Why did motorcycle swing arm suspension take so long?

MatthewB

Banned
In the 1930s very few motorcycles had rear swing arm suspension. In 1930 Matchless introduced their cantilever rear suspension.

17_06_2015_1930_Matchless_Silverhawk_01.jpg


Others soon followed, and by the mid-1930s these included New Imperial, UK.

New-Imperial-1937-76DL-GS.jpg


New-Imperial-1937-Spring.jpg


Here’s some history on the swing arm and on Velocette leading the charge https://thevintagent.com/2018/02/13/whence-came-the-swingarm-frame/

So, how do we get the swing arm into earlier adoption? I’d say regulatory is the way to go.
 
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Actually Indian sold motorcycles with swing arm rear suspension in the 1910s. They called it a cradle spring frame, and it used leaf springs on either side of the rear wheel, parallel to the ground, on the upper part of the frame.

1914 Indian Hendee Special with leaf spring swing arm suspension
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Cradle spring frames were an expensive option, and Indian quit offering them as an option in the early 20s.

I think reasons why swing arms didn't become widespread until the 1950s were because:

1. Swing arm frames were expensive.
2. The damping (stopping the suspension from excessively bouncing after hitting a bump) and shock absorption were poor. Until BMW's ISDT (International Six Days Trial - an off road riding event) competition motorcycles came out in 1936, all swing arm suspension used just springs, instead of oil filled or oil & air filled shocks. You can use a friction damping system (like was used in girder front forks with varying degrees of success) at the swing arm pivot to help control spring bounce, but you end up having to constantly adjust it, as the friction plates wear. Also, springs alone aren't that good for road compliance (they're either too soft, or too stiff/harsh for bumps of different sizes). Shock absorbers or hydraulic dampers in combination with springs work very well to overcome this issue. The thing is, most motorcycle manufacturers were small potatoes compared to the car manufacturers, so they they bought a goodly amount of components off the shelf, to save on the high cost having custom made parts pre-WW2. This was would have been thae case for shocks and hydraulic dampers (though HRD/Vincent did make their own hydraulic dampers - then again, their motorcycles were very expensive, so they passed the cost on to the customer). Almost all off-the shelf shocks were geared towards cars (and some airplanes), and as a result were too large and too stiff for motorcycle use.
 
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From what I gather, motorcycles were originally just motorized bycicles or scooters and were only marginally faster then regular bycicles. Therefore the standard bycicle frame- even the complete standard bycicle technology- was good enough for them. Only when motorcycles became faster and more powerful and later when special racing, touring or all-terrain motorcycles became a thing you had the specific technological challenges that forces you to come up with things like swing arm frames or even later single-arm and cardan transmission frames.
 

MatthewB

Banned
Why do you need to regulate swing arms? Why should you? Is there a safety issue? What's wrong with the existing suspensions?
Outside of custom special builds, new hardtail bikes are illegal today in many jurisdictions because they’re considered unsafe. Regulation before WW2 could have forced rear suspension of some sort, be it swing arm, plunger, etc.

That’s what happened with brakes, with regulation forcing manufacturers to employ two brakes, where they were offering just one brake until then. Of course you need to be specific as IIRC at least one firm simply put two separate braking systems both on the rear wheel, omitting the front brake the regulations intended.
 
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From what I gather, motorcycles were originally just motorized bycicles or scooters and were only marginally faster then regular bycicles. Therefore the standard bycicle frame- even the complete standard bycicle technology- was good enough for them. Only when motorcycles became faster and more powerful and later when special racing, touring or all-terrain motorcycles became a thing you had the specific technological challenges that forces you to come up with things like swing arm frames or even later single-arm and cardan transmission frames.

True, but by the 20s motorcycle manufacturers and riders had realized that they could not treat them like motorized bicycles. Even in the 1910s, you started to see frames that were beefed up compared to a bicycle. It's just that girder front forks combined with a seat supported by a spring (that allowed the seat to move vertically whenever a bump was hit, to lessen transferrence of the jolting to the rider), were considered adequate for most riding applications. Pre-WW2, most motorcycles were lucky if they could do 70 or 80 mph (a goodly amount of them could barely do 60 mph), and considering the condition of most roads pre-ww2, going that fast was pushing things.
 
This post is written from a U.K. perspective.

Cost comes into why swing arm or any rear suspension was not commonplace. Until roughly 1960 the majority of motorbikes were transport for people of limited means meaning retail price had to be low.
Even after ww2 when increasing power outputs and customer expectations demanded rear suspension the British manufacturers initially preferred modding existing rigid frames (triumphs sprung hub or bsa adapting a rigid frame to run a plunger set up) It was only when the mcandless brothers demonstrated the superiority, with what became the featherbed frame, of a swing arm setup did the other manufacturers follow.
 
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