A Tacho on the Cheap

This is really a nice idea - but you have to get the right unit.

Now this is a strange one - I go into into the workings out below - but if you can find a cycle computer that goes down to 100 mm on the wheel circumferance setting - several do, and cheap ones too -

then -

set it at 167 mm for the wheel circumference

So - at 1000 rpm - bike thinks you are doing 1000 x 0.167m which is 167m per min - so it makes that x 60 - which comes out at 10.02 kph !

Its asking a bit much to set a cycle computer to the exact 166.6666 mm for wheel circumference - so we have to live with the odd 0.02 figure.

The first I found to go this low was a Velomann V1 - 07 wired ( needs to be wired- they are cheaper anyways)

These were fairly pricey, though - up around £30 plus.

Trundling through impossible to read pdfs - Cateyes only go to 100 cm as do others. I was beginning to think it was just the pricey ones - but found a VDO unit that did.

The one I have ordered - and cheap too at £5 sale price from a cycle component store - was a VDO - A4 wired. German make as well. Its a cheap one - proper price £14 or so. You have to hunt around - and dig up the PDF instruction download - and even then they sometimes do not say !

Its hard life being retired - but I have a tacho for the engine - for £ 7.50 !

Apparently they are only two wires to these things - so you can lengthen the cord quite easily.

You only need one battery and they last a while remembering my bike days.

The pickup can mount on the main crank pulley - they cite 1 to 5mm for gap, so thats not too tight to work with. The Yanmar crank pulley is tapped with four M8 thread holes - pefect for a simple mount for the sender - pickup not really a problem.

I very much doubt if a wireless would have the range - and you also have two batteries to fail.

Its only going to do rpm - but I suppose the odo would approximate engine hrs - ie 3000 rpm = 30 kph - so every 90 k on the odo would equate to about three hours runtime. That info stays on I think.

( see below for mounting on the split coupling )

Now - thinking that it would be more out the way on the split coupling got me into another can of worms - how does the gearbox ratio work ?

For mounting on the split coupling - on a 2.21 ratio gearbox ( or other )

Now - when they say gear ratio 2.21 - what do they mean ? I was confused - but saw that they were quoting rpm at prop of 1540 ? Now - that x 2.21 = 3400 rated rpm. So - essentially, when the engine is doing 1000 rpm the prop is doing 1000 devided by 2.21 - 452 rpm.

2.21 reduction = 452.48 revs at the prop - per 1000 rpm.

2.21 x 166.666 ( our real 167mm figure ) = 368.3.

Set mm circumferance at 368mm ( 0.368 m )

Now when prop does 452,48 revs ( 1000 rpm at crank ) cyclo comp thinks its done 452.48 x 0.368 m - in a minute

= 166.51 metres. That x 60 = kph - which is 9990.75 - which is 9.99075 kph.

Near as dammit - 10 kph = 1000 rpm.

If you gearbox ratio is diffrerent - just multiply 166.666 mm by the ratio to get the circumference figure to match 1000 rpm to 10 kph.

But - you must have a cyclo unit that will set to 100mm up - some do some don't !

I shall keep looking when I have time - most of these units have their instructions accessable online - but some are horrendous to try to pour through in badly translated eight languages - and line drawings done by 5 year old's !

They are about !

The sender can go on anything that spins - size in not important as we are lying ! So long as it is at crankspeed - or a know ratio to that. The alternator pulley would be more difficult - if its not crank pulley size ?

I have a feeling that the wire from one of these would fire a normal dial tacho - but they don't come cheap.

For a Yanmar - the cheapest option is the remote, injection sencing TinyTach - about £75 ( clamps around the injector line ) - any guage is around £ 60 plus having to mod the alternator. Yanmar actual sender ( bolts to top of gear housing and sences flywheel - is mortgagable ) - ASAP I think do an aftermarker sender - but the thread is different, and its still £40 or more - plus the gauge !

So I'm happy at £7.50.

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