camber gain and fvsa lenght

[susuprbr]

Hi, I am trying to justify have a very long fsva lenght and very small camber gain. The RCH in the front is -1in below ground and +1in in the rear. The static camber is set to -3 degrees. Out fvsa lenght is 875in which translates to a negligible camber grain during compression. (0.0655degree/inch). assuming this geometry can't change how will this affect the cornering performance and is this acceptable?
Thank for the posts.

 

[GregLocock]

Depends on cgz, roll gradient, tire width, competition type and a whole bunch else.



Cheers

Greg Locock


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[patprimmer]

Tyre and chassis compliance spring to mind as variables to be considered.

Front to rear weight distribution and required underseer/oversteer characteristics and transitions also might be a consideration

Regards
Pat
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[cibachrome]

I'd be a bit more concerned with the low roll centers/axis. Were you planning to use oil pipeline stock for anti-roll bars, go with solid 'springs', lower the cg to 10 inches or just let it roll 8 deg/g ?

 

[susuprbr]

~12.5 in cg, 45/55 distribution, 7.5"
1.5 degress/g which is stiff because we don't have a anti-roll bar. 175 lb/in springs in the front 200 lb/in in the rear. formula sae competion. The -1in front an +1in rear induces understeer characteristics, reduce lateral load transfer during cornering and increase traction during cornering.

From what I have been able to research is that body roll normally results in positive camber gain on the outside and negative camber gain on the inside. The scrub radius therfore increases on the inside and decreases in the outside.
Camber gain due to fvsa lenght will gesult in greater negative camber. Out geometry has very low camber gain therfore it would seem with both combined that we would likely see positive camber gain on the outside and negative on the inside.
From what I understand is that due to our still roll rates our minimal camber change is acceptable. A anti-roll bar would have been nice to couple the chassis roll rates with the camber change but cannnot be done now.

That sound right?

Cheers

 

[GregLocock]

OK, I think you know enough about it to use the Michelin FSAE tire lateral load characteristics to answer your own question.



Cheers

Greg Locock


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[susuprbr]

yes i've been wanting to use that tire data but i never quite understood how to use it nor had the time.

 

[cibachrome]

I have posted a few Matlab applications in the FSAE/TTC forum website on how to treat the FSAE tiredata and how use it effectively for choosing brands(s), sizes, pressure(s) and rim widths. Considering that this is a heavily rearward weight biased car, your minor roll axis inclination isn't gonna be enough unless you use two different tire sizes on it.

You (as a student) will have to 'join' = 'buy' your way into the TTC, though, to get the data. 'Old School' denies its helpful. They just change parts and drive around until they run out of time or Red Bull. 'New School' gets the car PFC right off the hauler.

'New School' will get job offers, too, (in the "field", not in a field).

 

[GregLocock]

Michelin have published their FSAE data

http://www.michelinman.com/motorsports/

at the bottom



Cheers

Greg Locock


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[susuprbr]

Yes I do have the tire data for our tires. Where can i find this matlab code?

 

[cibachrome]

Sherman, set the WayBack machine. Mr. Bib's data (Round #1) is 3 years old, no MX or Mz, or Fx. And, the raw data has been laundered thru a MIC proprietary Pacejka-like model to produce the .xls deal. BTW: these tires don't conform very well to a TNO representation.

To get test Round #4 data for MIC, HOO, GDY, etc, you will need to buy it. Then you have to fit it to some kind of model (MF5.2, MRA, WAC-Spline, LapSim, HomeBrew, etc.), then run a vehicle+tire model in a tire and suspension evanuation procedure in a DOE fashion. I believe MRA just loaded their model fitting coefficients in the past few weeks. All the maths prototypes (no lisping intended) needed to produce the optimum tire and chassis parametrics from an ISO Test procedure format have been fondly seeded into the TTC forum threads. Students just need to add some bits to increase the model fidelity.

 

[susuprbr]

Without using tire data (I just don't have time to learn or look at it) is there any way of knowing that your outside tire will not reach positive camber with respect to the chassis under a certain acceleration? Our roll centers are actually -1.31 front and +0.32" in the back.
thanks

 

[GregLocock]

Sure, if you know the roll gain of the car, the static camber settings, and the FVIC (if that is how you want to think about it). You know the last, what about the other two?







Cheers

Greg Locock


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[susuprbr]

Does your camber gain due to roll not depend on your suspension geometry? I have tried to draw a model of1 degrees of body roll, but it's giving me weird answers. ( my tires are off the ground and the cambers are wrong but I'm seeing around 5 degrees per degree of roll ). I also drew up a model of the tire reaching 0 camber and determining the roll needed. I found 8.65727deg which does not seem right with me. I also found these two equations;
fvsa = (t/2) (1- rollcamber)
therefore
Roll camber =1-((2*fvsa)/t)
But that does not seem right as we would get a very large number
Roll camber = Wheel camber angle/chassis angle
But that does not seem right as if we increase the chassis roll angle we would expect a large camber change.

So as I expected have to do with the geometry. Our upper control arm ( if i'm using the right term) is 11.822in and our lower control arm is 15.549.

A quick post would be greatly appreciated, as I really need an answer by tomorrow morning.

thanks

 

[GregLocock]

The FVIC is the instantaneous centre of the suspension, in roll. By limiting the discussion to ICs and FVSA lengths we are assuming it is a swing arm, or equivalent, which is OK for small deflections.

Roughly speaking you have an infinite SVSA length, and the IC is more or less at ground level. therefore in a 1g turn when your car will roll by X degrees, dependent on CGZ and roll stiffness, that will then camber each tire by X as well.

Slight mods to the above due to the realities of your suspension will not change the fundamentals.







Cheers

Greg Locock


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