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Help with caliper alignment

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mr72

mr72

Joined
Apr 23, 2020
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Cedar Park, Texas, USA
When I rebuilt my Bonneville after the wreck, I put on Thruxton forks and had an adapter machined up to fit the Thruxton 320mm rotor to the mag wheel.

I noticed when I put it all together that the rotor was not centered in the caliper. In fact, there was just a tiny amount of clearance between the caliper bracket (carrier, whatever you call it) and the rotor surface on one side. See the pic:

1620136089618.png


Since it had some clearance, I decided to ride it and see how it went. Well, now the rotor has begun to rub and machine a bit off of the caliper, so it obviously doesn't always clear, guessing under cornering or braking load flexing causes it to rub.

I think the best solution is to grind a bit off of the caliper mounting tabs on the fork leg. I don't have the right tools to do this properly. I could always just go at it with a die grinder and micrometer and hope for the best. Wouldn't be the first time I have done this kind of modification but I wouldn't mind having a way to make more sure it was flat and square since any misalignment will result in the caliper not running true over the rotor.

I'm in Cedar Park. Anyone who can help?
 
As the inside pad wears the caliper should shift away from the disc giving it plenty of clearance , might pull that inside pad out and take a few thousands off of it and eliminate your problum . Either that or add several thousands more spacer on that side of the axle . While you have it back off be sure the wheel bearings are 100% too any slop there will screw things up too .
 
Hm. I'll check to see how centered the wheel is in the forks. This is all a custom job, obviously. But a simple washer might get the job done. I even think I have a couple. That would be the simplest solution, provided it didn't make the wheel run too far off center.

The part that doesn't clear is not on the sliding part of the caliper. It's the rigid carrier that is bolted to the fork. It can't move with respect to the rotor. I need to either mount the caliper further outboard by grinding on either the fork mounting tabs or the caliper mounting face, or I need the rotor to be further inboard, which can be done as you say with spacers.
 
Seems to me if you added a small spacer on one side wouldn't you have to take that much off the spacer on the other side?
 
Windmill said:
Seems to me if you added a small spacer on one side wouldn't you have to take that much off the spacer on the other side?
Click to expand...

No. The spacer the RH side butts against a shoulder on the axle, which is fixed in place by the cinch bolt after the axle nut is tightened. So the wheel positioning is essentially set according to the LH side spacers. I can add a spacer on the LH side and it will move the wheel towards the right without any issues, within reason.

FE_Rex, no the spacers are very different on each side. I had to add a custom spacer on the LH side to adapt the Thruxton fork to the Bonneville SE wheel.

I did a rough measurement with a regular tape measure from the fork leg to the rim surface on each side and it appears that I can probably afford 2mm or so of movement left. I'm going to try putting one axle washer on the brake side which should at least give me a touch of clearance. My micrometer croaked so I need to get another one and do precision measurements, once I get that then we'll see if I can get the wheel perfectly centered and how much clearance that'll give to the rotor. If it doesn't get it close to right, which I don't see how it will, then I may go ahead with the plan to try to grind off some of the fork tab width. Anyone have a reliable method to make sure that I get the same amount ground off each tab and that they are flat? I'm sure there's some power tool method those of you metal fabricators use to do this.
 
I think I see buttons indicating the rotor floats, correct?
Have you ridden it at all? does the rotor rub at all?

A floating rotor will find it's happy place. As long as the rotor isn't rubbing against anything, I wouldn't worry too much about it.
The only critical measurement is that the rotor runs parallel with the piston face(s) of the caliper.
 
Yeah it's a floating rotor. I have ridden it probably 1000 miles since I put it together (this picture was before I rode it, FYI). I think it is rubbing under some circumstances, can just see a bit of a groove forming on the caliper carrier/bracket. I haven't pulled it to see yet.
 
Do you have access to a disc sander? Could you make a jig with a price of steel angle where one leg is on disc sander table and other leg holds caliper bracket up against sanding disc? I think that would get you close enough if you verified table and sanding disk were square before starting.
 
FE_Rex said:
Do you have access to a disc sander? Could you make a jig with a price of steel angle where one leg is on disc sander table and other leg holds caliper bracket up against sanding disc? I think that would get you close enough if you verified table and sanding disk were square before starting.
Click to expand...

no, well I don't have a table disc sander. I have access to a rotary disc sander if I really want to take some material off real freaking fast.

I'm still not convinced that I would be better off modifying the caliper bracket vs. the fork leg. Thing is, this caliper bracket is an integral part of the caliper, and that's a wear item that I might one day have to replace. I would rather not have to redo this kind of mod in the event I replace the caliper, either with a new or rebuilt stock caliper or an upgrade. The Thruxton fork leg is already modified for this bike, and with any luck it will not require wear-related replacement. Further modifying of the fork would allow me to keep compatibility with a stock Bonneville SE caliper.

If I have to grind some of this off, my plan was to just pull the caliper off and use a die grinder to take off a couple of mm of material from the mounting surface of that tab and dress it with a flat file and a machined flat sanding stick (I have one for guitar fret leveling). I figure with a micrometer on the edges I can make sure that at least they match in depth. Then chase the threads with a tap and maybe Bob's my uncle. There's a lot of room for error in this plan. I don't know how a home machinist would go about reducing the thickness of these tabs.
 
> The Thruxton fork leg is already modified for this bike

Then, I'd think you just want to further modify it with a little grind grind grind, as opposed to having modified forks AND modified calipers. Also, there's probably more metal on the forks, so removing a bit would weaken that part less than removal material from the caliper?
 
The question of how I would do it. I have access to tools and machines that most don’t so I would put it on a mill and knock off how ever much I wanted off.
Now if I was at home and wanted to get it done... I would pull the fork leg off and wrap it in leather and put it in my vise. I would use the axle in the vertical position to square the leg in all directions. Then I would lay a level across both tabs to see how square it is to the axle to start with. Should be real close. Measure the thickness of a tab and start removing material with the weapon of your choice. I would probably use a flap wheel on my die grinder and a flat file working until I had almost removed the amount I wanted. I would then start removing material from the other one until they are real close to the same. I would finish with flat file across both tab together checking with level for straight edge and square to the axle.

Not sure that all comes across, but hopefully you get the idea.
 
Last edited:
bwdmax said:
Not sure that all comes across, but hopefully you get the idea.
Click to expand...

Yeah that sounds like a pretty good plan. Still depends on my ability to hand-dress it, which I guess is ok enough. Hard to check all of that for squareness in two dimensions especially when all of the parts are round. And this is the nut side of the axle; the axle fits through this fork leg with clearance, so it doesn't hold itself square. I'd probably be better off with a bolt I could cinch up to give me a vertical line 180 degrees from the plane I am working on. But anyway... it's still a pretty good idea, not really looking forward to pulling the whole fork off again.

I also considered using my drill press with a 1/2" or so forstner bit. Idea would be to clamp the fork to the base, square it up, and use the drill on the inside surface of each of the tabs to clear a little material. This might get me a more flat milled surface vs. using a die grinder free hand. Then dress with a flat file just like you said. In this case I could probably bolt the fork leg into the drill press base using a bolt and axle spacers through the axle hole and a nut and washer on the inside. For that matter, once I had it all fixed in the drill press, I could swap in a flat sanding disc or grinding stone and use that to make it more flat before the final dressing.

I'm not thrilled about pulling the fork leg off to do this but I guess I don't have a lot of choice.

First I'll try a washer :)
 
The hand dressing part is easy. The fork may have a flat part that the wheel spacer sits against that you can use to square against instead of the axle. I would skip the drill press unless you are certain in your clamps and have an end-mill. In the event a bit bites or the fork moves you might have a real problem to try and fix.
 
Reawakening my old thread.

Went out yesterday to finally do actual measurements to get more than an eyeball's worth of confidence about the right solution. The wheel is centered within the fork within 0.3mm of perfect, at least measuring from the outside of the fork leg to the rim on either side. The gap on the inside of the rotor to the caliper is 4 mm. So I need to either move the wheel 2mm off center, move the caliper 2mm outboard, or have my rotor adapter milled 2mm at the mating surface.

I don't want to run the wheel 2mm off center. If it was already off center to begin with, and that was the cause, then I'd be moving it. I could conceivably move it 0.6mm and it would be no more off center than it is now but just biased to the other side. But that will only get less than a third of the clearance I need. To pull the rotor and adapter off and have 2mm milled off of it is probably a bad idea and expensive anyway. I actually think its close to the minimum thickness I would trust, the same thickness as the aluminum center part of the floating rotor. I could have an entirely new part made up that has 2mm more offset.

Or, I could just hand-mill the mounting tab surface with a die grinder and dress it with a file. I think this is my only viable solution right now. The only real downside is that if I ever have to replace that fork leg, I will have to redo this mod on the replacement. The chances of me having to replace the fork leg are pretty small, I think.

I might try to do this job with the fork still on the bike. Maybe if I have enough time on my hands I'll pull that one fork leg.
 
I think I sorted this today. Measuring said I should be able to take 2mm off of the inner face of the rotor mounting tabs, so I pulled the wheel off and took the fender mount off so I could rotate the fork leg around. Tried the die grinder, but the stone just packed with aluminum and that didn't fly. Wound up using the angle grinder with a cut off wheel. Worked brilliantly. Dressed with a file afterward.

1626626673331.png


I took 2mm off, more or less, and put it all back together. Still doesn't precisely center. Could be off by another 1mm or so. But there's clearance on both sides of the rotor to the caliper. I'm not going to grind another 1mm off of the posts. I'm going to call this good and just ride it.
 
Is it because Thruxtons have 4 pots and you are using a 2 pot caliper? So the thruxton doesn't have travel in the caliper body, but the one you are using does, like a car caliper.
 
No, it's because the Thruxton uses entirely different wheel and triples to the Bonneville SE (mag wheel). The spacing is different. When I built it up, I had a custom carrier/adapter made to mount the Thruxton rotor onto the SE wheel placing the Thruxton rotor in the same spot relative to the wheel center as the SE rotor would be. The SE and Thruxton have different part numbers for the caliper, which nobody could ever tell me what the difference was since upon visual inspection they appear identical and the have the same pads, etc.; same Nissin caliper, but apparently there's a difference in offset between the center line of the caliper opening and the mounting surface. Or maybe the Thruxton forks have thicker mounting tabs than the SE, I don't know, I didn't measure mine before throwing them away. Or maybe it's a combination of both. But one way or the other, the SE caliper mounted to the Thruxton fork leg with the rotor in the stock position for the SE wheel results in the rotor running about 2-3 mm too far outboard to center in the caliper. To keep everything clean, the simplest solution seemed to be to take material off of the fork.

Still yet to test ride.
 
I rode it today. The alignment is much better:

1626713300635.png


It brakes just fine but I did notice it feels like there's a lot more travel on the lever before it bits vs. before, but I didn't do a back to back ride so who knows. I have been riding my project scooter and maybe those brakes just bite quicker. Hinkley Bonnevilles have long been known for having a soft brake lever.

I'll call this "done".
 
If the caliper is not centered it may be kicking the pistons back further than normal. Based on the ride report it appears to be ok. Maybe double stroke the lever to get better feel if you think needed.

Good job!
 
I may pull the caliper off and reset it by pushing the pistons in like a pad change, then make sure everything is lubed right and pump it back with the lever. We'll see. Maybe I'll just ride it.
 
Hey friends. I have been riding around with the newly realigned caliper and it is doing something strange. It makes a "clicking" noise irregularly when braking. I can hear it when parking or moving the bike with the engine off. Sounds like it might be the pad edge catching on some of the cross drilled holes in the rotor and shifting the floating rotor in the buttons. Not sure why it would have just started doing this. Not sure that's what it's doing, or really how to tell. Anyone heard something like this?
 
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