Re: Why Sliding block on racers

[Chris.Prael@xxxxxxxxxxx (Chris Prael)]

Will's, Simon's, and Richard's discussion of the properties and
"correctness" of the various ways of locating is very interesting. I
would like to bring up a few points that seems to have been missed.

1. A live axel and a Dedion rear suspension are geometrically
identical. They differ only in which mechanical components are attached
to the chassis and which are not suspended.

2. The transverse location system of a Dedion/live axel has three (not
two) functions:
    a. Setting the rear roll center;
    b. Controlling transverse location of the wheels and the mechanism
       that connects them;
    c. Controlling the roll moment arm (virtual), the distance between
       the roll center and the center of gravity of the chassis.

As Richard pointed out, the inside-out Watts link of the Alfetta
chassis is incorrect.  What makes it incorrect is that the roll center
is fixed relative to the axel, so moves relative to the chassis. This 
results in a variable length roll moment arm. This causes the center of 
gravity to have more leverage when the wheels are drooped and less 
leverage when the wheels are bounced.  (In other words, the geometry 
causes the chassis to roll a lot more when the wheels go down and a lot 
less when the wheels go up.)  This makes the rear cornering behavior less
predictable than is desirable.

Richard made the mistake, however, of claiming that the sliding block
system is geometrically correct.  It is not.  It is just as bad as the
inside-out Watts link in fixing the rollcenter to the axel and not the
chassis.

The Panhard rod is superior to the inside-out Watts and sliding block
linkage, and inferior to the right-way Watts linkage in that it fixes
the roll center half way  between the chassis and the axel.  This makes
the roll moment arm half as variable as it is with the inside-out Watts
and the sliding block linkage.  But it is still variable.

Another linkage which is similar to the right-way Watts linkage, but
allows a lower roll center is the Mumford system. This has been used in
Mallock race cars for some time.  (Mallock's are front engine, live
axel formula race cars that are highly competitive in a number of club
racing series in the UK. Yes, they compete successfully with rear-mid
engined race cars!)

Lastly, we should note what is wrong with the rear suspension geometry
in our beloved 750, 101, 105, and 115 chassis Alfas. (Well, I'm nuts
about them!)  In all of these chassis, the transverse location is done
by an "A" arm above the axel.  (Yes, that cast aluminum do-dad,
officially called a "trunion", in the 105-115 cars is really an "A" arm.) 

The fault with it is that it puts the rear roll center rediculously
high, which makes the roll moment arm much too short.  This makes the
roll characteristics of the rear suspension too stiff, which tends to
induce overstear. (Alfa designed a lot of roll induced camber change
into the front suspension to compensate.) 

Alfa switched from the hard ball joint as the pivot point in the
750/101 cars to the rubber bushed pivot point in the 105/115 cars. This
softened the effect of the high roll center, but does not "correct" the
problem.

Back to the original question. Aside from its geometric imperfections,
the sliding block location system has the fault of hanging greased
surfaces out in the air, under the car.  So, they grab all the gritty
stuff available and grind the locating surfaces.  The result is that
the steel block and the steel locating surfaces have to be replaced
quite frequently.  

On the other hand, If you HAVE to use the system, Alfa Ricambi was
trying to get rid of some sliding block replica packages a year ago.
The good part of the sliding block package it the ball jointed radius
arm and aluminum structure that replaces the trunion arm.

chrisp


> ------------------------------
> 
> Date: Fri, 15 Jan 1999 10:52:14 -0800 (PST)
> From: Will Owen <nashwill@domain.elided>
> Subject: Sliding blocks, etc.
> 
> In v.7, #393, Richard Welty wrote:
> 
> "the Alfetta/GTV-6/Milano watts linkage is actually not a "correct"
> implementation; it works pretty well, but the sliding block is actually
> a better way to implement the function centering the rear axle and
> controlling the location of the roll center in the rear.
> 
> "a "correct" watts linkage in a car would anchor the center link to the
> body, and the end links to the axle, which is the opposite of what Alfa
> did in the production cars. the proper implementation is much harder to
> manufacture and is not particularly space efficient."
> 
> What a sliding block does that that neither a Watts link nor a Panhard
> rod can do is restrict the axle's motion to purely vertical. The
> center point of the Watts link on the Alfetta/Milano axle describes a
> shallow S curve, kinda like the old Studebaker logo; the motion of the
> end of a Panhard rod is obviously a shallow arc. In the real world,
> these slight deviations from purely straight up-and-down are
> inconsequential, and more than compensated for by the high degree of
> free articulation given to the axle (in contrast to the earlier
> a-bracket, which wouldn't work at all without lots of rubber in the
> bushings), and the relative simplicity of the design.
> 
> "as i recall, the factory sliding block requires a lot of maintenence,
> which is why it's not on the street cars."
> 
> It also concentrates a lot of cornering forces into a very small area,
> and a lot of noise and vibration as well. 
> 
> Okay...now that I've gone and pontificated probably well over my head,
> would someone with some direct experience with the sliding-block axle
> like to comment?
> 
> Will Owen
> 
> ------------------------------
> 
> Date: Fri, 15 Jan 1999 10:58:55 -0600
> From: Simon Favre <simon@domain.elided>
> Subject: Re: Why Sliding block on racers
> 
> You're comparing two very different animals here. The watts linkage on
> transaxle cars was used with a deDion tube (no matter how you spell it).
> The GTA and GTAm, all had a classic live rear end. The sliding block
> was a very solid way to locate the rear axle during hard cornering, and
> it lowered the rear roll center. I believe you can achieve basically
> the same result with a properly designed and installed Panhard rod. The
> watts linkage is much more complicated, correct?
> 
> >Can anybody tell me why the GTA, jr,m's all used the sliding block rear
> >suspension as opposed to a watts linkage (like the alfetta)?
> >
> >I'm considering making a GTAm replica, but would rather put in a less 
> >maintainence watts linkage instead of the sliding block.  If there were 
> >a really good reason, then I would consider doing the sliding block...
> 
> ------------------------------
> 
> Date: Fri, 15 Jan 1999 14:05:52 -0800
> From: Richard Welty <rwelty@domain.elided>
> Subject: Re: Why Sliding block on racers
> 
> At 10:58 AM 1/15/99 -0600, Simon Favre wrote:
> >You're comparing two very different animals here. The watts linkage on
> >transaxle cars was used with a deDion tube (no matter how you spell it).
> >The GTA and GTAm, all had a classic live rear end. The sliding block
> >was a very solid way to locate the rear axle during hard cornering, and
> >it lowered the rear roll center. I believe you can achieve basically the
> >same result with a properly designed and installed Panhard rod. The watts
> >linkage is much more complicated, correct?
> 
> the panhard rod, sliding block, and watts linkage are all basically
> trying
> to do a couple of things:
> 
> 1) control lateral motion of the axle tube
> 
> 2) control the location of the roll center.
> 
> live axle vs dedion tube isn't actually all that big an issue; both
> need to
> be located and have the roll center pinned.
> 
> the degree of success varies depending on the method and the implemention.
> panhard rods work "well enough" for detroit, and with stiff racing
> suspensions, they work "well enough" for many applications (NASCAR
> stockers, for example.) they are not "theoretically correct", however.
> 
> the sliding block locates the roll center and limits lateral motion quite
> well, but at a cost in complexity and maintenence.
> 
> the watts linkage effectiveness depends on the implementation. the alfa
> implemention behaves as Will Owens describes. inverting this, so the axle
> is pinned on the ends and the link attaches to the body, produces geometry
> similar to the "ideal" sliding block implementation. neither sliding block
> nor the "ideal" watts linkage are common street car implementations.
> 
> richard
> 
> ------------------------------

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