R&D: 17" Rear Wheel Swap (RF600/Bandit 600)

Maintenance, repair, troubleshooting, etc.
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SpeedRacerOnline
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Fri Sep 11, 2015 3:02 pm

pdworak wrote:Even if it is 1 mm off, it really doesn't matter because the first time you apply the brakes, the 2 opposing pistons in the caliper will center themselves on the rotor and work just like normal afterwards. The only issues might be contact between the caliper and rotor or insufficient clearance on one side to get a new pad in, though I doubt 1 mm will make that much difference.
Ah, I've been watching this conversation and was sure Radare was probably right, but didn't see your mistake until just now. Your thought here would be right if it were a floating caliper like most cars have these days. With a floating caliper, there is a range of distance that the caliper can be located based on the center of the rotor and still work correctly. When the system is pressurized, the pistons will push the pads out until they contact the rotor. Because the caliper "floats" on slide pins, the pressure on one side being higher than the other because the caliper is off-center will force the caliper to move laterally until the caliper is centered, thus balancing the pressure.

However, our calipers are fixed, not floating. The caliper cannot move laterally to center itself over the rotor. If it is not mounted dead center over the rotor, there will always be uneven pressure from the pistons to the rotor. When the system is pressurized, the pistons will push out unevenly (one side farther than the other) to make contact with the rotor. Because your foot is applying even pressure on the entire system, the pistons with the shorter reach will be pushing with a higher pressure. This is because they are trying to move as far as the other side due to the same pressure being applied to both sides, but the rotor is obstructing them from travelling any further. In a floating caliper setup, that is what would make the caliper move laterally to even the pressure. In a fixed system, the caliper can't move, so the pressure will never equalize.

When the pressure is released, the pistons will try to retract evenly. However, the one side will likely drag because it would need to move farther (deeper into the caliper) than the other side to release because the caliper isn't centered over the rotor. They will still work, but the uneven pressure will generate uneven heat, and be more likely to drag (creating constant heat), both of which will wear pads out faster and unevenly, and make it more likely to warp the rotor.
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Casper
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Sat Sep 12, 2015 9:09 am

In theory what he is saying should work well. The caliper should be mounted in the same location due to the single 2.5 mm on the outside of the bracket (same as Radare's) and the overall spacing shouldn't change (15+33 instead of 14+34).


With floating calipers, it is true that the whole unit floats on pins in order to properly apply the pads at the same force on each side of the rotor. With non floating calipers, the same thing happens due to the pistons being allowed different locations, they are not balanced by distance but instead forces and will compensate for any offset nature of the caliper/rotor combo. This only becomes a problem when the groove in the caliper is too narrow and the caliper body then touches the rotor surface.


I would suggest he goes for what he is saying and post up pictures of the result, a lot harder to say something doesn't work when there is evidence of it working.
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radare
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Sat Sep 12, 2015 10:18 am

Am I really wrong on this, Casper? I gave this a lot of thought when I did the conversion.

Consider the following photo of the rear caliper:

Image

The green arrow points to the caliper centerline. It's the casting line which you can see running vertically on the caliper. The orange arrow points to right-hand spacer.

The right-hand spacer's position is set by the wheel spacer and the spacers to its left. It has a fixed position on the wheel. The rotor is bolted to the wheel and its position is also fixed by the wheel spacers and the spacers to the left.

The caliper bracket's position is fixed against the right-hand spacer. The thickness of the right-hand spacer sets the location of the caliper bracket. The caliper is bolted firmly to the bracket. That means, the right-hand spacer sets the position of the caliper.

If you leave the right-hand spacer at 15mm, the caliper bracket will shift 1mm to the right. The caliper will then shift that same distance. This will cause the caliper to be 1mm off-center (shifted to the right) relative to the rotor.

One of my concerns with a 15mm spacer is pad alignment, sure, but the bigger concern is with clearance to the caliper body. Looking at the clearance between the caliper body and rotor on the left-hand side, there is maybe 1 to 2mm clearance, that's it. If the caliper shifts rightward, the rotor will most-certainly rub on the casting for the caliper body. (This is a hard photo to take and I'm not certain it fully illustrates the concern).

Image

I've put thousands of miles on the conversion using a 14mm spacer and I know it works; it's proven in the real-world. I really do feel the correct way to do it is to use the 14mm spacer and center the caliper body.

But that being said, there is a lot more clearance between the rotor and the caliper body on the right-hand side of the rotor. If you really want to change spacers and run a mis-aligned caliper, you want to shift the caliper closer to the wheel, not further away. This would require you to find a 13mm spacer, which, I think the XJ has stock. So perhaps the solution here is to use the stock right-hand spacer and machine the caliper bracket a bit thicker to compensate. Keep in mind, the caliper bracket should not locate the position of anything; it should be used to fill the clearance left between all the precision-machined spacers and the swingarm.

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Casper
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Sat Sep 12, 2015 2:06 pm

Here is how I am viewing yours:
Image

You shaved the spacer 1 mm (green) and the caliper mount 2.5 mm on the right (red).

Random guess on the distance of the caliper bracket mounting flats... that number shouldn't matter as long as it stays the same.


Here is how I am viewing what he is saying:
Image

Keeping the spacer the same Kawi OEM width of 15 mm and shaving the caliper mount 1 mm on the left (red) and 2.5 mm on the right (red).

Since the shaving on the right is the same between both of your methods, the caliper mounting flats should stay the same.
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radare
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Sat Sep 12, 2015 3:12 pm

I see. Shaving distance off of both faces of the caliper bracket. My concern with that is, if they are not absolutely flat, you could end up with the wheel tightening down out-of-square, the caliper being out-of-square with the rotor and other issues. :twocents:

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Casper
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Sat Sep 12, 2015 3:52 pm

I would also be concerned about that, but if he is very good or very careful...
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pdworak
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Mon Sep 14, 2015 12:31 pm

I have access to a lathe and will machine down the end of a 1" steel round bar to snuggly fit into the caliper bracket axle hole. I will drill and tap a 1/4"-20 hole from the bottom of the caliper bracket to the axle hole. I can then chuck the bar into the lathe, insert the caliper bracket and use a set screw to hold it in place. I will machine one side of the bracket, then flip it to machine the other side. Both sides will be parallel to each other and perpendicular to the axle hole. This will be much more accurate than sanding with a belt sander and shaving with a metal cutoff saw.

But looking at your pictures, it appears using the stock Seca 13 mm spacer would work just fine. That way, I would only have to machine the caliper bracket 1.5 mm on the outboard side.

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radare
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Mon Sep 14, 2015 12:51 pm

pdworak wrote:This will be much more accurate than sanding with a belt sander and shaving with a metal cutoff saw.
I'm going to try and not take any offense from that statement. ;)

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pdworak
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Mon Sep 14, 2015 12:57 pm

No offense meant! I'm lucky enough to have access to some basic machining tools, so I can make this work. Your ingenuity and patience is greatly appreciated.

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Casper
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Mon Sep 14, 2015 8:16 pm

You have access to a lathe and you want to use OEM kawi spacers, why?

Just cut a few to spec from aluminium or stainless and reduce your total parts count.

Judging by the pictures, the stock Seca II spacer won't work. It's just not worth it to run the caliper that close without shaving 1 mm off of the caliper mounting bracket faces.
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