A Special Builder's Notes


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24 February 2012

The Engine Mountings

I've batted a few ideas around and have finally come up with a proposal for the forward mount that looks right. In effect, it's both an engine mount and a chassis brace. I'm not up on stress calcs and procedures but, looking at it, it seems to neutralise the effect of compression (the weight of the engine in the vertical axis) by an equal effect of tension (the weight of the engine trying to push the chassis rails apart in the horizontal axis). It's quite likely that I'm talking complete nonsense.

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So, today I'm going to slip off to a local company that specialises in laser-cutting. I'm blowed if I'm going to hack my way through 4mm steel with a saw. They've done work for me before; having got the design of the Austin 7 Special a bit wrong, my only recourse was to move the front axle forward and for this I devised a scheme around a pair of plates that bolted to the casting at the front of the 'A' frame. Works jolly well; not only does the car no longer look as though its tripping over itself, the ride has been smoothed out considerably.

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But back to the Hillman; the centre engine mount - the one which I envisage is going to incorporate the axle for the pedals, is a bit more tricky. I'm a bit pushed for space as the engine is so long and, a bit of a nuiscance, the designer has located the starter motor on the offside. When I'm back from the laser people (and a quick stop at the local micro-brewery which happens to be enroute), I shall measure up and assess the possibilities.

The laser people were charming and couldn't have been more helpful or, in my view, more reasonably priced. They understood my requirements and would have them ready in a week. The only hiccup was that I went to the premises that they occupied 2 years before and had another 10 miles to go to their new location. A couple of social engagements have postponed my wrestling with the centre engine mount.

A quick sketch of the proposed position of the pedals demonstrates that the pedal axle and the existing engine mounting will marry up almost perfectly. The distance from the bottom of the seat to the pedal plates is the same as my tourer which I find very comfortable.

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But, a desicion on the brake system has to be made pretty quickly because, if I elect to upgrade the front brakes to rod actuation, the centre engine mount will have to have provision for a second axle running transversely under the gearbox on which to hang the front brake levers. This axle will also house the rear brake balancing cable. It sounds complicated but a picture of the Hillman arrangement is worth a thousand words.....

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I'm beginning to warm to this idea because at the hub end of the operation is an assortment of levers and pushrods which are visually in keeping with the period and are interesting from an engineering point of view as I understand that the setup was adopted from a Rolls Royce design or RR borrowed it from Hillman - anyway, there is apparently some connection. I think I'll draw up for the cables and rods but I'll also check that the drums are cast iron and not the biscuit tin affairs I had on the Model A.

The drums are cast iron - that's a relief then. I've just been playing with the sketch to incorporate a couple of plates - one each side of the gearbox - that would make the pedal axle the engine mounting by carrying it right across to the nearside chassis rail. It would have to be some pretty special steel - a bit like the Hillman kingpins that you can't mark with a sledge hammer however hard you try. Then, the other end of the plates would support the cross tube I mentioned earlier that takes the rear brake balance cable. Here's what it looks like:

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and here it is a bit more clearly with 'A' carrying the load to the chassis rails.

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I think I'll go with this; it's clean and simple. One thing does occur to me though; if I want to remove the gearbox, I have to remember that the rear of the block is unsupported once the bell-housing is unbolted. I can only think that it was like this in the Morris Six and the Wolseley 6/80 because there's only one other engine and gearbox mounting and that's at the end of the output shaft.

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