RcTek : Information & Resources for the Model Car Racer

PB Dart Front Suspension Bell Crank Assembly Modifications

The front suspension bell cranks (sometimes referred to as rockers) on the PB Racing Dart need to operate unhindered and smoothly for the suspension to work.

There are two main problems with the assembly that the Dart comes with.

Front Bell Crank Pivot Pin

Part Number 72/209

The standard pivot pin is manufactured from 3mm diameter stainless steel that is not sufficiently hard enough to withstand the forces it is subjected to by the rest of the suspension, especially in a crash situation. There are a number of solutions;

Obtain a hardened piece of steel/titanium. Although a pivot pin from another car could be substituted, I have yet to find one long enough.

Re-drill the bell cranks and the mounting plates (#72/204) to accept a 1/8″ (3.18mm) piece of piano wire, which is widely available from model shops. The pin can be retained by using either grub screws or allen cap screws in holes that have been drilled and tapped in the mounting plates. E-clip grooves can be cut in piano wire (or other hardened pins), but they have to be ground in. For the average under equipped (but creative) author this is a precarious operation of using a Dremel® tool, fitted with a thin cut off wheel, and a lathe.

A variation on the above, which was used on the team cars, is to re-drill the the bell cranks and the mounting plates (#72/204) to accept a 4mm pin. A 4mm pin is ordinarily hard enough to withstand bending and can have e-clip grooves cut into it for retention purposes, although the slight size difference of 5/32″ piano wire provides a useful alternative.

Please Note: When drilling holes to fit an oversized pin you will have to use a drill larger than the pin. A 1/8″ pin will not go through a 1/8″ hole - there has to be around 0.10mm clearance. A sharp 3.3mm drill will generally suffice, but when the amount you are removing (drilling out) is only small, the spirals on the drill can get compressed by the slightly undersized hole. Materials from different manufacturers or even the same manufacturer are made to different tolerances and the only way to ascertain whether to use an over sized drill is to measure the material with some calipers or a micrometer.

Bell Crank Assemblies

Part Numbers 72/205 & 72/206

The other problem with the bell crank arrangement is that the shock absorber push rods (#72/207) are connected at an angle to the bell cranks. In use they are forced together, which can cause problems in an independant suspension system like the one that the Dart uses. There are a few solutions to this potential problem, the simplest being to create a little clearance by shortening one of the cranks to allow you to place either a brass or preferably a PTFE washer between them. Better alternative solutions using bearings are shown below, but ideally require access to machinery no not normally in the racers tool kit. See notes on modifications

Solutions

Bell Crank Layout

Both the following solutions use bearings, the only difference between them is the type of bearing used. There is therefore no reason to duplicate the drawing shown left, which shows the layout of the bell cranks and the position of the bearing as they would be on the car.

The two different solutions are illustrated below and for ease and accuracy would be best carried out with the help of a lathe (Creative use of a Dremel® considered if there no alternatives!). For simplicity of mounting in the lathe chuck the two parts of the bell crank should be separated. They are a press fit and therefore should present no problems using either a press (for the well equipped) or a vice and punch/drift (the rest of us).

Solution 1 - Deep Groove Ball Bearing

Ball Bearing Modification

Ball Bearing

The first solution (shown left) is an inexpensive one that uses a standard ball bearing (shown right). The ends of the pivots are machined so that one half contacts the outer race of the bearing and the other side contacts the inner race.

This solution offers a more or less maintenance free assembly. We have converted our Dart using this method, further information about this modification is available.

Solution 2 - Thrust Race

Thrust Race Modification

Thrust race assembly

The second solution (shown left) is a slightly more expensive one that uses the correct type of bearing for the problem with the bell cranks being forced together.

Although it would only (probably) give a marginal efficiency increase over the ball bearing solution, it does require less machining.

The ends could also be cut square using nothing other than a file and/or saw, the accuracy is not really an issue to be considered, as they rotate by such a small amount. This type of bearing would require occasional cleaning (application of an aerosol lubricant/maintenance spray such as WD40).

Where To Get The Bearings

The bearings are available from most bearing suppliers but bearings from 1/10th scale cars are around the dimensions you require. The standard ball bearings are usually used in wheels or wheel axle carriers, the thrust races may be used in clutches. If a thrust race of the correct dimensions cannot be found, those used in the eigth scale Centax type clutches are normally 5mm × 9mm × 4mm and would require a small bush to adapt the inner diameter.

Summary

If you disassemble the bell cranks to machine them, make sure that you re-assemble them as a pair!

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© 2001, 2009 by Darren Burnhill