Topic: Underslung Rear Caliper

[MidnightLamp]

Hey chaps,

Figure I'd post this here, as it's a somewhat more involved swap, and likely for those a little more "racy".

I was over at a friends house, who has just finished parting an uninsurable CB900C, and his rear caliper was left over. Looking at the rear wheel, I realized the shaft drive causes the brake and drive to be swapped from SOHC 4's. Now this has the effect of effectively reversing the arrangement of the brake caliper, such that when mounted on a CB750 rear disc arm (F), it is unmountable. However, when mounting to a K arm, the torque arm is in the correct location.

Advantages to this setup are:
-loading the arm in tension rather than compression (avoiding buckling and related flexing).
-lower weight distribution on rear end
-ability to use K swingarms (much cheaper than F ones)
-900C calipers, complete, can be had for $25 as opposed to close to $75 for the 900F calipers.
-Axle size is matched to 750F
-Small shims needed to line up caliper and disc

Parts that would be needed:
-Early or late K swingarm
-Some sort of disc carrier on the rear wheel
-F style torque arm
-Bushing for torque arm on K swingarm

My main concern would be cornering clearance, but this can be mitigated by using a single piston rear from the 900C's (80/81) instead of the twin piston rear on the 82. I have all the parts, can take some pictures later today!

Food for thought anyways, since a lot of people ask about disc conversions and the parts are getting harder to find.

Cheers

[Sam Green Racing]

You might run into problems with the alignment of the left hand shock and the chain.
I'm sure it's been done before but while you have both frames to hand check out if there are any diferences.

Sam.

[MidnightLamp]

Sam, I'm assuming you mean from the caliper offset pushing the wheel to the left on the swingarm? My front end won't allow any tire track offset so I made sure to check that. I've got a spacer on the sprocket side that centers the wheel along the headstock centerline. Based on this, the caliper fits well, but does need a spacer on either side of the caliper mount, as the width is not sufficient.

On my k2 frame, the bigger issue is the lack of frame clearance for a later swingarm (passenger footpeg mounts. In case it wasn't clear though, this setup will require a custom torque arm for the earlier shorter k swingarms.   

[Sam Green Racing]

That's not what I was thinking. Measure the inside distance where the spindle goes through the arm, Is the K arm not narower than the F2? or am I barking up the wrong tree?

Sam.

[H2Eric]

That's not what I was thinking. Measure the inside distance where the spindle goes through the arm, Is the K arm not narower than the F2? or am I barking up the wrong tree?

Sam.

Yes, it is

[MidnightLamp]

Sam: Just to verify, this is trying to avoid the use of the F arms in the K-chassis. The CB900C brake is reversed, and therefore will run underslung and not work with the F arms. This is to allow the use of the original K arm, or even a later K / hondamatic arm with a disc hub (kz hub, f1 hub, etc...). It's mainly just because I had the parts around, and the swap is a little pricey otherwise... =P

Picture time:

Firstly, a '76F wheel assembled with the '82 900c caliper (free caliper...courtesy of the 'If you throw out this pile of stuff, you can have the caliper' clause from above!  )

http://i1140.photobucket.com/albums/n576/Aenimal/IMG00283-20110912-1615-600.jpg
http://i1140.photobucket.com/albums/n576/Aenimal/IMG00282-20110912-1615-600.jpg

Next, a comparison of the 76F swingarm with a K3 arm. Both arms are placed with the ends of the swingarms touching on either side, to show the width differnce. It is under 3/8" of difference. between the two, and the difference is mainly due to the thickness of the chain adjuster part of the K arms. 

http://i1140.photobucket.com/albums/n576/Aenimal/IMG00277-20110912-1612-600.jpg
http://i1140.photobucket.com/albums/n576/Aenimal/IMG00279-20110912-1613-600.jpg

From the looks of things, it will just barely fit correctly in the '73 arm, as the widths are damn close, but the thickness of the axle mounts on the K arm causes slight issues. With a slight skim of the spacers, it could be made to work, I believe. I'll mock this up later.

I wouldn't really be using it with an old K arm, as I do have a '76 A arm to use or a '76 F arm to use. The 76 A arm seems to be about the same size as a K8 arm, whereas the F arm is skinnier, but has the different style chain adjusters.

Cheers

[MidnightLamp]

You need to skim an early k spacer lightly, and also lightly skim the caliper mount, but I just checked and you can in fact mount this to an early k arm using an F1 hub and 82 900c caliper.

[Sam Green Racing]

Sorry midnight, I thought you were putting a K arm to a f frame.
The F and A arms are cranked further out than the k and the offset causes problems when you try to fit the left shock.

Sam.

[754]

I an thinking of trying a 80 Susuki 2 piston rear caliper. It is made for the CB500 size Rotor.
 Anyone think there is an advantage to running a 2 piston, I know it should center up well, by design.
 I am not that keen on running the biggest rotors dont seem necessary.. not used on todays bikes..
 

[scottly]

Frank, if the 2 piston caliper is opposed, like a Grimeca, it may have less drag, since both pistons retract when the brake is released. I know my rear brake has no noticeable drag at all.

[754]

 Yes it is, I should have called it double acting.. not all 2 pistons are double acting..

[MidnightLamp]

http://www.wilwood.com/Calipers/CaliperList.aspx?subname=Dynalite%20Single%20III%20/%2

?

What about something like this, frank? a hanger from a different bike could easily be adapted. The 900C is a non-opposed dual piston setup. With a two piston floater, it's nice because the caliper can be solid-mounted to the arm.

Alternatively, one could use a non-opposed caliper and go for a floating rear rotor.

http://www.allamericanwheel.com/rotors.html

These guys sell ones with blank aluminum centers for around $50 (full floating), which you could machine out to fit a 76F hub.

[754]

 I have the Suzi caliper and rotor..same rotor size as CB 500 4 front.
 I also have an 05 R6 or R1 back brake and rotor.
 I am thinking the huge rotor on rear is just overkill and a bit big looking..

[MidnightLamp]

I also have an 05 R6 or R1 back brake and rotor.

This. I've been trying to do this for a while with the smaller rear rotor, but I can't find ANYTHING that'll even remotely fit onto a cb750 rear (most sportbike stuff is in and around 220mm from what I can see). I think the original intent was to make the rotors large for heat dissipation, wet braking and to deal with minor dragging.

[754]

 I have the wheel 5.5 inch and sprocket as well.
  I dont like the Comstar wheels much at all.

 Back in the day some folks used CB 750 front hub or a billet stronger re-creation on the rear. This require fat spacers in ether side for sprocket and rotor.. I think wider apart spoke flanges would be better.
 Then the 6 bolt pattern will take Honda Yamaha and Ducati rotors
 I think this could be done as a knock-off hub as well.

 Little known fact; early Yamaha rotors can be taken apart (unbolted) so can cb 750, (grind off rivets). Then you can take  a blank alloy sprocket and machine center out to fit on the rotor carrier..and have matching carriers on both sides.... might look good..

[Rookster]

Midnight,
Is the caliper bracket from the 900c as well or is it from 750F?
Scott