Tech Tip: How To Select A Smiths Speedometer Drive
When your riding your classic British motorcycle on the road there are many things that can get overlooked
One detail in particular is the speed and mileage that is displayed (and recorded) on your Smiths speedometer
A false sense of speed and mileage could cost you down the road..
To obtain the proper speed rating and mileage recording it all starts from the rear wheel at the speedometer drive
In today’s tech tip post we will be covering “How To Select A Smiths Speedometer Drive” for your classics British motorcycle
We will also cover some of the following topics today to help you understand a little more on how speedometer drives really work
Topics we will cover..
- Axle size
- Tire size
- Speedo drive type
- Ratios
- Fitment orientations
- and more
Let’s dive in
Getting to know the Smiths
"Smiths instrument van"
Back in England a company by the name of Smiths produced, designed and manufactured almost everything that had to do with counting and recording revolutions
Smiths had deep roots in the clock making business which allowed them to venture into vehicles and motorbikes both on and off the track..
Below are some components that Smiths produced for some of the classic British motorcycles we own today..
- Speedos (chronometric, mechanical and magnetic)
- Tachs (chronometric, mechanical, ATRC, and magnetic)
- Speedo wheel drives
- Tachdrive gearboxes
- Reverse and right angle drives
- Anti-vibration brackets
As you can see above Smiths played a key role in supplying parts - they where always known (and to this day) to supply quality components
For more information regarding the history of Smiths, visit there website
What is a rear wheel speedometer drive?
A speedometer drive is a unit that is fitted onto your rear wheel axle (sandwiches between the swing arm) that counts the revolutions of your wheel then displayed on your speedometer
Inside the speedo drive there is a small worm gear in line with your speedo cable that is rotated by a drive ring (usually has tabs on it) that is then operated by the rear wheel
Once your speedo cable is inserted into the drive the rear wheel will turn the inner cable which will the operate your smiths speedometer
Internally a smiths rear wheel speedo drive is extremely basic and very durable
Lack of greasing, cleaning and servicing usually cause the speedo drive to fail prematurely
4 things to consider
Now that we have covered the basics of a Smiths speedometer drive let’s go over the 4 things you need to know when selecting a new unit
Axle Size
Since Smiths speedometer drives are mounted on the rear axle, you must identify the outer diameter of your axle
Some axles have steps or different diameters on each side, so its very important to measure the axle side in which the speedo drive will be mounted on
Above is a photo of Malcolm measuring a Triumph rear wheel axle on the thread portion in which a speedo drive will be mounted onto..
The closest nominal fraction size (inch) would make this axle 3/4”
Tire size
Having the proper rear wheel size is just as critical as selecting the right speedo drive or having the right speedometer
Take a look at your tire dimensions and compare them with your parts book or workshop manual to confirm if your machine has the proper tire size
Your tire width and height will be shown on the sidewall of your tire as shown above (4.00 X 18)
“When in doubt, get the book out”
Which side, which way?
While most vintage Triumph, BSA and Norton models have speedo drives fitted on the RH (right hand) side of the motorcycle, some models had speedo drives fitted on the LH (left hand) side
Not to add to the confusion but some speedoemter drives even came “upside down” or backwards
Lets cover the 3 different speedo drives as found on most classic British motorcycles
Traditional speedo drive (RH)
This unit here is by far the most popular and traditional speedo drive that was used in production
Fits on the RH side of your motorcycle and hangs at the bottom
RH speedo drive "upside down"
This particular speedo drive fits on the RH side of your motorcycle but is "backwards" or "upside down"
This allows the cable to route up the rear of the motorcycle instead of underneath..
LH speedo drive
This unit here is fitted on the LH side of the motorcycle making this unit specific and unique to the Triumph T160 and Norton MKIII models
This unit is very similar to the “traditional” RH speedometer drive but on the opposite side
Ratios
The last important factor is identifying what ratio drive you need for your application
This can be found using a couple different methods
Some original smiths speedometer drives (and new) will have the ratio stamped on the inside portion of the felt retaining ring (see above)
Parts books and workshop manual are a good source in identifying which ratio drive you need
The speedometer unit and axle size will also give you clues of which drive and ratio your application may need
When selecting a speedometer drive you are limited to axle sizes, that is usually a tale-tell sign of what was originally fitted
For example, if you have a 3/4” axle and smiths offered only a 15/12 or 1:32-1 ratio drive with a 3/4” bore chances are that it’s the drive you need - just make sure your speedo matches
Need a new drive?
With all the info shown above, you are now able to select which drive you need
Since smiths is not making any new speedo drives I highly recommend replacing your original unit with a quality EMGO or L.F. Harris drive
I have found EMGO and L.F. Harris speedo drives to be exact and extremely rugged with excellent results
Here is a chart I compiled that will help with choosing a smiths speedometer drive - click on the side link to be taken to the product listing
Not all speedo drives and styles are not in production, should you need a drive not shown here I recommend checking on eBay of another parts supply house
Speedo drives will work with chrinometric, mechanic and magnetic instruments
Speedo types and applications
BG5330/168
- Triumph 650 unit twins (Bonneville, T120, Tiger, TR6, Trophy, TR6C) from 1967-1970.
- Triumph & BSA "triples" (Trident, Rocket III & T150) from 1968-1970 only
Smiths Part Number | Axle Size | Ratio | LH / RH | Link |
BG5330/168 | 3/4" | 15/12 or 1.25:1 | RH | Clink Here |
BG5330/171
- BSA unit twins (Lightning, Royal Star, Thunderbolt, etc) from 1967-1970
- Norton 750 & 850 Commando, Interstate, Roadster & Hi Rider models
Smiths Part Number | Axle Size | Ratio | LH / RH | Link |
BG5330/171 | 11/16" | 15/12 or 1.25:1 | RH | Clink Here |
BG5330/164
- Triumph OIF 650 & 750 unit twins (Bonneville, T120, T140, Tiger, TR6, TR7, Trophy & TR6C) from 1971-1978
- Triumph & BSA "triples" (Trident, Rocket III, A75 & T150) from 1971-1974
- BSA "Oil In Frame" twins (Thunderbolt, Firebird & Lightning) from 1971-1972.
Smiths Part Number | Axle Size | Ratio | LH / RH | Link |
BG5330/164 | 5/8" | 15/12 or 1.25:1 | RH | Clink Here |
BG5330/287
- Matchless, AJS & BSA singles / twins (C15, SS80, B40, SS90, B25, B44, Etc) from approx 1965-1970
- Triumph 350 & 500 unit twins (3TA, 5TA, Daytona, T100R, T100T, T100S, Tiger, Etc) from approx 1963-1974
- Triumph 650 unit twins (Bonneville, T120, Tiger, TR6, 6T, Trophy, TR6C, Etc) from 1964-1966 only
Smiths Part Number | Axle Size | Ratio | LH / RH | Link |
BG5330/287 | 3/8" | 2:1 | RH | Clink Here |
BG5330/170
- Triumph T160 from 1975-1976 as well as Norton Commando 850 MKIII models for the year of 1975
Smiths Part Number | Axle Size | Ratio | LH / RH | Link |
BG5330/170 | 11/16" | 15/12 or 1.25:1 | LH | Clink Here |
Things to consider
Before we wrap this post up, here are some quick things to consider before moving forward
Smiths rear wheel speedo drives will work with all types of speedometer instruments that where made including magnetic, chronometric, and mechanical units
Your speedometer should have a ratio or manufacture identification markings on the face that can be used to identify the ratio
To achieve a working speedo and speedo drive, you must have the correct speedometer ratio
The unit as shown above reads "SSM 5002/00B" & "1600" making this grey face gauge a 2:1 ratio (enter link to grey face)
To ensure a long lasting speedometer drive unit always remember to grease the unit regularly using a high quality / high pressure grease
It takes seconds to grease the unit but takes time and money to replace a bad unit down the road
Notice all Smiths speedometer drives feature a zerk fitting to accept a grease gun - take advantage of it!
Thanks for reading
Thank you for reading todays Tech Tip post
Should you be interested in reading more topics like this one, please click here
Don't forget to leave us a comment in the provided section below
Talk to you all next time
Take care - CBS
steve
Speedo drive zirk is a special one and too small for regular grease gun. I use a chain saw grease gun. Works great. Lowes for about $6.
Doug Bryant
Easy to read article. Will recommend to clients
Michael Tirrell
Thanks guys, getting my Bonney fixed up. This info helps.
John
Speedometer goes zero to about thirty rapidly back and forth when riding – how do I know if it’s the drive or speedometer that’s bad?
Classic British Spares
@Craig – thanks man!