Topic: Tire Envy. Rear tire enhancement plan...

[gregimotis]

So, I'd like to put a wider rear tire on my CB750 F1.  I can see It'll never take a modern sport tire, but man it's a slim little thing.  Heck, My GF's Honda Rebel has a wider tire than mine.

As I eyeball the situation the limiting factor is not the frame, but rather it's that bar which holds the rear brake in place.  There is clearance to the right (that's the frame side) to move that bar over a bit.  I haven't measured but I bet it's enough to go one wider on the tire.

Here's is my highly developed plan - The left side is stock, to the right is my proposed alteration. The tire is a cutaway of the front.  We are looking straight down through the bike.  (looked better on my desktop, but I think you can figure it out):




As I've said, I haven't measured yet - I'm more wondering If given a good fabrication and good steel, this could be strong enough to support braking.  For anyone who isn't aware, this bar prevents the rear brake from simply spinning with the disc and so supports the full brake load.

I mean, people chop frames all the time, right?  I could install it for testing and always go back to the original if it started to buckle...

[babyfood1217]

If you have got some scrap steeal around give a go, but be aware of a couple things...

The stock stabilizing bars are stamped in the center, making them stiffer, and slightly stronger (through work hardening.)
Flat stock will most likely be fine, but unless you have a way of stamping that groove into the new piece, you may want to consider either using thicker bar stock (heavier) or perhaps fabricating something else.  The force of the wheel on that bar while the brake is being applied is a pulling force (although I have seen setup where the bar/linkage is on top), so if you simply put a few bends in the bar, wouldn't the force being applied to it when the brake is in use make those bends want to straighten?  Is that force strong enough to consider this an issue, or am I letting my shoddy physics knowledge get the best of me. 

If it does work without a problem, then I'll do it too!

-chris

[eldar]

Well the 75/76 F bikes had the same size rim as the K bikes up to 76. I think. So I dont know how wide you can go with a tire. There has been plenty of discussion about it but most seem to figure a size up, maybe 2 depending on the maker, may be possible. If you can, maybe check with a shop and see if you can trial fit a tire or 2 and see if it is ok.

[Roach Carver]

when braking that bar will be in tension I believe. that wont be a good situation for a bar bent in two places. I would think if the force was strong enough it would try to pull the bar straight and into the tire. have you given any thought to moving the front bracket over?

[Roach Carver]

Also, the tire width is going to be determined by the rim width. I purchased a wheel and tire combo for my bike. Its a 3.5 x18 rim with a 140/70 tire. It fits the swingarm but just barely. everything else will have to be redone and adjusted for clearance.

[Sam Green Racing]

when braking that bar will be in tension I believe. that wont be a good situation for a bar bent in two places. I would think if the force was strong enough it would try to pull the bar straight and into the tire. have you given any thought to moving the front bracket over?

It's 4 am over here and I am very tired so my brain might be a bit scatty    Dose the F1 not have the brake anchor above the swing arm unlike the Ks drum brakes below the arm...to me that would be pushing , not pulling

Sam.

[Roach Carver]

when braking that bar will be in tension I believe. that wont be a good situation for a bar bent in two places. I would think if the force was strong enough it would try to pull the bar straight and into the tire. have you given any thought to moving the front bracket over?

It's 4 am over here and I am very tired so my brain might be a bit scatty Dose the F1 not have the brake anchor above the swing arm unlike the Ks drum brakes below the arm...to me that would be pushing , not pulling

Sam.

I missed that little piece of info. I dont have one of the fancy f's. If that is the case then I am wrong. That will never happen again

[Sam Green Racing]

 

[Quail "Owner of the comfortable k8"]

I had a 150/60/17 HD dunlap tire on my 78.  It just cleared a bent bar that worked just fine.  Then the center wore out 2000 miles later at 25 psi.  So I went down a few sizes and got 3500 mile out before it was bald in center and so chopped that the vib was numbing.  Then I read a post by Hondaman and realized all the funny handleing problems I had since I bought the bike was noted in his post about why wide tires do not work.  I bought the right tire and guess what?  The people that designed the bike put the best tire on it!  We Yanks love big tires.  It makes up for our small shoe size and short....  and YES thats why I drive a Vette.

[CB750R]

I'm running a 160/70 18, rear on my F2 with no rubbing, mind you I had to massage the brake caliper stay with a mallet, that sucker is rather thick walled tubing. I'd say just flatten the section out where its close the the tire and call it a day!

[KB02]

when braking that bar will be in tension I believe. that wont be a good situation for a bar bent in two places. I would think if the force was strong enough it would try to pull the bar straight and into the tire. have you given any thought to moving the front bracket over?

It's 4 am over here and I am very tired so my brain might be a bit scatty    Dose the F1 not have the brake anchor above the swing arm unlike the Ks drum brakes below the arm...to me that would be pushing , not pulling
Sam.

Yes, the F1 (Disk Brake) has the brake arm at the top of the brake (above the swing arm) unlike the K's (Drum brake) which is under the swing arm, so it is being pushed and not pulled under load.

I had a 150/60/17 HD dunlap tire on my 78.  It just cleared a bent bar that worked just fine.  Then the center wore out 2000 miles later at 25 psi.  So I went down a few sizes and got 3500 mile out before it was bald in center and so chopped that the vib was numbing.  Then I read a post by Hondaman and realized all the funny handleing problems I had since I bought the bike was noted in his post about why wide tires do not work.

You won't happen to know right where to find that post, would you? I'm picking up a "parts bike" next week that might turn into a "project bike." I was thinking of going "fat" all over with it, but that might talk me out of it...

[gregimotis]

I'm running a 160/70 18, rear on my F2 with no rubbing, mind you I had to massage the brake caliper stay with a mallet, that sucker is rather thick walled tubing. I'd say just flatten the section out where its close the the tire and call it a day!

So - thats interesting to me.  I run a 110/90 18.  Just eyeballing it, I've got not more than 3/4"(19mm)  clearance to the stay bar.  Given that, I should be able to get a 130 in there (10mm on each side) right now.  You got a 160 with a little help from the 'ole John Henry.


Can anyone tell me if the F1 and F2 are the same in this respect?

Could someone kindly give me a short cause and effect lecture as far as tire width/height and handling?

[ProTeal55]

I gots a 130mm on the back of my CB750 and it is as close as you can get (clearance wise)..

[eldar]

The later K bikes and maybe the F bikes seem to have more clearance. I have a 130 and there is still a little room in the swingarm.

[ohiocaferacer]

I've got a 130 on my CB500/4.

[gregimotis]

The frame around the tire has loads of clearance, it's this brake stay that limits things.  I think I'll measure it later today for a 130 - I'd really rather not mod the thing that holds the brakes in place if I can help it.   

I do have a pretty good relationship with a shop here, I'll get an opinion on the wheel vs. tire question as well.

[mlinder]

One thing the tire guy and I noticed, replacing my old 110 on the rear of my bike, that the very old 110 and a new avon 120 were nearly identical in width.
It was pretty weird. We went with the 120, because it fit, looked good, and as I said, was the same width as the ancient 110 on there. :/

[eldar]

Well I have this idea that just like shoes and pants, one manufacturer's size is not the same as the others. Just a theory though.

[mlinder]

Well I have this idea that just like shoes and pants, one manufacturer's size is not the same as the others. Just a theory though.

I think you're right, for sure. This guy said he'd noticed a trend though, that older tires of the same size in MANY brands were wider than their current counterparts.

[merc2dogs]

I think the manufacturer has a lot to do with it, I have two bikes with 120-17's , and there is a noticeable difference in size.

Ken.

[crazypj]

I'm running a 160/70 18, rear on my F2 with no rubbing, mind you I had to massage the brake caliper stay with a mallet, that sucker is rather thick walled tubing. I'd say just flatten the section out where its close the the tire and call it a day!

.




Could someone kindly give me a short cause and effect lecture as far as tire width/height and handling?

 If you fit a wide tyre to a rim thats too narrow it only increases the height of tyre, it wears out faster and it flexes more (which does nothing to improve handling.)
 If you cant fit wider rims, stay with a tyre thats close to stock size. If you have a look at rim width on G F 250 you'll see its wider than stock 750 ( probably 2.50 X 16 compared to 1.85 or 2.15 x18)
PJ

[gregimotis]

Quote from above post: If you fit a wide tyre to a rim thats too narrow it only increases the height of tyre


That's an interesting point.  I really don't want to replace the rims unless I could get wider, spoked, stock looking ones.  Any of you folks running 130's experience this, or will the rim allow the extra 20mm expansion?

[dusterdude]

have a 130-16 on my k,plenty of room in the swingarm,but had notch the chain guard and move the brake rod over a tad.

[eldar]

130s go on mine with no mods at all. The rim on the later Ks is wider, 77/78. So everything was probably offset more to accommodate a wider tire.

[ProTeal55]

Do your homework and you might be able to fit one of these bad boys on your scoot --->