Does Swinging arm design for a twin-shock really make a difference?

We take a look at the ones used on the Otter family...

 

Faber Mk 3 Frame and swinging arm...

I thought I would do a page on the difference between, and what makes a good swinging arm for your Otter, and other makes of twinshock machines.

The Harry Foster and the later Faber Mk1 Otter swinging –arms differ very little except for the Faber one being nearly an inch wider.

These were basically fabricated copies of the old BSA lugged design and of coarse the Triumph Cub arm has used in the making of the original Scott Ellis bike.

But Howard at Faber later changed the design in the later Faber Otter frames to be of only three components and even later nylon through bushes.

Did doing this change the character of the ride? Well a lot think so. And not for the better. But why? Was it the fact that doing away with the old BSA copy configuration made the swinging arm twist more? Or was it down to the lighter tubing used?

 

Mk1 Faber frame with the old layout...

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The old school swinging arms were made of very heavy poor grade tubing sweated into the appropriate castings. And not actually designed if that was the right word for any off road motorcycling.

But also the early-fabricated arms were made of heavy gauge steel tubing, and the example I have from a Cotton trials frame was I think made by an agricultural machinery manufacturing company…

Well you could plough a field with it and it would not bend.

A swinging arm is actually un-sprung weight, so does it really matter how heavy the item is?

Cotton Photo...

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This is the CoTTon swinging arm, Circa 1967..

This arm is twice as heavy as the rest of the arms here, the 12 or even 10 gauge tubing was only mild steel and was welded with and Arc welder (Stick), and I was told by someone that they were in-fact made by a local to Gloucester agricultural machinery company, and I know there were two or three in the area, but I don't know how true this was...

 Look at the design though,this has the "boxing in " effect at the front with the pivot tube bushes being carried in short tubes in front of the main brace tube, and again the engine mounting was run through with the pivot bolt...

 

 

 

This is my Foster Otter Frame using a Triumph Cub swinging arm and subframe...

 

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Twisting well that is probably the crux of the matter, do we want a bit of movement in an arm or not. A good example of swinging arms twisting more than they should is the Fraser Honda.

Built out of rectangular ERW tubing and a single pivot tube you would think it would be all right? But this arm just well, bent, and many a time I have seen a guy with a huge length of branch wood out of a hedge trying to prize the wheel back into an upright position. So what twisted it? The shear power from the engine or bad design?

So you see there is a lot of questions, so how do we answer them?

I will get back to the Otter family swinging arms, as I basically know about all of them so makes it easier to try and find and explain the difference between these.

Fraser Photo...

I will start with a quote or two, which I remember. I wont embarrass the ones that worded these but they are in these pages. “ Well the earlier bikes handled and felt better than the later ones, but I can’t put my finger on it”.

Well this was about the Faber Otter frame.

The other was, “Well the Foster Otter frames always handled a lot better than the later copies”.

As I said the only real difference between the first Faber Otter frame and the earlier Foster, was the width of the arm the Faber is almost an inch wider than the Foster as you can see. Below.

Photos…

 

This is the Foster swinging arm jig ,Yes it is crude, and when I first saw it I thought it was just a bit of scrap. But if you look at the photo below you will see that the arm fits perfectly, and there is a fixture for locating every bracket on the arm including the side stand spring peg...

 

The tubing used on this Foster Otter arm is slightly smaller section than the Faber arm the Foster is 1,1/8" and the Faber is 1,1/4"...

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And you will say what about the frame though?

Well don’t forget that I have the original Foster jig and also my own jig set up from a Faber Mk1 frame.

And I can tell you the angles are the same, it was not until later on the Mk 1 1/2 as I call it that Howard steepened the steering head angle a tad.

So does the width of the arm make a difference?

Across the wheel plates at the back the average between all of them is seven inches, give or take a thou or four. The difference between the Foster frames at the pivot end is an inch well slightly more, Foster 7 inches, Faber just over 8 inches.

And the Mills frame arms are again seven inches.

And they are said to handle better than any?

We will get to the Mills arms later.

I can’t see as the width at the pivot makes a lot of difference only to make the bike a bit slimmer.

I think it is the extra cross brace just behind the pivot that is the key.

In other words this extra cross member along with the pivot bolt running through the two tubes with the silent block mounting bushes or even nylon or needle rollers, sort of boxes in this area and makes it more rigid at the front of the arm. This goes one further on the Mills frames as a centre box also doubles as a back engine mounting, attaching that to the swinging arm as well.

So is this the key.

An example is the Mini-Otter frames.

As you probably know these are based around the Mike Mills frame concept with the addition of the Otter top and seat tubes. The Mk1 Mini-Otter frame had been tested in the IOM for two-three years

And I had gone back to the original Mills layout for the swinging arm, as I was using the silent-block bushes in the Foster Otter arms and thought it sensible to also use them again on the Mini frame.

So number two three and four Mini-Otter swinging arms, were made to this pattern…

 

 

This is the arm for the Mini-Otter I have gone back to...

with the pivot back into forward facing tubes. 

I may later connect the bottom rear engine mounting plates to this pivot bolt to see if it makes any difference.

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This is the arm that was fitted to the number one Mini-Otter bike,with the pivot bolt running on Nylon bushes...

You can see that I used oval tube for the blades on these arms.

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Brian thought he was loosing out by not having one on the first bike, so I built another, and he came over to have it fitted. Now I had ridden the first bike quite a bit when I first built it and thought how well it handled. But after fitting the new arm and just riding up to the end of the lane and back the bike felt, well different and more positive and not a lot had been changed on the bike except for changing the fork oil to new.

So was it all in my head or did that arm make a difference by just having the front end squared of as I call it…

To Be sure I am going to fit the first swinging arm back into the number six bike. (In build now,) to compare it with bike number two and five which have the same engines and exactly the same frames and set up…I will also pop into one bike just to test the Mills swinging arm to see if that makes a difference…

But we just need to get this “Lockdown” over before we can do anything…

Well I can build up five and six…

 

 

This is Mini-otter Two and frame number four...

 Built to Mike Mills frame specification...

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We will carry on with this theory later...

 But I can here some of you saying, it is not the swinging arm that makes a bike ride better or worse, or feel more positive or not.

It is down to the rear twin-shock suspension units fitted, that is what makes the difference!

Is it ?

Suspension is a totally different story, and we will cover that aspect of a bikes feel and grip later, and in depth...

 

 

Stay Safe……..

More later this only a start… Loads of photos later to better explain…