Considerations for adapting a 5-speed transmission to the Elan.
Ever since LotusElan.net started, and before, there has been discussion about adapting 5-speed transmissions to the Elan.
It strikes me that if there were a checklist of interface issues, that it would be possible to more easily examine the issues involved in a possible adaptation. Bear in mind that given enough effort, practically any conversion can be accomplished. But what we seek is the minimal effort to accomplish the conversion and to allow this to be done by the largest audience.
So here are some things to look out for. Experience will cause this list to be expanded, contracted, or modified.
Input shaft length. If the input shaft is too long, it has to be shortened. Too short is too bad. Complications are the depth of the 5-speed bell housing versus the TC bell housing and/or whether an adapter plate is used.
Input shaft diameter. The TC is .59 (15mm) or .67 (17mm). Probably 15mm should be used, since there are pilot bearings to fit the less common (I think) 17mm crankshaft hole. If the input shaft is different, it has to be machined down to 15mm if larger, or bushings provided to fit the larger TC crankshaft hole.
Input shaft spline diameter and number. If the donor gearbox input spline is different from the TC, a different clutch is needed at a minimum. There can be other problems also.
Gearbox to bell housing mounting bolt pattern. It would be nice if the gearbox just bolted up to the bell housing. If it doesn?t, there are four options.
First, there may already exist a conversion bell housing. If so, use it. There may be issues with clutch actuation which have to be resolved. Also to be considered is the starter motor mounting, its position, compatibility with the existing flywheel, and possible interference issues with the frame. In the worst case, a new flywheel and ring gear will be needed.
Second, an adapter plate could be machined. This will push the rear of the tail housing and shift lever position back, complicating fitting the transmission within the length of the original transmission. Also, it complicates positioning the gear lever appropriately.
Third, if material exists at the correct positions, the existing bolt holes (on the transmission, but maybe on the existing bell housing) could be filled in and new bolt holes drilled.
Last, a new bell housing would have to be designed, cast, and machined. If this is required, we should look at all the cities where we have members to try to locate a ?friendly? pattern shop who would be amenable to doing the job without charging normal commercial rates, which we cannot afford. A possible advantage here would be if the bell housing is cast in aluminum, saving weight over the stock iron bell housing.
Moving to the rear of the transmission.
First to consider is the length of the bell housing, gear case, and tail shaft. If this is close to the Ford setup, then go to the next section. Otherwise, a new drive shaft of the proper length will need to be patched onto the existing drive shaft. It is probably better to graft on the donor gearbox u-joint rather than use the Ford u-joint and have to cut and weld it onto the donor output.
Output shaft spline diameter and number. If the donor gearbox has a different number and diameter of splines, then it will have to be adapted to the Lotus drive shaft. You could use the Lotus u-joint cut and welded onto the donor gearbox, or use the donor u-joint cut and welded onto the Lotus drive shaft.
Speedometer cable. If the gearbox has the speedometer cable screwed into the side, a 90 deg box will have to be used to allow the speedometer cable to fit within the frame. A proper thread and length need to be sourced.
Gearbox speedometer drive gearing. This establishes the speedometer turns per mile figure. Depending on the source vehicle, the wheel size, tire size, differential ratio, and speedometer turns per mile, figures may be different from the Lotus figures, and produce incorrect odometer/speedometer readings. Also, there is a wide difference in the rev/mile of different tires commonly used (a 15% difference). A previous post
http://www.lotuselan.net/forums/viewtopic.php?t=16106 discussed this and provided a spreadsheet so that the tooth figures can be determined. Many gearboxes had a variety of drive/driven gear teeth. These may not be available, or not be the correct ratio. In that case, an external ratio converter is necessary. Some people may go to a mechanical to electronic conversion, and use an electronic speedometer. These often give a fine degree of adaptability, but the typefaces do not give the look of the older Smiths typefaces. In any case, the speedometer may need re-calibration since they are usually inaccurate.
Gearbox to frame mounting bracket. It is highly unlikely that the gearbox will bolt up to the frame. An adapter bracket would be needed. In the worst case, new boss would need to be welded on.
Dimensional problems. If a different bell housing is used, there may be interference issues with the bell housing and the chassis, either the standard Lotus chassis or the Spyder chassis. There may be problems with the starter motor, its position and mounting. This may require a new ring gear and/or flywheel. Hopefully, the starter does not interfere with the chassis. That might require a new starter. The width, height, or depth of the gearbox or the tail shaft may interfere with the Lotus or Spyder chassis (and is particularly severe with the early Spyder chassis).
Now, we come to the gear stick position. Even the original Elan was not perfect, with the early cars being a touch too forward, and the later (anti-sizzle) ones not being quite as precise. But this is dwarfed by the problems with most gearboxes. Usually, the gear stick is much to far to the rear. Some gearboxes have alternative tail housings with alternative gear stick locations. If the distance discrepancy is not too great (a couple of inches or so) you can use a metal plate picking up the gearbox on one end and the remounted gear stick on the other end. Keep in mind the shift geometry changes, with there being an up/down arc in addition to the usual forward/backward movement. This may eventually lead to interference in the frame tunnel or on the gearbox.
This link
http://nostalgiacars.co.uk/gearbox.htm shows how one company has approached the problem using a MT75 gear set and custom gear case. One of the ?standard? locations is quite close to the Lotus position.
Lets step back a look a this whole process. Basically, depending on the specific gearbox, we have a new car from engine block to differential, but still with the wide sedan ratios of the donor car. In addition, it requires a substantial amount of mechanical skill and ingenuity, not the remove and replace scenario that could be handled by the usual shop or self mechanic. I am envious of those individuals with the skills and equipment to do this type of conversion, but it limits the number of conversions to a rather small number.
What can be done to resolve this?
Consider a modest proposal, as it were.
Replace the existing gear set with a new gear set. In one fell swoop, this resolves problems with the input shaft length, shaft diameter, input spline diameter and number, output spline number and diameter, u-joint and drive shaft compatibility, as well as inappropriate ratios. Depending on the volume of orders and the flexibility of the machine shop, there can be one compromise ratio set for all users, or a selection of ratio sets.
Note that any sedan gearbox that handled 100 hp or more is a suitable candidate for a conversion, since modern alloys will be reliable under the horsepower outputs of even the strongest TC. These older transmissions often are more compact and lighter than more modern gear boxes dimensioned and stressed for reliability at 200+ hp ratings.
Replace the gearbox case with a new cast aluminum gearbox case. This resolves possible problems with replacement bell housing/adapter plates/new bell housing, starter motor positioning and interface issues, possible flywheel/ring gear incompatibilities, clutch actuation, frame mounting, and speedometer cable mounting. If the original gear case is cast iron, this results in a useful weight reduction.
The gear stick position problem has to be resolved, no matter which type of solution is chosen.
If well designed and executed, the Elan owner now orders and receives a box that they take down to their mechanic, who removes the original transmission, bolts in the new transmission, and the owner drives away with a 5-speed.
This solution requires a substantial amount of up front engineering as well as a commitment for a reasonable number of units. Given the realities of the retail market place (with current complete conversion kits costing from $3K USD with sedan ratios), this probably only makes financial sense if the development, manufacturing, and distribution are all handled within the LotusElan.net user group and other Lotus Elan owners and groups without involving a formal dealer/distribution network.
This concept can work, since an equivalent project put a 5-speed in the Lotus Elite. Those who believed and put their money up got a very nice 5-speed. Those who equivocated lost out.