Gearbox

It is a straight-forward matter to pick a set of ratios that will do justice to the branch of the sport that we intend to pursue. As three and four speed boxes were used, it is best if we forget about the former as the combination of gears was less, little is compatible.

First let us look at the outer shell in which to place these ratios. The original cases matched up with the crankcases of the 9E, and then were modified to match the unfaired cases of the 36A, but internally they were exactly the same. The only difference being the kart or minicar box which did not have the kickstart facility, the layshaft bush in the kickstart shaft being put straight into the box outer cover. Some cases even had a speedo gearbox facility driving off the layshaft, an ideal refinement for a trials bike, but for our exercise this can be blanked off using the special cover supplied by the manufacturers. Once the shell has been chosen to match with the crankcases, it is yet another simple matter to pick the internals to suit the task for which it is intended. The only consideration to cosmetic appearances is the year to which it should represent. Before 1962 the selector mechanism caused problems in that it was not strong enough, and should not be used, this being identifiable by the large nut holding the selector spring stop into the outer cover plate.

Post 1962, this nut was deleted and a set of uprated selector pawls added, together with a simplified mechanism (Fig 17 ). This is the box to go for, as it much easier to maintain and all parts are available.

Having been designed three decades ago the box has some draw backs, the biggest being the use of plain bearings, but this is far outweighed by the versatility of its internals. The main output bearing is a 6205 ball race, and when replaced should be substituted with a C2 which has less radial clearance than the C3 bearings used on the crankshaft. The gear selection problems can be eliminated by careful building and use of the uprated parts available from Invader Kart Engines, ie hardened selector pawls, stronger selector detent springs and modified cam barrel bearings. Invader also do a needle roller conversion for the first gear pinion, which is made to work with their 16T mainshaft to give a high first gear for racing and sports use. In the road racing role, the box can cause problems, as the plain bushes are not up to the stresses handed out by a tuned engine. A change to needle roller bearings, as used in the Starmaker box, would cure this problem but would require the services of a good machinist to install.

On the Minicar box, without the kick start facility, the lay-shaft right hand bush can be replaced with a shell backed needle roller bearing of 9/16 X 3/4 in. The left hand end of the layshaft, and the right hand end of the main shaft both measure 508 thou, which is neither a metric or imperial standard size. In this case the shafts need to be ground down to 0.5 in and used with a 1/2 X 11/16 in bearing. Invader tried the use of needle rollers, but found that the shafts (and not the rollers) wore out very quickly, as the bearings ran directly onto them, this then caused meshing problems which accelerated pinion teeth wear. They now prefer to use the bronze bush (the bush being cheaper than a shaft) and change it regularly. The only answer to this problem would be the complete redesign of the gear box castings, to accommodate a set of larger ball bearings, but this would still leave the problem of the high gear pinion bush, as the space available is very limited. Pinions also tend to be another problem area, being adequate for normal use, they do wear heavily when used for racing, wearing through the case hardening of the teeth in a very short time. Greeves understood this problem on the Mk 1 Silverstone, and produced an uprated set, which were identified by a ring cut in the gear end face.

Gearbox lubrication is an important point, the internals are going to be working over-time to cope with the extra power being put through it. A good quality oil must be used in order to protect the pinions from excessive wear. The grade recommended by Villiers was SAE 30, but this specification needs to be upgraded to SAE 80 or 90 to cope with competition use. For real high power work this spec should be further raised, and an EP 130 used, or a racing oil such as Castrol R40, or a synthetic oil such as the 75W fully synthetic racing gearbox oil from Morris Lubricants.

The oil level in the box is checked by use of a built in dip stick, but this stick acts like a tuning fork at high revs, and has a tendency to fracture with dire results. The stick is best removed from the cap, and a complete spare one kept in the workshop which should only be used during servicing.

Selection of the gear ratios available falls into two clearly defined standard areas, firstly the road and close ratios and then the wide types. If a 17T layshaft is used running with a 25T high gear pinion, it will close up the ratios. On the other hand, if 15T layshaft is used in conjunction with a 27T high gear pinion, it will space out the ratios, but other combinations are available to us. The main shaft is available as 14, 15, 16 and 17 tooth, and by careful selection of the remaining pinions from the list in Appendix B, a workable box can be produced to suit the task in hand.

If a 23T high gear pinion from the standard three speed box is used in conjunction with the 18T layshaft gear from the same source, the gears will close up even further, but they may be a little too close for normal use. To carry out this modification the integral pinion of the four speed box layshaft has to be ground off (the three speed shaft is too short to use in the four speed box) and splines machined into the shaft to accept the interference fit pinion from the three speed box layshaft. To achieve the intermediate ratio set, the same method is used, but this time the pinions are taken from the wide ratio three speed box.

The naming of certain gearboxes as "standard" and "close" by changing the mainshaft is a misnomer, the mainshaft controls only the first gear. Mainshafts are available as 14 tooth (standard) 15T (close) 16T (very close) and 17T (ultra close), 14 is common 15 is rare 16 and 17 are available new. The second and third gear dogs are reasonably interchangeable and come as 23T or 22T to give a slight variation, and can be used to give a higher second or third gear, and a sliding dog from a three speed gearbox may be used to provide an extra-high second or third. A full breakdown of gear pinion combinations is available in Appendix B.

What of the overall ratio? For a standard primary drive of 20/43 and a final ratio of 17/50 through an 18" rim and tyre (diameter of 26", therefore a circumference of 6.8 feet), this gives a speed in top gear at 1000 rpm of:

1000/60 x 20/43 x 17/50 x 6.8 feet x 60/88 = 12.23 mph
where 60 mph is 88 feet/sec

Now calculate the speeds in the gears for 15T mainshaft gearbox

Gear	Ratio	6000 mph	8000 rpm mph
4	1.0	73	97
3	1.27	57	76
2	1.78	41	54
1	2.54	28	38

By looking at the speeds in the gears it is obvious that gears 1-2-3 only just connect with the power band of 6000-8000. Perhaps we should coax the power to 8500 to allow an overlap, or maybe you can always change gear precisely at the right time. You will find a road speed chart in Appendix A for speeds in all gears, for all ratios defined in Appendix B, for final drive ratios 17-19/48-51 inclusive.

Overall gearing of the machine is looked after by the choice of gearbox and rear wheel sprockets, the front sprocket comes in 2 widths, for 420 (½"x¼") chain (17T and 18T) and 428 (½"x 5/16") chain (15T to 20T). The rear sprocket is an easier proposition in that any sprocket maker can machine a blank to fit your wheel, with any number of teeth on it. When fitting the gearbox sprocket smear the splines of the shaft, under the sprocket and the distance piece, with silicon sealer, this will stop oil creeping out and make the box cleaner in use.

This chapter is continued in the book with the following major sections.

Gearbox assembly
First gear method
Fourth gear method