18 front rim

Hi,
Please,
I need advice.
Can I mount the front rim by 18 and leave the back from 19??
Are there any contraindications?
thanks.
Piero

Yes of course you can but it will slightly decrease the trail effect and tire radius profile near edge on leans, so should sharpen up, ie: lower the effort to turn, is about all to expect. A good thing to some, not so to others but either way not that big a deal until on very edges of traction and bouncing frame straining handling, so mostly a non issue on street bike use in public. if ya want to experiment and fine tune further can try mix-match of bias and radial tires, such as BMW issued on theirs for a period w/o issue. I've found there is a difference in how radial or bias ply react to road crown and sides of ruts. One tends to climb them the other not but I forget which as was on my modern cycle I've not ridden in some years but to deliver to a friend. I may be nutz but I want no cure as I think and feel my Peel Combat with some mods is the best on very rough paths and off road that bounced the MX and trails bikes too much to tackle like my big heavy twin did w/o such jarring they have to stay on pegs where I could stay in seat, most the time. My normal Combat like yours kind of sucks striking lumps and ruts and speed bumps so gave up taking it through creeks or even across cow pastures at with any spirit.

Most people do it the other way, as there is a better choice of tires in 18" diameter.

Doesn't this come back to the story about the various commando fork yokes? I still haven't seen it stated anywhere what the rakes and trails of the standard commandos are. Apparently the steering on the first models was too quick and crashed a few of the less experienced riders in the UK when they rode over the 'cats' eyes' indicators in the centre of their roads? When you change wheel sizes, it is not automatic that the bike would be safe.

The Commando is fairly sensitive for smaller rim diameters.
I see no advantage in fitting smaller size rims, in particularat the front.
It is true that the choice of 19 inch rear tires is limited, but as long as 19 inch front and rear tires are both still available , I would stick to the original rim size.

It'll look like ass? :mrgreen:

Peter R said:

The Commando is fairly sensitive for smaller rim diameters.
I see no advantage in fitting smaller size rims, in particularat the front.
It is true that the choice of 19 inch rear tires is limited, but as long as 19 inch front and rear tires are both still available , I would stick to the original rim size.

Click to expand...

I have a 74 -850 with 18's front and rear. The 18 inch rear is the same OD as the original 19 inch tire but the 18 inch front is
about an inch smaller. It makes the steering too quick and gives me the feeling that it could go into a wobble at any time.

I plan on going back to a 19 inch front. Jim.

ludwig said:

comnoz said:

..I have a 74 -850 with 18's front and rear. The 18 inch rear is the same OD as the original 19 inch tire but the 18 inch front is
about an inch smaller. It makes the steering too quick and gives me the feeling that it could go into a wobble at any time..

Click to expand...

Odd .. I have the same combination and love it .
Even a few other mods to reduce trail further .
Bike is rock steady at any speed and in the curves , (almost) no chickenstrips .
Wobble can have many causes .
Are you wearing a loose jacket , maybe ? ..

Click to expand...

No jacket problems. I used to run 18s on my racebike also and they worked well but on the 74 streetbike it is not stable. The rest of the suspension is at stock heights with the addition of a Hyde fork brace and a iso type headsteady. Not too sure why it is unhappy, but it is. It could just be the particular tires that are mounted.
I do know that the rake on the factory frames can vary quite a bit and maybe I have a pretty steep one, but that is just a guess as I have not measured this bike. When I got it , it was a Dunstall 850 with a leaky tank and 800 miles on it so I doubt it had been crashed. Jim

Another thing on smaller foot print front is its brake grip goes down detectably. Commandos are likely the most tire sensitive cycles ever sold. If ya like to lean far over, whether ya really need to or not, just for fun, you may find lower front tire makes for rather too soon ground fouling. Seems so much variation in riders and Commandos and conditions only one way to find out for yourself, spend and diddle spend and diddle.

My favorite all around [non maxing out] tire combo is 110 x 19" on both ends. If pressing into max leans on max power and max sharpness 120 rear holds better for me [especially on THE Gravel] but does feel like a bit like running in a boot on a normal power Commando when I put it on my current factory Combat, til it got flat and road it 5 miles on THE Gravel, tempting with still good meat but its cords likely kinked to distort at speed if used again. !20 is a tight squeeze to get through swing arm/fender too.

I bought a protractor from a tool shop. Has a magnet which sticks it to the fork staunchion so the rake can be read off. It's useful for gathering data from bikes which handle well. You don't use it often, however it is very useful sometimes. The numbers are a better indication of what is going on with the steering geometry.

ludwig said:

acotrel said:

..so the rake can be read off..

Click to expand...

If yokes are parallel .
For 850 yokes , add 1.5 deg .

Click to expand...

Hi,
What does it means???
Please, let me understand.
Ciao.
Piero

The 750's are supposed to have 27-ish degree of frame stem "rake" while the 850's 28-ish rake as shown above. For rough off road work it is almost universal that a larger taller dia front wheel is fitted than on the back. For smooth tarmac dragsters it is almost universal to fit a lower smaller dia wheel. I've had both 750 and 850 on same day and conditions on mere pasture and on THE Gravel paths, the 850 felt sluggish 'resisting' to steer with a tendency to fall over trying to fight a turn, while the 750 felt immediate handy by comparison to do spin out dougnuts this way and that and not fight to prevent falling over. These were stock unmodified Commandos, not modified in anyway - like my modified one that made factory 750 feel too risky to leave beaten path. Off road cycles also have increased fork travel over road going cycles. Off road cycles generally have progressive springs that are soft-easy motion for a couple-3 inches then stiffen up dramatically for last couple couple inches of travel. If ya got 6 inches travel then can have an extra section of spring that's in between stiffeness.