I removed ~2.38mm from either side of the swing arm pivot area and also 2.38mm from the pivot/collars but after the bearings were fitted and the dust seals installed, it still needs just a little more clearance to fit inside the frame.
I removed as much as I could from the left side where the roller bearings live and still have enough depth to allow the dust seal to go on and sit fairly flush. Not sure what I'm doing wrong but I ruined 2 dust seals trying to install them without mangling them even after greasing the seals prior to installation. The right side has a little more wiggle room for trimming and it looks like I will need to remove just a tiny bit more on the right side of the pivot for everything to slide into place.
I found that, due to the close tolerance, the swing arm needs to be coaxed into place by inserting the nighthawk pivot bolt into the swing arm bolt hole in the frame in order to align the collars the final few mm to line up the collar with the hole and be able to insert the modified cb swingarm bolt.
The cir-clip on the right side of the pivot needed to be removed to allow the dust seal to seat just a little deeper after trimming the swing arm (this shouldn't be necessary with the narrower 5mm seal in my parts list). I think the cir-clip is used to control the depth of the dust seal, not to prevent the bearing from walking out. (I interchangeably refer to both sides as dust seals, even though one is a dust seal and the other is an oil seal, in my mind they do the same thing... prevent dirt from getting in and also holding the collar in place.)
The Nighthawk swing arm has 3 collars in the pivot area and it's not clear to me why there isn't 1 instead of 3.
#1: 40mm trimmed to 36.4mm (left side, roller bearing)
#2: not measured and not trimmed (center)
#3: 12mm trimmed to 7.6mm (right side, regular bearing)
Since I used a bench grinder to manually shave the collars, I had to occasionally measure the work with digital calipers to try to be less than .05mm difference in height on the collars and also stop frequently to let the metal cool down because it got too hot to hold after just 20-30 seconds of grinding. The trimming process took a long time and I ended up coming back to trim a little every day for a few days until the swing arm finally fit. I will need to order new dust seals since the ones I installed got mangled from all the test fitting. I tried hammering them on which deformed them a little so I'll try to grease the next set of seals and coax them in by hand ONLY.
I hope I didn't contaminate the bearings with metal dust from all the grinding after installing the bearings.
The modified CB pivot bolt does not have any noticeable play in the frame hole (even though it's reduced 1mm) but I will be trimming the carbon fiber tube to sleeve the original pivot bolt hole because that 1mm difference in the bolt hole will cause an obvious clunk when I go over bumps and that bolt jiggles in the frame mounting hole.
With the swing arm fitted, I'm satisfied that the wheel swap is doable and will go ahead and start ordering the remaining parts. The first item that I ordered was a 150/70-17 Continental Conti-motion Radial tire. I was afraid the tire would be too wide but ordered it anyway. Go wide or go home?
Nope.
Using this tire comparison site, I was able to determine that the 150/70-17 tire had very nearly the same circumference as the old CB tire so I shouldn't see very much speedometer variance with this new setup. Stock Nighthawk sizing is 140/70-17, but the 4" wheel is better suited to a 150. I read a few posts about people barely fitting 150 tires into the CB frame so I had a fair chance. In the end, I think the slightly extended length of the swing arm afforded a little more room for the wider tire as well as the smaller 525 chain compared to the stock CB750's 530 chain.
I'm wary of using eBay shocks, but there are plenty for sale on eBay/Amazon (are these basically the same company with a different user interface?) Unfortunately, all the name brand stuff such as Progressive, YSS, and Hagon are easily triple the price of the mid priced eBay stuff and I have trouble thinking that a preload only name brand shock is worth $2-300 when the one I ultimately chose has preload, height and rebound adjust-ability for about $120.
My wallet is a fair amount lighter after all the purchases over the last 3 days, this is probably the largest project to date for the CB750c.
One unintended consequence of the swing arm swap is that the wheelbase has elongated about 3 inches, so I will need to mount the rear tire as far forward as possible to get the wheelbase closer to original.
shopping list:
* * *
I mangled my new tire and my nighthawk wheel while trying to spoon that new 150 tire on. The key thing I forgot was to keep the tire near the center of the wheel as I try to push the bead on by standing/kneeling on the tire as I pry it on. As a result, I have a slightly dented wheel, scratched up paint and a wavy bead. Going to fill the tire and attempt to seat the bead and see if it holds air but another rear wheel and/or tire could be in my future if this doesn't work out. I'll hold my breath for now and hope for the best. My takeaway is to avoid using a cheater bar on the tire irons (which I bent) because the tires should not need that much force to get on the wheel.
I aired up the tire to 50 psi and hoping that wavy bead starts to seal. I warmed up the tire with my heat gun and I'm leaving it pressurized to at least 40 psi. After a few hours, the leaking is reduced to a slow leak which I think will go away after a few more days. It's cold outside, so I'm keeping the wheel in the house for now.
After a week I discovered a slight tear in the rubber in the bead area so I sealed it with a little silicon and it's been holding air all week.
***
Follow up notes:
The front sprocket had a tiny amount of axial play. If I gently twist the sprocket I can feel a fraction of a millimeter of free play before the spline catches. Once I greased the spline, the play was no longer detectable. (edit: sprocket brands may have an effect. I got a 15t sprocket from a different manufacturer and this one clearly fits much better, I guess it pays to buy a higher quality sprocket)
I ended up cranking the shock preload almost to maximum to get the rear sag to where I want it. I think a slightly heavier spring would work better and a longer/taller shock than 13.5" better still, but instead of buying a longer shock (hint: there's not as many options when looking for taller shocks), I accidentally came across a product called 'shock absorber eye adapter' that will add about 1.5" to my existing shock. This is likely enough additional height to make up the difference from swapping the swing arm.
Since I have these shocks almost maxed out on the preload, I was afraid they would be too stiff like the old shocks I used to have on this bike but they feel really good on the few short rides I've taken thus far. Not at all harsh or floaty which was my only 2 choices with the old shocks. A longer ride will reveal much more but the bike rides better than ever now.
I removed as much as I could from the left side where the roller bearings live and still have enough depth to allow the dust seal to go on and sit fairly flush. Not sure what I'm doing wrong but I ruined 2 dust seals trying to install them without mangling them even after greasing the seals prior to installation. The right side has a little more wiggle room for trimming and it looks like I will need to remove just a tiny bit more on the right side of the pivot for everything to slide into place.
I found that, due to the close tolerance, the swing arm needs to be coaxed into place by inserting the nighthawk pivot bolt into the swing arm bolt hole in the frame in order to align the collars the final few mm to line up the collar with the hole and be able to insert the modified cb swingarm bolt.
The cir-clip on the right side of the pivot needed to be removed to allow the dust seal to seat just a little deeper after trimming the swing arm (this shouldn't be necessary with the narrower 5mm seal in my parts list). I think the cir-clip is used to control the depth of the dust seal, not to prevent the bearing from walking out. (I interchangeably refer to both sides as dust seals, even though one is a dust seal and the other is an oil seal, in my mind they do the same thing... prevent dirt from getting in and also holding the collar in place.)
The Nighthawk swing arm has 3 collars in the pivot area and it's not clear to me why there isn't 1 instead of 3.
#1: 40mm trimmed to 36.4mm (left side, roller bearing)
#2: not measured and not trimmed (center)
#3: 12mm trimmed to 7.6mm (right side, regular bearing)
Since I used a bench grinder to manually shave the collars, I had to occasionally measure the work with digital calipers to try to be less than .05mm difference in height on the collars and also stop frequently to let the metal cool down because it got too hot to hold after just 20-30 seconds of grinding. The trimming process took a long time and I ended up coming back to trim a little every day for a few days until the swing arm finally fit. I will need to order new dust seals since the ones I installed got mangled from all the test fitting. I tried hammering them on which deformed them a little so I'll try to grease the next set of seals and coax them in by hand ONLY.
I hope I didn't contaminate the bearings with metal dust from all the grinding after installing the bearings.
The modified CB pivot bolt does not have any noticeable play in the frame hole (even though it's reduced 1mm) but I will be trimming the carbon fiber tube to sleeve the original pivot bolt hole because that 1mm difference in the bolt hole will cause an obvious clunk when I go over bumps and that bolt jiggles in the frame mounting hole.
With the swing arm fitted, I'm satisfied that the wheel swap is doable and will go ahead and start ordering the remaining parts. The first item that I ordered was a 150/70-17 Continental Conti-motion Radial tire. I was afraid the tire would be too wide but ordered it anyway. Go wide or go home?
Nope.
Using this tire comparison site, I was able to determine that the 150/70-17 tire had very nearly the same circumference as the old CB tire so I shouldn't see very much speedometer variance with this new setup. Stock Nighthawk sizing is 140/70-17, but the 4" wheel is better suited to a 150. I read a few posts about people barely fitting 150 tires into the CB frame so I had a fair chance. In the end, I think the slightly extended length of the swing arm afforded a little more room for the wider tire as well as the smaller 525 chain compared to the stock CB750's 530 chain.
I'm wary of using eBay shocks, but there are plenty for sale on eBay/Amazon (are these basically the same company with a different user interface?) Unfortunately, all the name brand stuff such as Progressive, YSS, and Hagon are easily triple the price of the mid priced eBay stuff and I have trouble thinking that a preload only name brand shock is worth $2-300 when the one I ultimately chose has preload, height and rebound adjust-ability for about $120.
 |
| I scuffed these shocks trying to extend it by 1/2" |
My wallet is a fair amount lighter after all the purchases over the last 3 days, this is probably the largest project to date for the CB750c.
One unintended consequence of the swing arm swap is that the wheelbase has elongated about 3 inches, so I will need to mount the rear tire as far forward as possible to get the wheelbase closer to original.
 |
| 150 rear fits perfectly! |
shopping list:
- JT Sprockets 43T steel sprocket
- Oil seal (part is NLA, search for: "Oil Seal 22X31X5" )
- Dust seal (part is NLA, search for this slightly shorter one: "Dust Seal 22X35X5" )
- NICHE Gold 525 X-Ring
- 150/70-17 rear tire
- 340mm Rear Shocks PSB V7R1 (from ebay)
- 14T front sprocket 525 (looks like a NH front sprocket fits, don't need a custom cb750c sprocket)
I cut off the 530 chain and cut the carbon fiber sleeves for mounting holes in the frame which the pivot bolt goes. I still need to replace dust seals on the swing arm before I complete the swing arm installation. Not too bad and as close to a direct swap as you can get without actually being a direct swap.
I mangled my new tire and my nighthawk wheel while trying to spoon that new 150 tire on. The key thing I forgot was to keep the tire near the center of the wheel as I try to push the bead on by standing/kneeling on the tire as I pry it on. As a result, I have a slightly dented wheel, scratched up paint and a wavy bead. Going to fill the tire and attempt to seat the bead and see if it holds air but another rear wheel and/or tire could be in my future if this doesn't work out. I'll hold my breath for now and hope for the best. My takeaway is to avoid using a cheater bar on the tire irons (which I bent) because the tires should not need that much force to get on the wheel.
I aired up the tire to 50 psi and hoping that wavy bead starts to seal. I warmed up the tire with my heat gun and I'm leaving it pressurized to at least 40 psi. After a few hours, the leaking is reduced to a slow leak which I think will go away after a few more days. It's cold outside, so I'm keeping the wheel in the house for now.
After a week I discovered a slight tear in the rubber in the bead area so I sealed it with a little silicon and it's been holding air all week.
***
Follow up notes:
The front sprocket had a tiny amount of axial play. If I gently twist the sprocket I can feel a fraction of a millimeter of free play before the spline catches. Once I greased the spline, the play was no longer detectable. (edit: sprocket brands may have an effect. I got a 15t sprocket from a different manufacturer and this one clearly fits much better, I guess it pays to buy a higher quality sprocket)
I ended up cranking the shock preload almost to maximum to get the rear sag to where I want it. I think a slightly heavier spring would work better and a longer/taller shock than 13.5" better still, but instead of buying a longer shock (hint: there's not as many options when looking for taller shocks), I accidentally came across a product called 'shock absorber eye adapter' that will add about 1.5" to my existing shock. This is likely enough additional height to make up the difference from swapping the swing arm.
 |
| shock adapter for additional height |
Since I have these shocks almost maxed out on the preload, I was afraid they would be too stiff like the old shocks I used to have on this bike but they feel really good on the few short rides I've taken thus far. Not at all harsh or floaty which was my only 2 choices with the old shocks. A longer ride will reveal much more but the bike rides better than ever now.
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