rec.autos.simulators

    How does one measure chassis rate? Is it the deflection relative to the
origial plane? Does it take into account how the other corners are deflected
relative to the original plane or is it measured, statically, with the other
3 held in their original position?

PS - I initially had a smart arse answer here since I though "WTF would the
units of chassis rigidity be FFS" and thus WTF would ratio mean? :) Then I
saw "pdotson", a familiar name, and thought a little more seriously on the
subject. Glad I did! :)

   Anyway, all I know of this subject is that the word always seems to be
the more rigid the better. I imagine the reason is that the more rigid the
chassis, the more accurate the handling can be adjusted via
springs/shocks/sways? I'd imagine "acceptable" is greatly subjective.

I'm building a new kart chassis.  It's designed already, and I've
welded the basic structure together.  The idea of the design is to
apply the idea of a rigid race car chassis to a racing kart.  The
center section of the kart is designed to be rigid.  All the flexing in
the chassis will be isolated to the front and rear tubing.

So this is new gound I'm breaking here in terms of karting, but I
figured there might be some car chassis design guidelines regarding
chassis rigidity for varyious types of cars.  And if there is, then
someone on RAS would know.  This information is probably in Milliken,
but I don't have access to that book right now.  I could add as much
structure to the center section as necessary, but don't want to go
overboard.

As for measuring chassis rigidity, it's a common thing.  The units
would be force per unit of deflection - like lb/in or kg/mm, same as
wheel rate.  Rigidity is measured like you guessed, pin down three
corners, apply a force to the fourth corner, and measure deflection.  A
car with a stiffer suspension would need a stiffer chassis to properly
distribute roll torque.

I know stiffer is better.  But, stiffer is also heavier, more
expensive, more time consuming, and more restricitive to the design.
If I knew that street cars typically have a chassis rigidity of 1% of
wheel rate, then I'd use that number as a target for my kart chassis.
If the ratio is more like 1:1000, then I'm in trouble :)

BTW, what is FFS?

Thanks,
Pat

    So in the case of this kart of yours, this "centre section" is like the
chassis and the tubing is like suspension. Or is there seperate suspension?

    How do you make adjustments? ie oversteer/understeer?

    For frigs sake! :)

It operates like any other kart, and will use mostly the same
adjustments use on other kart chassis.  Weight percentages mostly as
this one is designed for ovals.  Caster, camber, and front ride height
is also fully adjustable.  This chassis is different in that it has
some provision for adjustment of its overall stiffness.   The roll
couple distribution, which is already radically different compared to
other kart chassis', can also be adjusted.

Pat Dotson
www.kartcalc.com

-Larry

I don't think comparing kart and cars chassis is a good idea. Cars have
suspensions, karts don't.

    To just guess, I'd say if the chassis is already pretty stiff then the
most important thing would be the stiffness of the front tubing compared to
the rear so as to get a neutral handling kart. But I guess that's pretty
obvious.

   I suppose what you're asking is how stiff (thick or long/short) should
the tubing be relative to the chassis?

   I'm betting trial and error for both general stiffness and handling
balance will be required. Even in sims, you can be as theoretical about it
as you like, but simply making adjustments and trying them out is all that
counts ultimately.

   So is this kart a kit? Any instructions/recommendations? How much
lattitude is there to stiffen chassis. What sort of tubing options?

  I imagine that wheel/tyre type is a big part of all this too.

  My idea would be to keep it all as tight as possible up to just below the
point where boucyness = poor grip.

   Sorry for the rambling and lack of a particular answer - I haven't
thought too much on this issue b4. :) I wonder if chassis flex is moddeled
in sims so far?

I would tend to agree with this statement.
A Kart uses a SOLID rear axle... this requires that the chassis flexs enough
to allow the rear inside wheel to lift (leave the ground) so as to allow it
to turn more rapidly, since the outside wheel has to turn more times than
the inside wheel to make a corner.  Without the chassis flexing enough to
let the inside wheel (lift) turn faster....  your turning radius is going to
grow.  It will feel like serious understeer.

My 2 cents

Cheers
==--==

Secondly, there are lots of race cars that use a solid or locked rear
axle that don't rely on lifting/unloading the inside rear tire to turn.
 The chassis that I am building at the moment is for oval racing.  Rear
stagger is used on ovals to allow the solid rear axle to roll around
the corner WITHOUT lifting the inside rear wheel.  Lifting the rear
wheel on an oval is usually a bad thing, and a ticket to the back of
the pack.  When I build a sprint chassis, it will be designed to lift
the inside rear wheel.  The oval chassis won't lift the left rear.

...so I guess no one has the answer :)

Pat

Right, the front/rear relative stiffness is the key to getting balanced
handling.  That's the easy part.  I'll be able to get whatever I build
to handle well by adjusting weight percentages, etc.

If by "is it a kit?", you mean did I buy plans and materials from
someone, then no.  I designed the chassis, bought the tubing, then cut,
notched, and welded it together.

Seems like I once noticed a "chassis rigidity" variable buried in ISI's
F12XXX .ini files.  Although, maybe I just dreamed that...  Maybe there
is, and I can just compare that value to the wheel rate in the INI
file.  Thanks for the idea!  I'll check it out.

    LOL - even though this was not what I meant to suggest, I'll gladly take
the credit for it! :)

http://fsae.com/eve/ubb.x/a/tpc/f/442600868/m/89010136711/r/26610330521

Pat

   "Was this spring I think".

    Had to think about that for a sec! :)