Topic: Rear wheel rubbing frame - Options? Longer rear shocks?

[the architect]

My build is 90% there.  But, I still have a few details to attend to.  Getting it running this weekend revealed another detail that needs attention.  When I did my frame design, I was certain I did the necessary diligence to ensure I had clearance at the back to allow for suspension travel.  Well . . . .  famous last words, right? 

With one rider, the rear wheel rubs the frame on bumps if set to anything besides the stiffest pre-load setting.  With a passenger, bumps cause rubbing at even that setting.

1975 CB750K.  Rear shocks are stock, just cleaned up.  Swingarm is rebuilt and functions as it should.  Front forks have been rebuilt with Progressive springs, new seals and 10w fluid.

What are my options?  I designed the bike with 2-up riding in mind.  The stock rear shocks measure 13-5/16" center of eye to center of clevis, and I'd like to pick up 1/2" to 1" of additional clearance, just to be safe.  Can I find longer shocks?  I've looked a bit but turned up nothing, yet.  Or, can I get shocks with less travel?  Can I modify my existing to work?  I thought about unscrewing the top eye mount (it's threaded on) and shimming it (washers, spacers, nuts, something) to pick up some additional length.  I could also conceivably shim it between the top of the spring and the top spring seat (inside the shock cap cover thing) to slightly reduce travel. 

I'm trying not to spend much to make this work.  Any thoughts are sincerely appreciated 

[domer]

rear shocks could be worn... but you dont have #$%* for clearence to begin with! longer shocks i think are your only choice... that or hard tail the sucker... should have bent the frame to conform with the tire, but too late for that.
im not an expert, but maybe some one can chime in on the effects of longer shocks on steering geometry, cant imagine much harm, but with 2 up im not sure just a half inch will do it.
maybe rebuild the current shocks with stiffer springs?
just brain stormin here...

[steam-powered man]

move the lower shock mount ahead just a bit, will gain some.  popular mod on bultacos, CZ's too i think.

[the architect]

I can't do any mods to the frame or mounting points.  The frame is freshly powdercoated, and I'm not about to destroy that new finish -- it's never even seen the open road 

When mocking up the build, I had the rear shocks on the bike with no springs, which allowed me to fully compress the shock.  From there, I calculated how much I *thought* I needed to compensate for the amount of space a fully-compressed spring would need.  Needless to say, I left too little room    It's funny, too (and not in a good way) -- I had planned on welding that rear frame hoop sloping up, to clear the wheel.  But then, my ciphering lead me astray . . . . for shame.

[the architect]

move the lower shock mount ahead just a bit, will gain some.  popular mod on bultacos, CZ's too i think.

shoe -- are you thinking there could be some adapter bracket or something to accomplish this relocation?

[MCRider]

I can't do any mods to the frame or mounting points.  The frame is freshly powdercoated, and I'm not about to destroy that new finish -- it's never even seen the open road 

When mocking up the build, I had the rear shocks on the bike with no springs, which allowed me to fully compress the shock.  From there, I calculated how much I *thought* I needed to compensate for the amount of space a fully-compressed spring would need.  Needless to say, I left too little room    It's funny, too (and not in a good way) -- I had planned on welding that rear frame hoop sloping up, to clear the wheel.  But then, my ciphering lead me astray . . . . for shame.

Generally there is a rubber bumper on the shock shaft, which is there to prevent full shock compression. But the fact that its there would indicate that the spring will not reach coilbound and keep the rubber bumper from coming into play. Hence there should be no allowance given to the travel limitations of a potential coilbind.

Sorry for your trouble. I see so many bikes that have been lowered or altered in such a way that I think they must have clearance problems, like with the edge of a license plate, or an inner fender.

If not moving the top forward, maybe moving the bottom forward? Like with a reverse lowering kit.

Dang, somebody here had some shocks that came with length adjustment. Not much, but you may not need much. Seems the shocks carried 3 letters in their name. Not cheap.

[MCRider]

Also, going to a 17" wheel with low profile tire, certainly along with the longer shock, would gain clearance without kicking the a--end up so much.

A 16" with low profile would do it too, but I hate 16"ers. The usual 16" conversion uses high sidewall tires and you don't reduce the overall diameter by much if any.

The 17 is standard on so much, even late CBs.

[MCRider]

Shimming the spring won't reduce travel unless you induce coilbind.

[MCRider]

here's the shocks. YSS  Buy the length to fit and it comes with an adjuster to boot.

http://forums.sohc4.net/index.php?topic=55256.0

[the architect]

here's the shocks. YSS  Buy the length to fit and it comes with an adjuster to boot.

http://forums.sohc4.net/index.php?topic=55256.0

Wow -- those are super fancy and, unfortunately, out of my price range. 

I appreciate all the tips.  You're right about shimming the spring.  It would act to prematurely induce coilbind.  The rubber bumpstop isn't reached at full suspension compression, in its current state.  If I tried this, is coilbind an issue/problem?

The reverse lowering kit may have some potential, even if just a stop-gap measure until I can get some longer shocks.  The bike actually had a lowering kit on it when I bought it.  I didn't realized what the lower brackets were, so I sent them with everything else to get powdercoated.  So, they even match the frame and swingarm.  I may give this a closer look this evening.

I've had some limited success (hit and miss) at finding adjustable length shocks.  We'll see if some of these options might pan out.  I like the idea of adjustable height shocks, so I can fine-tune the rear height to just clear the frame without being locked into a set height that may or may not be optimal.

So, what about the idea of shimming the top eye mount?  I'd hate to decreace the thread engagement at that mount, but it could pick me up some critical fractions of an inch.  Drawbacks?

[Duke McDukiedook]

You'll have to chuck those stock shocks anyway, they turn to crap after 5,000 miles on average.

Honda had to cut some corners to get the bike at the $1500 price point.

[the architect]

You'll have to chuck those stock shocks anyway, they turn to crap after 5,000 miles on average.

Honda had to cut some corners to get the bike at the $1500 price point.

Good to know!  The bike shows 20k on the odometer, so if they're still original, they're exceeded their usefulness x4 

[andy750]

Ive had good success with 1982 CB750 shocks - they are quite firm and better than the stocks for 2 up. I have them on my road racer and can go 2-up no problem when they are on the firmest setting. For riding solo I turn them down a notch. See pic.



cheers
Andy

[the architect]

Thanks, Andy!  I was just about to ask about the DOHC shocks.  Dynoman's website is showing 2 different part numbers for Progressive shocks, depending on whether you're looking for a sohc or dohc bike:  http://www.dynoman.net/chassis/progressive.html

Do you happen to know the center of eye to clevis dimension of your shocks, Andy?

I'm comparing similar photos, and it's hard to visually determine whether there's much more clearance.

[MCRider]

here's the shocks. YSS  Buy the length to fit and it comes with an adjuster to boot.

http://forums.sohc4.net/index.php?topic=55256.0

Wow -- those are super fancy and, unfortunately, out of my price range. 

I appreciate all the tips.  You're right about shimming the spring.  It would act to prematurely induce coilbind.  The rubber bumpstop isn't reached at full suspension compression, in its current state.  If I tried this, is coilbind an issue/problem?

The reverse lowering kit may have some potential, even if just a stop-gap measure until I can get some longer shocks.  The bike actually had a lowering kit on it when I bought it.  I didn't realized what the lower brackets were, so I sent them with everything else to get powdercoated.  So, they even match the frame and swingarm.  I may give this a closer look this evening.

I've had some limited success (hit and miss) at finding adjustable length shocks.  We'll see if some of these options might pan out.  I like the idea of adjustable height shocks, so I can fine-tune the rear height to just clear the frame without being locked into a set height that may or may not be optimal.

So, what about the idea of shimming the top eye mount?  I'd hate to decreace the thread engagement at that mount, but it could pick me up some critical fractions of an inch.  Drawbacks?

Yeah those YSSers are droolworthy.

Do you mean the rubber bumpstop isn't reached because the wheel hits the frame?  Bumping that stop is the goal. I can't imagine any shock/spring combo that would allow the spring to coilbind before hitting the bumpstop. No first hand knowledge, just can't imagine it. I think that would be a dangerous condition. Breaking the spring would be my fear.

With the lowering blocks in place, they should bolt to the shock mount and the chain adjuster stop bolt, if you could weld a mounting point to the front of the bracket, in front of the current original mounting point, mount the shock to that. But I bet even a small move forward will really kick the tail up.

Shimming the top eye, I heard somewhere a long time ago for a threaded joint to be effective you need to engage 8 threads. More is little advantage, less gets weak quickly. No documentation on that, just picked it up somewhere.

[the architect]

May have answered my own question, regarding the length of the '82 shocks:

http://www.bikebandit.com/1982-honda-cb750-shocks/c/a643958?mg=995&t=1

Bike Bandit is showning them available at 13" and 14" lengths.

[the architect]

here's the shocks. YSS  Buy the length to fit and it comes with an adjuster to boot.

http://forums.sohc4.net/index.php?topic=55256.0

Wow -- those are super fancy and, unfortunately, out of my price range. 

I appreciate all the tips.  You're right about shimming the spring.  It would act to prematurely induce coilbind.  The rubber bumpstop isn't reached at full suspension compression, in its current state.  If I tried this, is coilbind an issue/problem?

The reverse lowering kit may have some potential, even if just a stop-gap measure until I can get some longer shocks.  The bike actually had a lowering kit on it when I bought it.  I didn't realized what the lower brackets were, so I sent them with everything else to get powdercoated.  So, they even match the frame and swingarm.  I may give this a closer look this evening.

I've had some limited success (hit and miss) at finding adjustable length shocks.  We'll see if some of these options might pan out.  I like the idea of adjustable height shocks, so I can fine-tune the rear height to just clear the frame without being locked into a set height that may or may not be optimal.

So, what about the idea of shimming the top eye mount?  I'd hate to decreace the thread engagement at that mount, but it could pick me up some critical fractions of an inch.  Drawbacks?

Yeah those YSSers are droolworthy.

Do you mean the rubber bumpstop isn't reached because the wheel hits the frame?  Bumping that stop is the goal. I can't imagine any shock/spring combo that would allow the spring to coilbind before hitting the bumpstop. No first hand knowledge, just can't imagine it. I think that would be a dangerous condition. Breaking the spring would be my fear.

With the lowering blocks in place, they should bolt to the shock mount and the chain adjuster stop bolt, if you could weld a mounting point to the front of the bracket, in front of the current original mounting point, mount the shock to that. But I bet even a small move forward will really kick the tail up.

Shimming the top eye, I heard somewhere a long time ago for a threaded joint to be effective you need to engage 8 threads. More is little advantage, less gets weak quickly. No documentation on that, just picked it up somewhere.

I'll have to remove and count the available threads for the eye mount, tonight.  Thanks for that 8-thread go-by.  I know it's just word-of-mouth / rule-of-thumb, but I'll still take a look.

Yeah, the tire its the frame before the bumpsstop engages.  Maybe I can shim the bumpstop to limit travel.    It's nice that a solution like that would be completely hidden from view by the shock cover, too = still looks clean.

[MCRider]

Sounds like you've got some shock alternatives.

From your picture, I think if you moved the lower forward you may actually have to go with a shorter shock! In other words, you'll gain too much clearance. Not bad on a dirtbike application.

Stacking up another bumpstop on the shock shaft would give some relief to the tire, but those bumps are going to take some mighty hard hits on a day long ride.

I still vote for a 17" wheel with low profile tire. I've seen them think they look good on a cafe racer application.  Its always easy to spend the other guys money.  

[andy750]

I think those 1982 CB750 shocks are ~13` (not 14` though - I tired the 14` ones with the piggy back reservoir but found that with everything else stock I would get some back-end hopping on severe braking likely due to the trail being off. 2 up that wasnt a problem but just remember to turn them down when going solo).

Here is a bad pic of the 13` progressive shocks on my K4 - they give much the same clearance as stock - Ive used them for 2 up (with camping gear) with no issues.  It looks to me as though your frame to rear tire height is particularly short...maybe an optical illusion.

My understanding was that if jacking up the rear by an inch then you have to lower the front forks by the same amount...you need to consider trail and rake when adjusting front and rear. Maybe Ron (MCrider knows more about this than me).

Andy

[MCRider]

I'm certainly not a rake/trail theory expert, just seat of pants. If the rear goes up, it will turn in quicker, done on racers a lot. And increases ground clearance if that's an issue, again often on racers. Dropping the front = raising the rear except for ground clearance goes the other way.

But all else equal, if rear goes up, I'd raise the front. Which is difficult to do. Which leads me to a smaller rear wheel/tire combo to restore the original ride height and rake/trail.

But top priority is to get the tire to clear. a couple of real good thwacks and the tread can come undone.

[number13]

Hi Architect -

Been following your build, we are almost neighbors,
I live in Third Ward. Regarding your clearance problem, is
a longer swing arm an option? I know there are different
lengths for 750s, but can't recall which is longest.

[Toxic]

Shocks from an early 80's CB900F are about 1 to 1 1/4" longer.

Let me know if you want me to measure exactly.

They can be found real cheap and I got mine for $10 in real good shape.



Obviously mine are moved back some.

[the architect]

Shocks from an early 80's CB900F are about 1 to 1 1/4" longer.

Let me know if you want me to measure exactly.

They can be found real cheap and I got mine for $10 in real good shape.



Obviously mine are moved back some.

Toxic --

Absolutely!  Please measure and post your dimension.  I would appreciate it greatly.  Have you ridden it with those shocks on it?  If so, how's the feel?

Hi Architect -

Been following your build, we are almost neighbors,
I live in Third Ward. Regarding your clearance problem, is
a longer swing arm an option? I know there are different
lengths for 750s, but can't recall which is longest.

number13 !! -- You're in the 3rd Ward!?!  We are practically neighbors!  I live in the Heights and I work in the Sixth Ward.  I'd still love to get a Houston CB ride to happen sometime.  I think we have enough people to make it fun.

[Toxic]

eye to eye

cb900f  14.5  VS  13 "

Now I can't say for sure that the 13 inchers that came off the project were stock or not.

In the pic, my rear mounting point is 5" rearward.

Sorry ask me in a month about the ride.  I'm getting close but not quite there yet.

I'm going to road test it with both riding positions so I'll post my impressions.
The original mounting point is still on the swingarm.

[MCRider]

Architect: that 14.5 could be the ticket. If too long maybe you could use your lowering blocks too.

For discussion purposes, changing the angle of the shock changes its effective damping characteristics. Has to do with the amount of shock travel that occurs per the amount of wheel travel.

But your first priority remains tire clearance.