Author Topic: rev counter ratio  (Read 2762 times)

Offline bsaalf

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rev counter ratio
« on: 29.05. 2009 23:30 »
hi all, i have a 4-1 rev counter fitted on my 1960 a10 and the other end of the cable drive is fitted to the timing casing which runs off the end off the magneto, and i was just wondering if this was the wrong ratio rev counter as the rev counter always seem to be reading high rev's compaired to the engine sound, do anyone know what the correct ratio is for this bike !

Online trevinoz

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Re: rev counter ratio
« Reply #1 on: 30.05. 2009 01:53 »
Alf, the oil pump driven tacho is 3:1.
Trev.

Online RichardL

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Re: rev counter ratio
« Reply #2 on: 30.05. 2009 03:27 »
Well, well, well!. The crank is going around twice for every revolution of the mag, so I would think you need a 2:1 tach. Is the tach of the magnetic or chronometric variety. I'm thinking the former because if it were chronometric you would expect either a 2:1 for mag driven or a 3:1 (as Trev says) for oil pump driven. I'm not a tachometer expert, but it seems unlikely Smith's would go far afield to make a 4:1 chronometric that did not work with bikes of the day. Maybe another member knows of a bike that uses a 4:1, but it seems it was not  n A7 or A10.What does it say on the face? Speaking of faces, I'm prepared for egg on mine if I am way off here.

Richard L.
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Online groily

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Re: rev counter ratio
« Reply #3 on: 30.05. 2009 05:02 »
Royal Enfield used, I am told, a 4:1 tacho drive in some applications. Not quite sure how and why and what, but someone of my acquaintance was bemoaning the scarcity of this very object only last week in relation to a 700c twin. If a person has a 4:1 anticlockwise drive set of bits, I'd say it could be quite valuable! Also prepared for egg if above is bunkum.
Bill

Offline bsaalf

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Re: rev counter ratio
« Reply #4 on: 30.05. 2009 11:15 »
ok guy's firstly i have only had the bike a few weeks its my first one so don't know to much about them but i am learning thanks to this forum now the rev counter hasn't got anything written on it at all just 4.1 then underneath that rpmx1000 it looks new to me unlike the old smiths speedo next to it !
it go's to 8000 rpm and between 7000-8000 its colored red i would say its modern but not sure what it is made for my main concern is i would like to know the true rpm of the engine as i am riding it as this one seems to rev very high and tickover is above 2000 rpm any suggestions cheer's

Offline a101960

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Re: rev counter ratio
« Reply #5 on: 30.05. 2009 13:54 »
First of all I have never seen a chronometric tacho that is of the same vintage as an A10 that fitted to a bike that had a with a red sector on the face. I would suggest that it is likely to be a car unit. Many of the smaller sports cars of that era, and indeed the early Mini Cooper rally cars were so equipped. However all is not lost. If you were to send your tacho to one of the specialist repairers it would be a simple matter for them to calibrate it to the correct ratio. Not only that a new face could be fitted with the correct ratio on it. I had my done last winter at an all up cost of £70 including the the new face. All of the chronometric tachos and speedometers are identical inside and can be calibrated to what ever it is you require for your bike. Indeed a tacho can be converted to a speedo and vice versa. In your case the drive is off of the magneto therefore it will need the correct ratio for this drive which is going to be 2:1 because the magneto runs at half engine speed. As has been stated already the oil pump drive is 3:1. At first sight £70 may seem quite a lot of money, but bare in mind that what you will get is a completely re-built tacho of the correct specification. There are both tachos and speedos out there at auto jumbles and on ebay which are going to cost a lot more than £ 70 to buy, and in all probability unless you were to be exceptionally lucky they will require attention. If I was you I would send my tacho off and get the job done properly.

Online RichardL

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Re: rev counter ratio
« Reply #6 on: 30.05. 2009 14:56 »
Alf:

This quote from a Triumph forum (Google never ceases to amaze me):

"Tachos from 66 on, 500, or Trident, 4:1 ratio will work. Beware BSA items, they are 3:1. However, BSA speedos will interchange."

Then, a quick look at eBay finds the tach that's pictured and can be further seen at the long link that follows.  However, if it is this, I think you might have mentioned the clunky needle.  Nevertheless. I am willing to bet that what you have is a magnetic drive aftermarket piece. If so, please don't go putting a lot of money into it for gear conversion, as a new one of that type for your bike will likely cost less. For now, you can just divide by 2 in you head up through 4,000 RPM, after that, you're off the scale.

 http://cgi.ebay.com/NEW-VEGLIA-TYPE-TACHOMETER-FOR-TRIUMPH-T140-T160-etc_W0QQitemZ120313456105QQcmdZViewItemQQptZUK_Motorcycle_Parts?hash=item1c033da5e9&_trksid=p3286.m63.l1177

Pictures would be a big help. Maybe your great grandson can help with the camera.  ;) *smile* *eek* (Sticking my neck out on your sense of humor.)


Richard L.



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Offline bsaalf

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Re: rev counter ratio
« Reply #7 on: 30.05. 2009 20:51 »
that looks exactly like it including the clunky needle thanks richard looks like i will have to purchase another one with a 2.1 ratio regards alf..

Online trevinoz

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Re: rev counter ratio
« Reply #8 on: 31.05. 2009 02:32 »
Alf, I misread your post. Check the ratio of the gearbox on the timing cover. Normally it is 1:1 so you need a 2:1 Tacho.
Trev.

Online RichardL

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Re: rev counter ratio
« Reply #9 on: 31.05. 2009 02:50 »
Oh my. I forgot about the little right-angle-drive gearbox attached to the timing cover (I have no tach but, hopefully a little tact). By all means, do a little ratio test on that gearbox, though I'd bet Trev is correct, having some sense for his experience..
Richard L.
Plan on signing up for the world-wide 2020 DISTINGUISHED GENTLEMAN'S RIDEon September 27, 2020 (if it's not cancelled and we are free to move about by then). Watch website at https://www.gentlemansride.com for details.