From: hotrod@dixie.com (The Hotrod List)
Subject: Re: Watts Link
Date: Sat, 09 Jan 93 05:55:17 GMT
Organization: Dixie Communications Public Access.  The Mouth of the South.
To: hotrod@dixie.com
Reply-To: hotrod@dixie.com
Posted-Date: Saturday, Jan 09 00:54:52
X-Sequence: 3323
X-Gifs-To: met@sunset.cse.nau.edu
X-Gifs-From: ftp.nau.edu

On Jan 8, Bob Wise wrote:

> Conclusions:
> 
> 1) The bell will be mounted to the frame, the tie rods to the ends
>    of the axle.
> 
> 2) The center of the pivot will be centered on the differential,
>    and the tie rods will be parallel to the ground and to the
>    axle as viewed from behind the car with the car static.

Cool. This should give you the best results.

> Further questions:
> 
> 1) It seems that placing the pivot _exactly_ at the diff center
>    point will be easy left-to-right but difficult top-to-bottom,
>    due to weight.  It needs to be centered with the driver in,
>    fuel in, etc.  Is building it to be adjustable top-to-bottom
>    worthwhile?  Where are the significant loads on the bell's
>    pivot bearing?  Top-to-bottom or side-to-side?  Intuition says
>    that the major loads are side-to-side, so not much strength
>    will be sacrificed by a top-to-bottom adjustability.

I think that you may be overconstaining yourself here. You want the
pivot to be on the longitudinal centerline of the axle, all right- but
is it not necessary, and may not be desireable, for it to be _exactly_
at the vertical location of the axle centerline. The point is that the
pivot location can move freely in the vertical, allowing you to tune
the suspension- all that constrains it is the ground clearance of the
tie rod that hangs below it by half the bellcrank diameter.

You will want to be able to adjust the vertical location of the
pivot, within some decent granularity- and be able to adjust the tie
rod ends to maintain your parallelism. The rear roll center ought to
be a little bit higher than the front roll center (too much higher,
and you get too much roll understeer), but how much higher depends on
your car, your track, and your driving style. So pick an ideal
guesstimate, and leave yourself +- 1" to play with- that's only 3
holes to drill at each point.

Loads will all be side loads, all right. And may be sizeable, if you
spin and meet Mr. Tirewall- you might want to decide how much load you
want to design for, and how you want to control the failure modes when
it happens- it's a hell of a lot easier to replace a sheared pivot bolt
than to have to reweld the whole pivot truss that ripped loose from
the chassis. Sort of like the wire melting, to protect the fuse...

> 2) Anyone have any rules of thumb for computing the diameter of
>    the bell based on expected axle travel?  What is the acceptable
>    amount of rotational travel for the bell?  45 degrees?  I assume
>    that 90 degrees is too much.  Is it?

That's a hell of a question. I don't have a clue, and I didn't see any
mention of it at all in my references. I have seen people using an
bellcrank as short as 6" overall in a light E-Prod car that works very
well, and I think that the crank in the Roush Trans-Am cars was barely
any longer- but their suspension travel was maybe 3". If anybody has a
good answer on this, I'd honestly love to hear it. I've never seen
this documented, which I find pretty amazing. 90 degrees is clearly
too much, and my gut feel is that 45 is stretching it- but only
testing would really tell you. My gut feel doesn't mean _shit_.

With a 6" bellcrank, you can get your roll center down to 7" and still
have 3"+ of ground clearance. Where do you want to put it, and what
does that leave you with for ground clearance?

-skod

--
Scott Griffith, Sun Microsystems Lumpyware
expatriate SCCA New England Region Flagging/Communications worker
(and sometimes driver, of anything that turns both right and left,
and can pass tech...) Return Path : skod@trenco.Eng.Sun.COM

----------
Posted by: emory!Eng.Sun.COM!Scott.Griffith (Scott Griffith, Sun Microsystems Lumpyware)
Newsgroups: alt.hotrod,wiz.hotrod