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Poll: CG vs. Wheelbase

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adrive7 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote adrive7 Quote  Post ReplyReply Direct Link To This Post Topic: Poll: CG vs. Wheelbase
    Posted: Dec/13/2008 at 2:45pm
Ok folks,

Higher CG with a shorter wheelbase, or a lower CG (- ~6") with a longer Wheel base(+ ~6")?

Discuss.


Edited by adrive7 - Dec/13/2008 at 5:11pm
-Joe
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Post Options Post Options   Thanks (0) Thanks(0)   Quote thompm1 Quote  Post ReplyReply Direct Link To This Post Posted: Dec/13/2008 at 4:19pm
how about lower CG and same wheel base
Michael

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Post Options Post Options   Thanks (0) Thanks(0)   Quote dillon_b12 Quote  Post ReplyReply Direct Link To This Post Posted: Dec/13/2008 at 4:19pm
Lower CG, longer wheelbase.
 
Wheelbase can become an issue if made overly long because of high-centering on obstacles but I think lowering the CG is a more important concern.
 
The lower you can get that CG the harder the car can be driven in to turns, hopefully generating some oversteer along the way.
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dillon_b12 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dillon_b12 Quote  Post ReplyReply Direct Link To This Post Posted: Dec/13/2008 at 4:21pm
Agreed.  Lower CG FTW always but I don't think lengthening the wheelbase is always a great idea.
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adrive7 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote adrive7 Quote  Post ReplyReply Direct Link To This Post Posted: Dec/13/2008 at 5:09pm
We were leaning towards the lower CG as well. The options are drop the engine to the floor and push the gearbox out the back, or raise the engine and run the gearbox vertically. Option 1 keeps the entire drivetrain nice and low, it just pushes the back end out about 6 inches.

Thanks for the quick replies. We need some more people on this forum. BajaSAE.net >> SAE forums.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote CLReedy21 Quote  Post ReplyReply Direct Link To This Post Posted: Dec/13/2008 at 10:08pm

We run an artificially high CG and a medium to long wheelbase on our cars so that they have a very loose rear end.  However you end up with a car that needs to be driven at 9/10ths to handle properly but if driven at 10/10ths is very roll happy.  We run the swingarm so our cars are different animals from the get go.  Another swingarm team, USF, runs a super low CG and their cars are very slide happy and handle great (thanks for the test drive guys :)

Overall the CG and the wheelbase have a huge effect of the vehicle dynamics, but often can be mitigated with suspension tuning and tire selection.  Our cars usually handle pretty well, but with shocks that are slightly too stiff or tires that are too soft the cars will either not turn enough or will simply flip over after kicking the rear end out.  Whichever design you choose just try not to push either dimension to the extreme and you should be able to tune your car to handle the way you want later as long as your car isn't super long or precariously tall.
-Chris Reedy
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jarmumd Quote  Post ReplyReply Direct Link To This Post Posted: Dec/13/2008 at 11:15pm
Here are some fun facts from my time in baja... In our 2005 baja car, I ( a 6' 5" 250lb driver) created a lower CG than our shorter lighter ( and better) driver.  I'll try to dig up some pictures of the tests that I did.  My theory is that taller heavier drivers have more leg mass than torso mass and that drives the CG down.  We ran a rear steer system that was based on vehicle roll.  So CG was really important.  I think you want a CG height that is comfortable for you to induce oversteer into the car.  If the car is too long, I think you end up with too much polar moment of inertia in the car.  Although I don't agree with the look of either of the TTU or USF cars, i think you can say that there must be a good CG vs wheelbase since Auburn, ETS, old Sherbrooke, RIT and Michigan (kinda short for my taste)...  Seems to be a sweet spot for vehicle design, but I certainly won't deny that other designs work just as well...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mikez Quote  Post ReplyReply Direct Link To This Post Posted: Dec/14/2008 at 2:22am
Originally posted by adrive7 adrive7 wrote:

We need some more people on this forum. BajaSAE.net >> SAE forums.

Agreed.  Glad you guys like it.  We always wanted something better when I was doing Baja, so I decided it was time to go there.  Hopefully we can continue to raise awareness about this site at the 2009 competitions.  I'll continue to support these new national Baja forums as long as people make good use of them.

By the way, there's more cool stuff in the works coming later in 2009.  If there's anything else you guys would like or might find useful, feel free to suggest.

Thanks,
-Mike
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Post Options Post Options   Thanks (0) Thanks(0)   Quote karman1970 Quote  Post ReplyReply Direct Link To This Post Posted: Dec/14/2008 at 5:19am
One of the primary rules of racing is to get the CG as low as possible. The driver makes a big difference in our light-weight cars. The only things that should limit the wheelbase are turning radius and concerns over high-centering. Our first car was around 80". Cut it down to 66, then 60, and even 54 on our still-born #2 car last year. We're back up to 66" this year. The 6 extra inches of wheelbase probably won't hurt, unless it's already pretty long. BTW, we put the engine on top of the trans, usually because of packaging issues. The CG only changes a couple inches.

Edited by karman1970 - Dec/14/2008 at 5:29am
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adrive7 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote adrive7 Quote  Post ReplyReply Direct Link To This Post Posted: Dec/14/2008 at 10:41am
Originally posted by MikeZ MikeZ wrote:

Originally posted by adrive7 adrive7 wrote:

We need some more people on this forum. BajaSAE.net >> SAE forums.

Agreed.  Glad you guys like it.  We always wanted something better when I was doing Baja, so I decided it was time to go there.  Hopefully we can continue to raise awareness about this site at the 2009 competitions.  I'll continue to support these new national Baja forums as long as people make good use of them.

By the way, there's more cool stuff in the works coming later in 2009.  If there's anything else you guys would like or might find useful, feel free to suggest.

Thanks,
-Mike


Might I suggest a sticky thread of features people would like to see?
-Joe
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dillon_b12 Quote  Post ReplyReply Direct Link To This Post Posted: Dec/15/2008 at 12:27am
Originally posted by jarmumd jarmumd wrote:

Here are some fun facts from my time in baja... In our 2005 baja car, I ( a 6' 5" 250lb driver) created a lower CG than our shorter lighter ( and better) driver.  I'll try to dig up some pictures of the tests that I did.  My theory is that taller heavier drivers have more leg mass than torso mass and that drives the CG down.  We ran a rear steer system that was based on vehicle roll.  So CG was really important.  I think you want a CG height that is comfortable for you to induce oversteer into the car.  If the car is too long, I think you end up with too much polar moment of inertia in the car.  Although I don't agree with the look of either of the TTU or USF cars, i think you can say that there must be a good CG vs wheelbase since Auburn, ETS, old Sherbrooke, RIT and Michigan (kinda short for my taste)...  Seems to be a sweet spot for vehicle design, but I certainly won't deny that other designs work just as well...

I agree that cars don't have to be swingarm or overly long to handle well.  The old green Sherbrooke car is still one of the best handling Baja cars I have ever seen.  That car wasn't overly wide or long, handled ridiculously and was a trailing arm car.  

The swingarm cars do have a certain advantage in handling but a properly designed and built independent car can do the same with other desirable suspension characteristics.

I also agree that tire choice is extremely important.  This year we are putting new tires on all of our old cars and testing each set individually on our new car to hopefully find something better than we run now.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mikez Quote  Post ReplyReply Direct Link To This Post Posted: Dec/15/2008 at 2:55am
Originally posted by MikeZ MikeZ wrote:

If there's anything else you guys would like or might find useful, feel free to suggest.

Originally posted by adrive7 adrive7 wrote:

Might I suggest a sticky thread of features people would like to see?

Absolutely.
Suggestions for BajaSAE.net:
http://forums.bajasae.net/forum/forum_posts.asp?TID=26

Thanks.


Edited by MikeZ - Dec/15/2008 at 2:56am
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