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A lot of people like the look of fatter tires, and are contemplating which size will fit between the forks or swing arms.

While the look is good, the function is often something less than optimal.

Fatter tires, Mounted on the correct size rim, will slow handling down ie: make the bike slower to respond to steering inputs) than thinner tires. Not saying this is bad or good, just that there are consequences to geometry changes on the bike. This is due to a number of reasons. Larger diameter tires increase train on the front. The larger diameter circle in the rear has a lower effect on the rear wheel steering effect, adding to the slowness. These results are not unacceptable, just considerations on what fat tires have on the handling and traction of your bike.

There are other considerations that go beyond the handling effects though. The cross section of a tire on a proper rim forms a rough semicircle. The sidewall shape, on a proper width rim, is much straighter than on an undersized rim (Undersized in width. When you make a turn there is lateral (sideways) force put on the tire. A straighter sidewall is stiffer, and resists being deformed by force. A fat tire on a thin rim, however, cannot resist the lateral forces as easily, so the tire moves back and forth laterally from these forces, making the bike squirmy feeling. If you have tar snakes on your roads, and you ride on them in the summer, you can feel this squirm quite a bit. The profile of the tire, instead of being rought a half round, is now a half oval, in the long direction. This makes the contact patch narrower, as the tire is less flexible, and can reduce traction.



So the best way to improve safety and handling is to have the appropriate width rim for the size tire. Attached below is a graph of recommended tire sizes and also, the standard rim widths that most of our bikes use.

The narrowest acceptable rim is fine, but going up a size or two on the rim will improve handling. for instance, my CB360 has 3.00x18 Tires on a WM1 (1.60 inch wide) rim. If you look at the chart, the smallest acceptable rim is 1.60. So it is acceptable. In my restoration, I replaced it with a WM2 rim (1.85 inch wide) on the same size tire. As a result, the tire actually looks wider, but I found the bike is more responsive and steady when riding. the wider rim added lateral stiffness to the tire. I also increase the stock rear to a WM3, up one size. My motorcycle had a noticeable increase in secure, predictable handling because of the rims. My Shocks still stock from the factor, the fork stock with fresh oil.

So the trick to good handling, or at least the best handling, is to have the right width rim for a given tire size. As a general rule of thumb, the widest acceptable rim will provide the strongest lateral strength. Of course, tire construction and design make it difficult to compare one combination to another, but a wider rim is still the right choice.

There is more, I wanted to get this thread started, and additions to this thread are welcome.


[attachment=2:193555pl]REcommend tire size 1.jpg[/attachment:193555pl]

[attachment=1:193555pl]Wheelrimwidths.jpg[/attachment:193555pl]

[attachment=0:193555pl]morad_s_section_profile.jpg[/attachment:193555pl]
 

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66Sprint
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I hate to have to correct the new posting, but rather than have it misunderstood, here goes......
The dimension given as "A" for overall diameter is INCORRECT in the examples chart.....At least as I see it......

This MIGHT help clarify my position.....

"C" is the nominal rim diameter (TRUE) and for example would be an 18" rear and a 19" front on a CL350

"B" and D" are the width and height of the mounted tire respectively.....
ON a 3.50 X 18 tire, both "B" and "D" would be 3.50", and on the modern "rough equivalent sizing" (90/90 X 18), the tire would be 3.54" wide ("B") and 3.18" tall ("D")......This would make dimension "A" on the wheel assembly I just discribed 25" using a 3.50 x 18 tire and 24.36" if using a "metric" 90/90 x 18......

The reason I'm assuming the chart numbers are incorrect is the mounted tire/wheel diameter ("A") is "C"+"D"+"D" and I find it hard to believe that a 6.50 width tire is only going to have a 4" height (aspect) unless "B" is also about 4+" wide.....In essence a 165/60 tire ( sounds more like a go-kart wheel to me with an 8" diameter and 4+" width).......

Can we edit the drawing to remove the chart and its confusion?
 

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Discussion Starter #4
Good catch. Missed that. Correct the chart.



Sent from planet Earth using mysterious electronic devices and Tapatalk
 

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Ok…I confuse easily, so listen slowly.

My SL350 K1 calls for a 3.25 x 19 front

I can get a Shinko SR241 in a 3.50 x 19 or a Shinko 244 in a 2.75 x19.

I'm thinking the 3.5 is a better fit…It's also the Trials pattern, similar to the originals…

I'm I correct?
 

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66Sprint
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Check what rim width the manufacturer recommends for that size tire.... IF your rim is that width, you are fine to run that tire size...
IF your rim is narrower, the tire may mount, but will sit "pinched" at the bead and thus may actually present LESS tire surface to the road than the smaller tire......
 

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drdubb said:
Ok…I confuse easily, so listen slowly.

My SL350 K1 calls for a 3.25 x 19 front

I can get a Shinko SR241 in a 3.50 x 19 or a Shinko 244 in a 2.75 x19.

I'm thinking the 3.5 is a better fit…It's also the Trials pattern, similar to the originals…

I'm I correct?
Sorry, not correct. You would be much better off using 2.75" tyre on the WM1 front rim of CL (WM1=1.60")
Oh, 66sprint, that chart you said is wrong is for tractor tyres, that's why the rim sizes look weird
 

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Discussion Starter #9

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Mydlyfkrizis, thanks for the tire/rim size charts, but I didn't see where you addressed the opening to your discussion: I am one of those people wondering just how wide I can go with a tire/rim on my CB350 cafe build. I'm trying to get the best handling possible. I don't need to stay race legal (I understand that to be 80/90 on a 2.15 rim up front and 110/80 on the rear), so I want all the rubber I can get to the road. For the front, I'm adapting a GSX600 triple tree and fork to get better damping and stiffness along with disk brakes, so there is plenty of room for a tire up there. That fork will also give more trail for stability at speed (racers are running up to 4" of trail).

My question is: how wide can I go on the rear? I've read that some folks go to a 520 chain to get more tire clearance. I want to stay with spoke wheels, and probably want to stay with 18" rims. I believe I can go to a 130/70 on an 18" rim for the rear. Can anyone on the forum verify that? Can I pair that with a 110/80 x 18 up front? Sport racer tires are much more common in 17" sizes; has anyone tried those? How do they look on a CB350?
I'm looking at Battlax tires (running those on my BMW K1200LT with good results). I appreciate any advice you folks can give me!

Ps if this is a duplicate reply, I apologize.
 

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Discussion Starter #11 (Edited)
You are putting a very stiff, high braking force fork on a bike that is not designed for those forces. Look at a GSX600 Frame and steering neck support. Compare to the pressed steel frame on a CB350. A stiffer fork will transmit more shock and loading to the neck of the 350. It will work for a while, but at some point, the neck on the 350 will stress fracture, probably under braking. Not much fun. The frame on the 350 is also quite flexible....there is no way to tell how this will affect the handling, but usually it is negative. Modern bikes use perimeter frames to absorb these forces, not downtube frames. You may find the bike handles much worse than stock. If you have not ridden a well put together stock cb350, then you will have nothing to compare too. You may think it handles well, but probably doesn't.

A K1200 is a bigger, heavier bike. I used Battleax Tires on my CB750, and they worked well. I then replaced with stock Dunlops K505, the OEM tire actually worked noticeably better. I would of said the Battleax tires were great, until I tried the OEM size/model. Manufacturers pick tires sizes and models specifically, often have special tire models purposely built.

You are not going to go fast enough for sport racer tires to heat up well and have good traction. The stock, smaller tires are more loaded and heat up to proper temperature faster. Because something works well on a 100HP 4cylinder bike does not mean they will work well on a 34 HP, lightweight bike.....

So all in all, I do not know how wide can you go, and can't recommend you do it at all.

It is your bike, your life, so do whatever you please.

Not trying to rain on your parade, but trying to convey decades of experience. I have the scars and memories of my attempts. Hopefully no scars for you.
 

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The Kawasaki 750 H2 triple ran a 1.85"wide rim with a 3.25 x 19 front, and a 2.15" wide rim with a 4.00 x 18 rear. Felt like driving on liquid snot. Was called the Widow Maker. I rode a CB 450 for years with 2.00" wide rims front and back with an Inoue 3.50 x18 front and an Inoue 4.00 x 18 back. Had to push the bars to get it off the vertical, then it would fall into the turn. Was very predictable. The flexi flyer frame was more of a problem than the fudging on the tire/rim size. My point, horsepower, frame, frame geometry, rider skill level and handling preferences all go into what works for you. My mentor said everything is a compromise in engineering. Taught me to take out the compromises that didn't work for me, make the ones that do. 4.00 inoue rear with a 3.25 ribbed front felt the best but pushed the front end much sooner than running the 3.50. My working hypothesis from 40 plus years ago.
Ran tubes for smaller cross section tires, adjusted tire air pressure to get the wear pattern I wanted. Made the tire feel as if it was on a wider rim. My opinion.
 

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This helped me to realize that getting a fat tire up front might look cool, magazines and photos...., but if I want to have handling and safety, I would be going in the wrong direction and should at a minimum stick with the stock setup, and for improvement increase rim size. Thank you
 
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