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 

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Talk to you all next time

Take care - CBS