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Moving A Cub Engine Over To The Left


danw43
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Clearly you don't understand the term offset in relation to wheel building. What Duncan was suggesting was that you move the rim relative to the hub.The centreline of the rear rim will be slightly off the centreline of the bike but will track parallel to the front wheel.

What you seem to inferring is that you misalign the rear wheel spindle so that it is no longer at right angles to the bike centre line. That is not what was suggested..

Been building wheels since early 70's so probably do understand what the term offset means. Agree with you that to maintain the wheels tracking parallel as you say you need therefore to "misalign the rear wheel spindle so that it is no longer at right angles to the bike centre line" This will then have an effect on sprocket wear as the engine and rear wheel sprockets will no longer be running true on the same centre line. Getting there slowly aren't we?
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Going back to the original question. On my original Sammy Miller Cub the engine was offset to the left, primarily to give better chain clearance and tire clearance on the swinging arm. The SM swinging arm kit comes complete with the rear wheel sprocket spacer which obviously moves the sprocket further away from the line of the tire. Up front on the Miller frame the front engine mount is widened to allow a distance piece to be inserted to the left of the motor. 2 other distance pieces are used on the lower mount and also the rear. Front and rear sprockets have been displaced the same distance and therefore line up.

The lower tube has been subjected to some serious "SM" wellie!! to allow the oil pipe assembly and drain plug to be accessed Personally I thought this was VERY CRUDE. Also by moving the engine over the front of the exhaust pipe is very tight against the front downtube.

My current frame doesn't use any offsets, and the lower tube has had a tubonectomy which as OTF said gets rid of all the oil issues

TONY

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Been building wheels since early 70's so probably do understand what the term offset means. Agree with you that to maintain the wheels tracking parallel as you say you need therefore to "misalign the rear wheel spindle so that it is no longer at right angles to the bike centre line" This will then have an effect on sprocket wear as the engine and rear wheel sprockets will no longer be running true on the same centre line. Getting there slowly aren't we?

No we are not getting there at all.. you are the only one that seems to think that the wheel spindle needs to be mis-aligned.

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Still don't see how the rear wheel if offset / dished can follow in the same track as the front if you don't misalign the rear spindle but obviously all those who move the engine and rear sprocket over are deluded and that includes Sammy Miller and you are right. I doff my cap to you but I will still build my bikes with the wheels following the same track you do what you like. I'm off to answer my phone B)

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Offset as I understand it in this context means moving away from the centre line of the motor cycle. Offsetting the motor to the left of the centre line of the motor cycle or as others have mentioned offsetting the rear rim to the right away from the centre line of the motor cycle frame. Offsetting the motor and rear sprocket still allows the wheels front and rear to follow on the same track or centreline whilst offsetting the rear rim means that the front and rear wheels must follow two different tracks parallel to each other but not on the same centreline. To maintain both wheels following the same track as against parallel tracks the only way I can see to achieve this is to misalign the rear wheel spindle. If you are happy to have front and rear wheels following different though parallel tracks by offsetting the rear rim from the motor cycles centre line than that followed by the front wheel then fine but I was always taught to check wheel alignment by aligning straight edges along the rear wheel and measuring the distance either side of the front wheel which should be the same either side thereby maintaining both wheels following the same track. Something I check on a regular basis. If you move the motor over to the left, in the case of the cub, to increase chain clearance you must also move the rear sprocket to left by the same amount to maintain chain alignment so as not to prematurely wear the sprockets or worse have the chain come off causing damage.

If the above means "OTF is getting confused with the term offset?" then so be it. Perhaps I am?

Edit: thanks for the sketch Suzuki250 but isn't that what I said ?

Edited by old trials fanatic
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The following is a conversation (by e-mail) I had with Charlie Prescott some time ago which has some relevance.

Charlie,

In a recent post on trials central someone commented that Sammy Miller felt the front and rear wheels didn't need to be in line and that a little offset made it easier to balance. On a BSA, to get reasonable chain to rear wheel clearance, the engine looks as though it's C of G of the engine is significantly offset from the centreline of the two wheels! Are the otter's wheels exactly inline?

Charlie's response

Yes the Otter wheels are in line? Or should be!

The engine is offset to the left, and does look more bulky that side, But the centre of weight is not that much different ,because most of the bulk is only the primary chain case, and with PVL ignition fitted? this item could be slimmed down. As you know on modern trials bikes this situation does not arise as the engine gearbox layout is a lot different, and the clutch is in a different position and not outside the gearbox drive sprocket, Progress Ahh.

Regards Charlie.

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Hi Guys,

Tracking????

Yes, you can run the back wheel , over in the frame, but parallel to the front wheel ,although the centre of the rear wheel tread has now been moved over!!

This means the rear tyre is now not running central to the front tyre, but both, wheels are running in the same plane.

Does this effect handling?

Well it just means that by moving the back tyre over to the right slightly, you have moved the weight of the centre of gravity over into the opposite direction ,that you have moved the engine, so you have increased the difference of balance more to the right,so throwing the weight of the engine unit more to the left!

The flywheel is already of of the centre line of the frame, but most Brit bikes run like this anyway!

If you could move the weight of the outside running clutch, you would eliminate this situation.

But then the bike would be classed as a Special because you could not have done this before 1965???

So if you can leave the wheels running in line. We do with the Otter frame, as the design caters for the chain clearance. Another plus for one of these frames. Then I would say that !!! . Yes bsaotter.com

Regards Charlie.

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So if you can leave the wheels running in line. We do with the Otter frame, as the design caters for the chain clearance. Another plus for one of these frames. Then I would say that !!! . Yes bsaotter.com

The gear box sprocket position cant move relative to the engine, so the engine must still be offset ?

The problem with engines like the cub, b40 etc is they were not designed to run with 4.00 tyres.

Edited by b40rt
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There is one variable here that you have all completely overlooked, the thing does not run on rails or in a grooved track. Also when ridden the handlebars are constantly being moved one way or the other so creating misalignment.

Thousands of Cubs have been built with all the above ideas and more incorporated in their design and are being used weekly winning everything form club trials to the Scottish Classic without any thought being given to wheel alignment.

This topic was started by someone requesting advice regarding chain clearance on a 4.00 tyre, It is time to cease worrying about things that do not matter and focus on enjoying the ride.

Edited by trialsrfun
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Hi Guys.

You can ponder to much on what other people do to there bikes, and freak out, for what other people. Say.

A works trials Cub, trust me was a compromise, mine was a Works ,replica from the factory. And the nearest you could get to a works bike, and they kept replacing the parts that went wrong on a permanent basis! Even when we I turned the bike in to a Sprinter.

Charlie Rynolds told me one day that a Cub with a 350 back tyre was well superior at finding grip with the power the bike supplied ,than over powering it with a 400 tyre. And all you did by fitting a full sized tyre was loose power from the blocked up tyre.

Regards. Charlie.

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One question.

I don't know about rules for those bike but I notice on my TY that there is a big difference between a tire mounted on a 1.85 rim than one mounted on a 2.15 rim so my question is

Can't you use a narrow rim to have more clearence ? Mick told me they tried very narrow rim back in the days with succes.

Guy

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Hi Guy,

Yes you can run a narrow rim ,but the tyre,tire, is still 4" wide so the problem still exists.

Look the Triumph Cub was never designed has a trials bike, all the works did was convert a road commuting bike , after several privateers had done the homework for them.

All they wanted was publicity to sell more bikes, so any work, and very little was done on these bikes

All parts had to come from the Triumph spares bins, and very little modification was done.

Some Bsa parts were used as these were by now available, but even this was hushed up.

The only reason the Cub has become what it has today, is because it is a light weight little Pre 65 , trials bike . This is why so much time and money has been spent by small back yard engineers, developing this machine beyond recognition ,to what a Cub once was, a cheap ride to work commuter bike.

So any of you taking a road Cub frame or compleat bike, are going to have the same problems and struggle to build one of these little bikes into a trials machine, has we always have done.

Unless you purchase the available expencive parts , that are not Realy Cub anyway.

So in the end you decide, money pit or compromise .

Regards Charlie.

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