19" wheels

Time Warp said:

The Vincent had 20's both ends ?

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Good high speed tires came in 20" back then.
Manx Nortons then had a 20" rear tire too.

Still available too, if you hunt high and low enough.
Found a Michelin for something a while back...

Rohan said:

No need to get narky - plenty here didn't know there is a patch of dead air behind the front wheel. ?

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That would depend on the profile of the tyre/tire compared to a base line square section.

That would be the early Dunstall Dominator, 650ss or Atlas based.

Vincents mostly had 19' rear and 20" fronts. Racing bikes such as the Grey Flash and the Black Lightning used a 21" front and 20" rear. Touring Vincents used and 19" front and 18" rear.

Glen

So there must have been some educated theory and thought given to rim diameter.
I would imagine profiles were more limited.

Lets not mention the 16 in front era with 17 or 18 rears.

It must be pushing 40c/104F outside. :shock:

Time Warp said:

So there must have been some educated theory and thought given to rim diameter.
I would imagine profiles were more limited.

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As speeds came up, they reduced the tire sizes ??
And yes, profiles were all (?) 100%, for a long time.

rags said:

I've been told running two 19" wheels (stock) can cause higher speed weaving - tank slapping. Anyone know of this?

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I think somebody has been told a mistruth by somebody else.

concours said:

Rohan said:

Checked your steering bearings lately ?

All later Commandos use those simple ball bearings, which are not really designed for hard use/side loadings.
If they wear, then the steering may not be all that flash.
1st thing to look at anyway...
And inexpensive and simple to replace.

Are you actually getting a weave, or is this just reading angst ??

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+1

The sealed ball bearing steering neck design with no provision for preload is a horrid design, (as far as I know, the ONLY motorcycle on the planet so equipped) and will be retrofitted with tapered rollers this winter. JMWO :mrgreen:

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I haven't even ridden it yet. It was a basket case and I'm totally rebuilding it. The bearings are new (I've renewed every bearing in it). A fellow commando rider told me he fitted a 3.60 to the front and had no further problems. A long time ago I rode a Triumph on regular long runs with 7 or 8 commandos, all stock as far as I know. Never heard of the problem before.

Thanks all. I havent even ridden it yet as its a basket case ground up rebuild. A fellow commando rider told me his had the problem until he fitted a 3.60 front tyre. I used to run with 7 or 8 commandos on long runs and never heard of it before.

rags said:

Thanks all. I havent even ridden it yet as its a basket case ground up rebuild. A fellow commando rider told me his had the problem until he fitted a 3.60 front tyre. I used to run with 7 or 8 commandos on long runs and never heard of it before.

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LOTSA folklore, misinformation. Is your rear rim/spokes in a usable condition? Or rusted to need replacement? Are you looking to keep it mostly stock? Or keep adding chassis/brake/engine upgrades to emulate modern? Share some pics with us?

Forgive my ignorance. How do I upload pics to a reply? One rim is bran new and the other in almost new condition (tiny amount of rust you'd have to really look for). Stainless spokes. Not building stock, but close to, just neater and cleaner looking. Shortened fenders. One off leather seat (using stock base). Grey frame. Ceramic Black exhaust system with shorter pea shooters than stock. No electric starter or indicators. Minimum wiring (wires hidden in aluminium bars). All motor and drive train rebuilt. Very little chrome. Satin finished alloy (no polish). Bottom mount smaller headlight. Single central mounted electronic Smiths speedo (no tacho) All fasteners stainless. All new wiring. Pazon system. Single fire coil.
Neater rear light (cats eye bridging over frame loop). Lack of polished alloy/chrome made up for with metalflake paint (not done yet but plan on yellowy-gold). Single carb. One off chain guard (or non at all).

Rake and trail establish how a bike will handle. Wheel size, yoke offset and steering head angle all affect the trail. If you have a lot of trail the bike becomes more stable to the point where it might stand up and turn the wrong way under brakes. As the trail is decreased at certain head angles the bike might tend to steer quicker, tighten it's line as you ride around corners and apply power. For some bikes, reducing wheel size by one inch in the next model, manufacturers reduce the head angle by half a degree. With a commando you have a situation with the isolastics where the head steady affects the handling. If you reduce the wheel sizes by one inch the handling should become more stable (and possibly safer). However if you want quick steering staying with 19 inch wheels and skinny tyres might be less stable but better. - Depends on what your roads are like.
As far as handling is concerned a standard Vincent is a very dangerous motorbike - everything has to be adjusted spot on.

The only time I ever had straight-line wobble was above 70mph that got worse with speed (I tried twice to punch through it). Once I got rid of the old, square-profile rear tire and made sure it was balanced, it's never again showed any signs of head-shaking. Iso's set at .010" the whole time.

Nathan

Let's suppose that increasing the wheel's diameter induces a problem. If one continues to increase the diameter (a bit at a time), would that then increase the degree of the problem?

Perhaps, if so, that would explain why Jeandr hasn't replied - his Commando with 21" wheels has spat him off and killed him.

Say What? Someone fit 21" wheels on an isolastic Commando and killed him! I don't take that as any evidence its a bad ideaast knowing what I do about isolastic hinged handling limits and lean fouling on 19 & 18" wheels I'd suspect this experienced rided was elated by the better handling and lean angle so induced him to exceeed isolastic rebound tolerance of hi loads applied and released so flung him off at higher speed than 19" tires would allow before too scary to ride. In rigid mounted off road cycles with 21" front they do claim poorer hwy turn handling so maybe too dangerous in Commando on pavement. I see Harleys with pimp size front wheels for show but don't know if that limits their pavement handling.

Perhaps you need to measure the rake and trail on your bike and compare it with those that handle well ? Under brakes the bike should become stable, however not so much that the bike stands up and turns the wrong way. Going in the other direction, if the bike feels skitterish, it is usually letting you know when it is about to chuck you onto your head. The Vincent motorcycle however does not give much warning. The dampers under the seat and the bushes in the forks must be properly adjusted or it can come from nowhere and deck you. I know the commando isolastics can work well, however to my mind it is a source of movement and that is where danger can start. I suggest fitting a 21 inch front wheel might not be smart. If I did that I'd be very aware what the handling was doing the first few times I rode the bike. The trouble with this stuff is that it is not always predictable and the problem can be there waiting to grab you - the first time you ride over an uneven surface, or a ridge in the road. Even tyre size and tread shape is a factor. Take care - if you are prepared for the bike to go pear shaped you can usually control what happens. If it catches you by surprise ..... ?
Do you realise that the 30s Norton Inters had 20 inch front wheels ? - horrible handling pieces of garbage compared with the featherbed. I've read about how they tended to stand up in corners - one of the signs of too much trail. One thing I did notice was that the steering on the 70s water-bottle Suzuki GT750 was dangerous - friend had a very fast one at Phillip Island back then, which made him go a bit slower - scary !

Takes some stupid heros to learn what you reviewed Alan. Its well known 21" on front only is no no for road handling but maybe not if both ends 21" The gryo and flywheel effects may overwhelm 21" paved handling with one or two of em. Smaller tires are easier handling till hi speeds can get unstable compared to bit larger OD. If you can keep the pressure hard on isolastics and not hit a bump or wind gust that momentarily removes the full tensioned all slack out pressure then a Commando can handle as good or better than anything out there - until it does get knocked a bit out of full held tension so gets the rebounds of rubber mounts springing differently at each end. Solids can take this loading/unloading better- up to point the frequencies resonate then unpredictable things hit them too a lot faster harder.

Hm, most non paved operation uses mostly straight steering so maybe 21" front make the bike fall over easier and that's what bother them on pavement unless wanting to get into the hi side inducing state of rear patch pressure, hm...

Well that set off quite a discussion! When I posted the question I was building my Mk3 and was curious about the rumours I heard. Back in the 80s I ran on a hell of a lot of long rides on my Triumph with a lot of mates who had Commandos, some of which got a regular hammering and all had stock wheels, some with worn tyres and isolastics etc. I never heard of them weaving. Mines been on the road now for quite a while. It's difficult to go fast often in cop and speed camera infested W. Australia but the few times I've got it up to around 100mph I haven't had any dramas. Thanks for the feedback.

Spoke to a fella the other day who said he was riding a commando and had to back off half-way around a corner and got the big fright. I think the steering geometry on most road bikes is set up to be neutral. The effect of riding 2 up on a commando which makes it safer, makes me believe that the standard steering is near the limit of safety. It is only fractions of a degree change in the rake which can change the handling - make the bike more or less stable. I don't think the isolastics help the situation.
I read somewhere that the first commandos would go into a tank-slapper if someone rode over the cat's-eyes in the centre of some roads. Then Norton changed the geometry to make the commando more stable. The comment was made that an experienced rider would simply ride through the problem, however it caught a few beginners out.

acotrel said:

Spoke to a fella the other day who said he was riding a commando and had to back off half-way around a corner

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What do we conclude from this ?
That he was going too fast for the corner ??

Other bikes don't do this ???

I've arrived at a (thankfully small) variety of corners on a variety of bikes doing warp speed or part thereof,
and found this wasn't such a good idea - and survived by a variety of methods.
Putting the foot down and getting around speedway style on a couple of gravelly corners undergoing unmarked roadworks
has worked better than I had dreamed of, although I wouldn't like to try to reliably repeat that...