Topic: CB750K2 rear brake engages with passenger

[hotelstationery]

When I got the bike the rear brake pedal had to be pushed a long way before it engaged, so when I put on my new tire recently I adjusted it according to the manual, with the brake engaging at one inch of travel.  I assume that means it starts to engage and not is fully engaged at that point.  I was much happier with it having less travel, being easier and more natural to use.

On Saturday I gave a friend a ride home, my first passenger on this bike.  When she got on the rear brake became engaged and the bike wouldn't go.  It was dark and cold and I didn't have another vehicle so I just unwound the brake adjuster six or so turns and it was enough to be free.  On the way back home it was back to how it was when I got it.

I've never had this happen on any bike before.  I've also never had a bike come with a rear shock adjusting tool, so I've also never adjusted the rear shocks before.  Is that all that is required?  Is this the right sort of tool?

http://www.z1enterprises.com/ItemDetails.aspx?itemDescription=Spanner+Wrench&item=TU15-1698

What else could have caused this?  I'm a gentleman and didn't ask, but I'm guessing the passenger was a maximum of 140 pounds.

jeremy

[Don R]

I'm not sure, but check your brake stay and auctuator rod very closely. Also be sure the brake mechanism isn't sticky in the housing. Also have you inspected the shoes?  The old ones delaminate and could lock up.

[PeWe]

It must be the rod that is too tightened. It have to be adjusted when wheel is moved backwards due to chain slack or additional weight as a passenger.

[hotelstationery]

I had the shoes out for inspection a couple of weeks ago and they are fine.  The rear shocks are on the middle setting, so I guess I can stiffen them up a bit and maybe it won't be so bad.  The brake stay and actuator appear fine, what sort of thing would I be looking for?  When I replaced the rear wheel last week I set the chain slack and adjusted the rear brake according to the manual, so it starts to grab at one inch of brake travel.  Maybe I'm too sensitive and need to back it off more.

jeremy

[bs175dths]

The chain needs to be adjusted with the weight of the rider on the bike, but I have never heard about the brake needing the weight to be adjusted.

If you have it adjusted as you like it (adjusted per the manual) see if you can get some volunteers to sit on the bike while you look at how the additional weight affects the rear brake rods and such.  Something is amiss if the added weight of any passenger (being polite, and I don't even know the lady, ) causes the brake to be activated.

What I am imagining is, the brake is somehow rotating clockwise (while facing the right side of the bike) when a passenger is added.
Maybe look at your brake stay and tighten the fasteners.
I am not too familiar with the CB750 family, but is it possible, a shorter version of the brake stay has been replaced on your bike?
Or, is the brake pedal somehow being depressed when a passenger is added?

Just thoughts, but they may give you a direction.
Good Luck and give us updates.

-Ride On

[hotelstationery]

I know the chain is supposed to be adjusted with a rider on board, but I have a shortage of volunteers, so I just do it on the center stand.  I will try to rustle up a helper or two and see how the parts move under weight.

[754]

Check the angle of the brake arm, and look at the wear indicator.. If the torque. Arm is the wrong length it might affect it. A pic of the setup would help.

[hotelstationery]

Here are a few pictures, I hope they capture enough to get an idea of what is going on.  As far as I know the bike is stock all around but since I'm not the original owner it's hard to say for sure.  The adjuster nut on the end of the actuator arm is the only thing I've moved at all.  I guess the brake light switch as well, but I don't think that's the problem.

IMG_20141105_191101288 by JeremyIllingworth, on Flickr

IMG_20141105_191226256 by JeremyIllingworth, on Flickr

IMG_20141105_191138871 by JeremyIllingworth, on Flickr

The wheel was off last week and everything is on nice and tight.  I've never noticed any of the little arrows for the brake wear indicator on the drum.

jeremy

[70CB750]

Cant' tell but did you line up dimples on the arm and on the shaft?

[DH]

If the pic shows brake components at rest, i'd examine the internal components (shoes, etc.) again. The manual says that the arm and rod should form an 80-90 degree angle with brake applied. Pic is showing about 90 at rest. Internal actuating cam may be rotating too far, causing it to hang up in an "over center" position.
Wear of the shoes at the point where they contact the cam combined with wear of linings and overcompensation of all related adjustments might be a cause. That indicator is on the arm  at the top, a small red arrow that should not line up with its matching index mark when brake is fully applied. In an extreme case, the pedal could've been installed on its shaft in a manner that alters the function of the shoes, (jumping a tooth) on the shaft to gain more adjustment. All assuming you have a stock non lowered suspension.

[madmtnmotors]

Put it on the centerstand, remove the shocks, then move the rear wheel/swingarm up and down. Watch for binding on the linkage between the drum hub and the pedal. My money is that something is binding on the brake link arm (the arm with the adjuster nut and spring at the hub). That stay arm bolt (the fixed arm that is bolted to the hub with a cotter pin through the bolt) looks pretty close to the link arm. Does the stay bolt hit the link arm when the wheel is raised (simulating a compressed suspension with the shocks removed as described above)?

Also make sure the rear axle is tight making sure the wheel can't move fore and aft. This would throw the brake adjustment out of whack too.

[MCRider]

Yes it is very true that due to the geometry of the parts involved the brake becomes tighter as the wheel moves up. The only way to fight this is to tighten up the shocks. IT was part of my pre-ride to adjust the brake according to the load anticipated. Or just live with the situation.

Jokermachine has a really cool adjuster that's machined with a large diameter knurled knob that makes this easy.  Also adjust the brake light accordingly.

Be sure to check all the things suggested, line up of the dimples, good meat on the shoes etc.

[madmtnmotors]

Yes it is very true that due to the geometry of the parts involved the brake becomes tighter as the wheel moves up.

I have never experienced this and would seem to be a dangerous condition if the OEM design exhibited this sort of behavior. Something is wrong for this condition to exist, at least to the extent described by Jeremy.

[madmtnmotors]

Do you have stock exhaust? If not, what sort of centerstand stop do you have to prevent the centerstand from hitting the chain when in the up position? The stock exhaust should prevent the centerstand from hitting, aftermarket exhaust typically do not and require separate stop bracket, usually an "L" angle off the left rear footpeg.

[madmtnmotors]

Put it on the centerstand, remove the shocks, then move the rear wheel/swingarm up and down. Watch for binding on the linkage between the drum hub and the pedal. My money is that something is binding on the brake link arm (the arm with the adjuster nut and spring at the hub). That stay arm bolt (the fixed arm that is bolted to the hub with a cotter pin through the bolt) looks pretty close to the link arm. Does the stay bolt hit the link arm when the wheel is raised (simulating a compressed suspension with the shocks removed as described above)?

Also make sure the rear axle is tight making sure the wheel can't move fore and aft. This would throw the brake adjustment out of whack too.

BE SURE TO PUT THE SHOCKS BACK ON BEFORE MOVING OFF THE CENTERSTAND! Otherwise you'll have a heck of a time getting it back up...


(I hate it when that happens) 

[MCRider]

Yes it is very true that due to the geometry of the parts involved the brake becomes tighter as the wheel moves up.

I have never experienced this and would seem to be a dangerous condition if the OEM design exhibited this sort of behavior. Something is wrong for this condition to exist, at least to the extent described by Jeremy.

I do think I'm right on this, though open for discussion. The first time I noticed it was back in the mid-70s. Like the OP, I like to set up the rear brake tightly. I was traveling at high speed on a smooth road. I went through a dip that compressed the rear suspension to bottoming. At the same time I felt the bike slow down for just a moment. Then all was normal again.

It goes with the same phenomena that the chain will tighten and loosen as the wheel goes up and down. I think we are all in agreement n that. There is nothing wrong, its just a function of the compromise in geometry. IF you think of it, the pedal pivot is fixed on the frame and can't move. The wheel travels up through its arc and increases or decreases its distance from the brake pivot, just like it does re the swingarm pivot. This resutls in a pull or release on the brake arm on the brake plate.

So most of us just unconsciously set the brake at a  compromise that minimizes this feeling. The test is to remove the shocks and lift the wheel through its travel and observe. Though I have not done this, I am confident from my observations of what I would see.

(I see that MadMntn suggests the same thing, but he is looking for something wrong. I submit that with everything right, you will see what I'm getting at, the stationary brake pivot will pull and release on the brake arm on the brake plate. If you were also pressing down on the pedal while the wheel is going up, you will see the brake tighten up.)

This is at least partial explanation for rear wheel hop when applying the brake hard on a bumpy road. The brake alternately locks and releases.

When I was roadracing I had to do the same thing. If it was a smooth track, you could tighten up the brake pedal lash. If it was bumpy, you had to allow more lash for the deeper wheel travel. When heeled over in a bumpy turn you can feel the wheel going up and down telegraphed through the brake linkage.

As an aside, rear disc brakes don't have this feeling as they are hydraulic and don't have to deal with compromises in geometry.

[MCRider]

BMW tried to address this problem by making the relationship of the parts, brake stay, pull rod etc, into a true parallelogram. Sadly the parts on the CB750 are far from that.

[754]

A bit off topic Ron, but sight your sprocket centers, you should find that the swingarm pivot is slightly above a line between sprocket centers .( At least I think it should be)
 Depending on where this is will affect wether chain tightens or not during riding.
 Make sure you have slight slack at tightest point of chain, when seated on bike..

[cougar]

I absolutely agree with MCRider! In my younger years I raced dirt bikes and noticed (as he mentioned) the braking changing as the wheel moved up and down (with OEM all steel linkages). I removed the adjustable brake activation rod and replaced it with my own design cable activation unit. No more brake intermittent brake locking as the suspension moved. So it would be a matter of adjusting the actuating rod to suit how your going to be riding, 1 or 2 up.   ...cougar...      P.S. Or you could alter the bike to use cable operated or hydraulic brakes. PITA

[MCRider]

I absolutely agree with MCRider! In my younger years I raced dirt bikes and noticed (as he mentioned) the braking changing as the wheel moved up and down (with OEM all steel linkages). I removed the adjustable brake activation rod and replaced it with my own design cable activation unit. No more brake intermittent brake locking as the suspension moved. So it would be a matter of adjusting the actuating rod to suit how your going to be riding, 1 or 2 up.   ...cougar...      P.S. Or you could alter the bike to use cable operated or hydraulic brakes. PITA

Thanks. I meant to include cable with my comment on hydraulic. This may be at least part of the reason Honda converted the CR750 from the OEM rod to a special cable on the rear brake.

In practice, under hard braking the bike will pitch forward and extend the rear suspension. This direction is opposite of the direction when the brake would tighten up. In fact pitching forward reduces the pedal input transferred to the rear brake arm, which is the safer direction.

But anything that compresses the suspension, like a full load, will tend to move towards locking the brake.

[MCRider]

A bit off topic Ron, but sight your sprocket centers, you should find that the swingarm pivot is slightly above a line between sprocket centers .( At least I think it should be)
 Depending on where this is will affect wether chain tightens or not during riding.
 Make sure you have slight slack at tightest point of chain, when seated on bike..

Correct, I'm onto that. Usually most manuals say set the chain with weight on the bike, which puts the wheel at its furthest distance from the CS sprocket.

[madmtnmotors]

I understand the offset geometry, the forward pivot of the brake activation linkage would have to line up with the swingarm pivot. Not very practical, so compromises have to be made. Same holds true for the chain and sprockets. My contention is, that this compromise would not "lock up" the rear brake simply with the addition of a passenger, even if the shocks were shot and the suspension bottomed out, provided the brake pedal free-play is properly set. Certainly not to the point that the adjuster would require "six turns out". I have had the suspension bottom out while carrying a passenger and have never had a rear brake lockup as a result.

Something is wrong.

[MCRider]

I understand the offset geometry, the forward pivot of the brake activation linkage would have to line up with the swingarm pivot. Not very practical, so compromises have to be made. Same holds true for the chain and sprockets. My contention is, that this compromise would not "lock up" the rear brake simply with the addition of a passenger, even if the shocks were shot and the suspension bottomed out, provided the brake pedal free-play is properly set. Certainly not to the point that the adjuster would require "six turns out". I have had the suspension bottom out while carrying a passenger and have never had a rear brake lockup as a result.

Something is wrong.

He needs to check everything that has been suggested, no quarrel there. We don't know if maybe he has some shorty shocks and a lowering kit, or what other variables there may be. And from what's been reported here lately, seems 40 year old shoes are coming apart from age.

However, my money is on, yes it could be made to lock up, if the adjustement was too tight to begin with. His was not an issue of locking up after being underway. He couldn't even get it to move, after being loaded. I wonder if his "six turns out" is six full turns or 6 clicks. 3 full turns is within the range I'm speaking of.

We'll never know till we hear back from the OP.

PS: I see from the pics, no lowering kit. He had the wheel off for a tire, so all bets are off till its gone over closely.

[DH]

For the guys who know this system well....I don't know if this is significant, but i just looked at the pics again, and didn't see the small splined washer/dust cover that has the little indicator arrow stamped into it, Possibly a previous owner left it out ? Or did the k2 even have an indicator? (honestly don't know) Makes it a little suspicious that someone may have been in there before though, Eh?

[754]

Is the front still mounted to tye original mount.
 On dirt bikes the brake anchor is often mounted to frame not swingarm. The way I read about it, said less chance of lockup, while going over bumps..