whole car into some fun programs...

[Racecomp Engineering]

Originally Posted by fika84

Please tell me you're building this with an Arduino?!?? I started collecting parts to do this... accelerometers as cheap as shit these days!! 6 axis w gyro and temp built in

You should let us know how this progresses!

Ummm...I was about to post the same thing.

I have an arduino sitting next to me right now. I have all sorts of ideas, not enough time!

- Andy