The on-going saga of "as the sprocket turns"
In this installment of “as the sprocket turns” we’re going to discuss one of the most overlooked aspects of a motorbike – the hub. In this case, we’re going to be looking at the rear hub because it pulls a lot of duty on an ironhead.
Like many older Harley products, the brake drum is bolted to the hub. On big twins, this allows for the drum to stay with the suspension whilst a wheel is removed – and on older twins, interchangeable front to rear. On an ironhead, the rear drum stays with the rear hub and is NOT interchangeable with the front. Adding to the fun; Harley specified shouldered bolts that are the locating member for the assembly. In other words, the drum does not register on the center bore of the hub or by a locating pin – it is fully free to move and the only thing that clamps it tightly are the shoulder bolts.
These shoulder bolts are a light press into the hub and should NOT be replaced with standard, full thread bolts. This will allow the drum to shift its registration and can easily lead to an out-of-round assembly. As it is; it is difficult to find a stock hub that doesn’t reach the outer edge of allowable runout.
On a normal bike, you may never notice this – especially if you keep your chain and your rear brake adjusted “loose.” Once you start tightening things up, you may find that you have multiple tight spots in the chain and or tight spots on the brake adjustment. Much of the time, these are due to worn parts, and some of the time they are due to the hub itself. And, the faster you go or the more power you put through the hub – the more you’ll notice issues. Vibration is certainly one of them; loping brakes is another; and the list goes on.
In sum – you don’t likely have to do anything we’re about to describe on a stock street bike. As you go to silly land – or if you just like “blue printing” components – then you may want to deal with the hub.
An excellent write up on this whole thing can be found here:
http://xlforum.net/forums/showthread.php?t=1525902
So, let’s walk you through it.
In the first photos, you can see the hub as it came back to Chuck from powder coating. The original registration surface for the drum has been coated – and 99% of people would just run the hub as is. For this build, we went a little further.
IMG_4190.jpg
IMG_4191.jpg
First, we determined that the two bearing bores were within .005 of being concentric to centerline. We also took the time to mark out where the lowest run out was with the hub and drum assembled. Once we knew we could true the hub off a common centerline and have the bearings not kill themselves, we made up a mandrel and mounted the hub in the lathe. The left side of the hub was registered to a 4-jaw universal chuck and the mandrel was a light press fit into the right side of the hub and held to center by a live-center in the tail stock. The drum mating surface was then faced to dead true with the center line.
The drum was also faced and the whole assembly put back in the lathe to cut the friction surface dead true. To do this, you will need at least a 10” lathe; 12” if you leave the sprocket attached (more on this later).
IMG_4206.jpg
IMG_4212.jpg
One more thing – Chuck did have to clean up the powder coating in the hub holes for the shouldered bolts. DO NOT just scrape these out. Remember, the light press fit is all that holds the hub true to drum. So, we very, very carefully took out the excess powder with a letter drill just below the shoulder bolts OD. To do this, we mounted the drill bit in a spare Jacobs chuck and ran it through by hand. Take your time and do not remove material from the hub – just the layer of powder.
IMG_4209.jpg
In this installment of “as the sprocket turns” we’re going to discuss one of the most overlooked aspects of a motorbike – the hub. In this case, we’re going to be looking at the rear hub because it pulls a lot of duty on an ironhead.
Like many older Harley products, the brake drum is bolted to the hub. On big twins, this allows for the drum to stay with the suspension whilst a wheel is removed – and on older twins, interchangeable front to rear. On an ironhead, the rear drum stays with the rear hub and is NOT interchangeable with the front. Adding to the fun; Harley specified shouldered bolts that are the locating member for the assembly. In other words, the drum does not register on the center bore of the hub or by a locating pin – it is fully free to move and the only thing that clamps it tightly are the shoulder bolts.
These shoulder bolts are a light press into the hub and should NOT be replaced with standard, full thread bolts. This will allow the drum to shift its registration and can easily lead to an out-of-round assembly. As it is; it is difficult to find a stock hub that doesn’t reach the outer edge of allowable runout.
On a normal bike, you may never notice this – especially if you keep your chain and your rear brake adjusted “loose.” Once you start tightening things up, you may find that you have multiple tight spots in the chain and or tight spots on the brake adjustment. Much of the time, these are due to worn parts, and some of the time they are due to the hub itself. And, the faster you go or the more power you put through the hub – the more you’ll notice issues. Vibration is certainly one of them; loping brakes is another; and the list goes on.
In sum – you don’t likely have to do anything we’re about to describe on a stock street bike. As you go to silly land – or if you just like “blue printing” components – then you may want to deal with the hub.
An excellent write up on this whole thing can be found here:
http://xlforum.net/forums/showthread.php?t=1525902
So, let’s walk you through it.
In the first photos, you can see the hub as it came back to Chuck from powder coating. The original registration surface for the drum has been coated – and 99% of people would just run the hub as is. For this build, we went a little further.
IMG_4190.jpg
IMG_4191.jpg
First, we determined that the two bearing bores were within .005 of being concentric to centerline. We also took the time to mark out where the lowest run out was with the hub and drum assembled. Once we knew we could true the hub off a common centerline and have the bearings not kill themselves, we made up a mandrel and mounted the hub in the lathe. The left side of the hub was registered to a 4-jaw universal chuck and the mandrel was a light press fit into the right side of the hub and held to center by a live-center in the tail stock. The drum mating surface was then faced to dead true with the center line.
The drum was also faced and the whole assembly put back in the lathe to cut the friction surface dead true. To do this, you will need at least a 10” lathe; 12” if you leave the sprocket attached (more on this later).
IMG_4206.jpg
IMG_4212.jpg
One more thing – Chuck did have to clean up the powder coating in the hub holes for the shouldered bolts. DO NOT just scrape these out. Remember, the light press fit is all that holds the hub true to drum. So, we very, very carefully took out the excess powder with a letter drill just below the shoulder bolts OD. To do this, we mounted the drill bit in a spare Jacobs chuck and ran it through by hand. Take your time and do not remove material from the hub – just the layer of powder.
IMG_4209.jpg
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