By Dave Porter, Customer and Technical Support at The Bonneville Shop
A Guide to Cables
As a customer service tech advisor, I have observed that control cables are products that have been historically somewhat problematic for the end user to deal with and seem to generate consistent inquiries about proper fitment and operation on the various applications on a motorcycle. What could be so complicated about fitting a simple control cable? I’d like to share some of my experiences with throttle, clutch, and brake cables as they apply to vintage British motorcycles.
Guide to Cables: Throttle cables
Factors to consider:
- cable anchor position
- throttle grip assembly
- handlebars
- cable routing
“We all know that there are many changes that are made over the long lifespan of a vintage motorcycle, some very subtle [and] some changes were not…”
Throttle cables are perhaps the most complicated cables to fit due to variables in the handlebar dimensions, throttle twist-grip manufacturers, cable routing, and inner wire free length required for proper carburetor throttle slide function. When a cable manufacturer specifies an original part number for a throttle cable (or any other cable for that matter), the cable is constructed to the original overall length, sheath length, and inner wire free length under the assumption that the cable will be fitted to an original stock motorcycle for which that part number applies to.
We all know that there are many changes that are made over the long lifespan of a vintage motorcycle, some very subtle, such as the cable anchor position on an aftermarket throttle twist grip assembly compared to a factory original Amal 364. This difference can alter the desired inner wire free length downstream at the carburetor’s throttle slide and have an effect on the ability of the slide to completely drop to the resting position in the mixing chamber of the carburetor.
Some changes were not as subtle, such as a higher-rising or wider, non-original handlebar fitted to the motorcycle. This will have an effect on the overall length of the throttle cable and may require a custom-made cable to get the fitment right. The routing of the cables also has an effect on the proper operation of throttle cables and can make a cable seem too short if it is not correctly routed under the petrol tank and free to move without being pinched.
One scenario that is somewhat common is when a new Amal Concentric carburetor fitted with top cap adjusters is installed on a Triumph twin, and there is not enough inner wire free length for the original cable to let the slide drop all the way. From the factory, the throttle and choke cable barrel adjusters were omitted from the carburetor caps. They were drilled out and the special shouldered “top hat” cable ferrules sat directly in the drillings, adding back the 5/16” of free length needed to allow the slide to drop to the bottom resting position. I use a 15/64” drill bit for this procedure.
BSA and Nortons from the late 1960s to the mid-1970s retained the adjusters, as they had sufficient clearance under the petrol tank to not interfere with cable fitment. A Triumph twin throttle cable is readily identified by the shouldered ferrule and inline cable adjuster.
Guide to Cables: Clutch Cables
Factors to consider:
- cable routing
- cable ties
- control levers
- handlebars
- lubrication
“In many cases, there were different specifications for handlebar fitment depending on the intended market the motorcycle was produced for.”
Clutch cables are subject to heavy loads and must be lubricated at the nipples on both ends of the cable wire. The barrel that fits into the bore on the control lever needs to freely rotate and not bind at all. Sometimes it is helpful to lightly file the outside of the barrel if it is prone to binding. When the clutch lever is pulled in to disengage the clutch, the lever is working against the load imposed by the clutch springs and a bound cable barrel will cause the inner wire to worry back and forth until it finally work-hardens and starts to fray.
Like throttle cables, clutch cable routing is critical to the operation of the clutch. Large, sweeping bends helps prevent friction between the cable inner wire and sheathing, which will help produce a much lighter pull at the clutch control lever. Cable ties are useful for maintaining cable routing, but heavy-duty “zip ties” can be overtightened around the clutch cable, resulting in unwanted friction.
When ordering a clutch cable for your vintage motorcycle, be mindful of the effects of aftermarket control levers and handlebar dimensions. In many cases, there were different specifications for handlebar fitment depending on the intended market the motorcycle was produced for. Most Home-market (UK-spec) models were fitted with lower handlebars. The US-specification models with wider and taller handlebars would usually require a longer clutch cable. Some of the parts books from the mid-1960s onward discern between the Home-market and US-specification cables, but not all.
Guide to Cables: Brake Cables
Factors to consider:
- cable routing
- lubrication
- brake shoes
- handlebars
“If the front brake cable seems to have too much free inner wire length, this may be a sign that the brake shoes are worn and require replacement.”
Brake cables are subject to the same lubrication requirements as clutch cables and should also be inspected after installation to ensure the barrel swivels easily in the control lever. Again, if the barrel is bound up in the lever, it will eventually fail where the barrel is molded to the wire. Routing most front brake cables is straightforward but pay attention to the cable’s position to make sure it doesn’t get squashed between the steering stop lug on the headstock and the lower fork yoke. 1969-1970 BSA and Triumph twin-leading shoe brake plates have a linkage that can be adjusted to optimize the brake shoe and actuating cams interface.
If the front brake cable seems to have too much free inner wire length, this may be a sign that the brake shoes are worn and require replacement. As the friction lining material on the brake shoes diminishes, the brake control lever has to pull more cable, resulting in a soft front brake. This can be adjusted out to a degree by using the inline adjuster on the cable, as well as the barrel adjuster on the control lever. As is the case with throttle and clutch cables, variations in handlebar dimensions will also have an effect on the fitment of the front brake cable.
Hopefully this blog will help explain how to avoid some of the pitfalls when ordering cables for your BSA, Triumph, or Norton vintage motorcycle. Thanks for reading!