Mossetti 1/32 Sports Chassis

By Chris Frost

The 1/32 Mossetti sports/GT chassis that a number of people are now using has a number of interesting features. These chassis come in both kit form and ready built from Canadian raceway owner and ISRA Delegate Ernie Mossetti. The chassis has a great many intricate parts, the accompanying diagrams are intended to illustrate how the various parts move, they are not intended as accurate drawings of each part. In fact there is not just one Mossetti design, the design has evolved over several years and looking at various people’s chassis there are a number of differences. The chassis is cut from steel, there is no sheet brass used except on one batch of chassis where there are small pieces forming the back part of each side of the center section ( see the main diagram - the right hand side shows where the brass goes, the left had side shows the all steel version - at the risk of stating the obvious on any one chassis its the same on both sides). Unlike some chassis kits that leave the builder to decide where to put the hinges and stops, most of these elements are built into the Mossetti chassis. This results in a considerable number of intricate parts cut from steel sheet, this intricacy perhaps explains the price. Ernie Mossetti has a relatively small number of sets of steel parts cut at a time, and each time he gets a new batch cut, there is the opportunity to incorporate changes. The most obvious changes between batches are in the motor box area, - the earlier ones had larger boxes to fit the larger motors that were available at that time. This also means the earlier ones had significantly less metal at the rear of the centre section of the chassis, and extra weight in this area seems to aid stability in cornering. The shape of the front of the chassis has also changed.

The best place to start describing a chassis is what connects the motor to the guide. In this chassis there is a central spine connecting the guide to the motor box - unlike a conventional 1/32 sports chassis there is no hinge in the middle. This spine is cut out of steel, and above it is a length of round steel wire (see diagram - Detail A). This wire runs from just in front of the motor to just behind the guide, and is fixed to the main chassis at each end. It is also supported by a pivot block midway along its length - this allows the wire to rotate but not to move either sideways or up/down relative to the spine. As the wire is fixed to the spine at either end the relative rotation will be very small. So what does this bit do? Well as far as I can see, the main purpose of the wire is to increase stiffness in beam deflection (See the diagram showing types of chassis flex) without stiffening it much in twisting or in steering. Could a simpler arrangement produce the same effect? You could certainly produce similar stiffness for these 3 types of flex- would that result in similar handling? Good question, the only way to be sure would be to build one and try it!

The chassis side rails (all part of the same sheet as the central spine and motor box) are cut through just in front of the motor box. Each side rail is connected to the motor box is by wire and brass tube (see diagram - Detail B). The L shaped piece of wire is not soldered to the tube, so it can rotate and slide lengthways in the tube. The way these rails are connected allows some free fore and aft movement, so the motor box can steer relative to the guide. There is also a tiny amount of side to side movement. This fore and aft movement is free up to the point of hitting the stops, the movement is limited by the gap between the channel section piece of brass and the motor box. This gap is adjustable (to some extent) by means of unsoldering the piece of brass and moving it backwards or forward the desired amount. The newly built ones have only microscopic amounts of fore aft movement. Some people have noticed the amount of movement increases as the chassis wears. I’ve not heard any opinions on whether adjusting the amount of movement makes any noticeable difference to the performance - most people didn’t notice this had occurred. Apart from this fore aft movement, the side rails are free to hinge relative to the motor box,. This means that the side rails are contributing to the torsional stiffness of the chassis (in theory they would contribute just over twice as much stiffness in these directions if they were soldered onto the motor box). They also have a damping effect. The side rails cannot be contributing much to the beam stiffness of the chassis - in fact if you press down in the middle of the spine it doesn’t take much force to make in touch the track.

This arrangement is very similar to Ernie Mossetti’s 1/24 chassis. The layout of a fixed central spine with side rails free to move fore and aft is similar to other 1/24 chassis such as the SCD Sniper and the Czech chassis from a couple of years ago (The current Czech chassis are rather different with the guide tag attached to the central spine, and the guide end can flex sideways relative to the rest of the chassis). However the way the side rails are hinged and the extra piece of piano wire above the central spine are unique to the Mossetti designs.

So why does this steering back end work ?- good question! (You could also ask is the sideways guide movement in the latest Czech chassis achieving the same effect - but I don’t propose to try and tackle that question in this article) I’ve heard suggestions that it is to do with the force from the rear tyres turning the chassis about the guide pivot. Perhaps the amount of steering is too small to have any effect, and the flex and damping in other directions is the important bit. Alternatively it may isolate the guide and back wheels so that any slight unevenness in the track at one end doesn’t upset the other end of the car. Then there is the idea that more flex allows the chassis to generate more grip from the tyre, this would normally make the car tend to tip in corners, but this arrangement happens to produce extra stability so you can have the grip without the tip. Well whatever the reason, these cars certainly seem to work.

I wonder if anybody really has enough in depth understanding of why slot cars handle to make changes to the position of stops and how much the various bits need to flex on purely theoretical grounds, or if most of it isn’t a case of developing by trial and error. By a theoretical explanation I mean something rather deeper than "last time we had that handling problem we put a bit of lead there, and it worked so let’s try it again". Its vital to know what to do to correct a handling problem, but unless you know why it works you can only design new chassis by guessing what changes might work, building it and seeing if it goes better or worse than the old design.

The center section of the chassis is all part of the same sheet as the central spine and side rails, but is only joined at the front. The two halves of the centre section are joined at the back by a piece of wire at the rear, this acts as a down stop so that they cannot drop below the level of the central spine. The rear centre section is free to move above up relative to the spine (rather like "plumber" movement in he pans) within the limits the flex of the chassis will allow until it hits the cross bar for the pans. This is quite unlike a conventional 1/32 chassis where the center section is pivoted to the spine (and in some chassis pivot has to be kept a good fit because the handling deteriorates if there is any slop in the pivot). It is also unlike the Mossetti 1/24 chassis where the corresponding part of the chassis is cut out. In the Mossetti 1/32 chassis lightening this center section does not appear to be a good idea, the weight in this area seems to improve stability.

Another interesting feature is the skids. Most chassis run with skids on the front corners of the chassis. Some people use bits of shim soldered on ( and occasionally these peel off), others cut the chassis plate and let in pieces of piano wire. The Mossetti solution is a round hole in the chassis with a small (about 1.6mm diameter) ball bearing soldered in. (see diagram - Detail C) This is adjustable by melting the solder and setting the height of the ball to the required ground clearance.

The pans are attached by fairly conventional hinges at the front (see diagram - Detail D). The tube is let into slots in the steel chassis plates, thus they are mounted lower and attached more strongly. In the earlier chassis the L shaped piece of wire is free to rotate in both pieces of tube, which allows both "bat pan" and "plumber" movement (see Types of Pan Movement diagram). The wire can also slide in the tubes also allow both side to side and fore/aft movement of the pans. The side to side movement is restrained by the way the parts fit in the chassis, the fore/aft movement is controlled at the rear end of the pan. (see below). In later designs the wire is soldered on to the front cross piece and only rotates in the pan tube. The clearance in the tube will allow some "plumber" movement. Soldering the wire prevents any side to side movement at the front of the pan. Apparently the 1/24 chassis all have the wire soldered at the front.

The pans are joined at the rear by a fairly conventional hinge arrangement

(see diagram - Detail E).

The pans are free to hinge with a piece of tube to act as a stop to the "bat pan" movement in the normal way. The pivot tube at the rear is soldered on top of the pans. The later chassis have small extensions on the chassis side rails which carry the up stops for the "plumber" movement and limit the fore - aft movement of the pans. The down stop for the cross rail is either where it sits on top of the spine, or each side. There were various opinions as to the merits of these small extensions, and some people had the stops set slightly differently. Free fore and aft movement in the pans seems to be an increasingly common feature of 1/32 designs these days - another idea frequently used in 1/24 racing that has been developed to work in the smaller size chassis.

The pans have several windows cut in them so that they can easily be lightened. None of the 1/32 cars I’ve seen had any weight removed from the pans, but many of the 1/24 version run with parts of the windows cut out. An unusual feature of the pans were little pieces of steel let into the pans to support the pin tubes some distance above the top surface of the pans. There’s nothing new about moving the pin holes up - its pretty much conventional practice to space the pin tubes up above 2 or 3mm above the bottom of the chassis to reduce the risk of pins pulling out the bottom of the body - but this is certainly a light and strong way of achieving it. 

When I originally wrote this article BSCRA rules didn't allow pins outside the 64mm width limit - a fact that caused more than a few owners embarrassment at scrutineering time.  The chassis was designed to meet the international ISRA rules, which allow the pins outside the limit - subsequently BSCRA rules have changed to allow the pins to protrude outside 64mm, so this is no longer a problem.

Many thanks to Mike Walters, Brian Sanders, Charlie Gooding and Keith Gibson for letting me examine their cars in such detail. In the next article I’ll be taking a closer look at the Mossetti F1 chassis.

Chris Frost

 Based on an article first published in Slot Car Racing News in 1998

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