Someone Double Check my Math! Spacers for Calipers on 919 with F3 Wheel - Wrist Twisters
 
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post #1 of 12 Old 01-01-2015, 04:42 PM Thread Starter
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Someone Double Check my Math! Spacers for Calipers on 919 with F3 Wheel

Here are my measurements;
1.11" Right Caliper Mount on Fork to Rotor
1.06" Left Caliper Mount on Fork to Rotor

0.16" Rotor Thickness (both)

0.508" Caliper Mounting Surface to Inside
1.025" Caliper Opening Width

Theoretical Perfect Center of Rotor to Caliper Fork Mount
1.11 (Caliper Mount on Fork to Rotor) + 0.08 (half Rotor Thickness ) = 1.19 to centerline of rotor

To Center Caliper
Minus Half Caliper Gap Opening
0.5125
Minus Caliper Mount Surface to Inside
0.508
Equals Spacer Thickness needed
0.1695 on Right

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post #2 of 12 Old 01-01-2015, 05:18 PM Thread Starter
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Left
Theoretical Perfect Center of Rotor to Caliper Fork Mount
1.07(mount to rotor) + 0.08 (half rotor thickness) = 1.15 to centerline of rotor

To Center Caliper
Minus Half Caliper Gap Opening
0.5125
Minus Caliper Mount Surface to Inside
0.508
Equals Spacer Thickness needed
0.1295 on Left

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post #3 of 12 Old 01-01-2015, 05:26 PM
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I just did this myself recently, and didn't go quite so precise as you are doing here.

My thinking was based on the fact that there are pistons both sides of the disk, which will adjust to the new disk location even if it is even slightly off-centre.

I measured the disks, outer edge to outer edge, on the 919 wheel, then the F3 wheel, found the difference [around 9mm IIRC], halved it [=4.5mm], and used that as my shimming thickness guide. Ended up with a couple of stainless washers at each caliper mount point and called it good. Loctited it all together, 1600km ago.

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post #4 of 12 Old 01-01-2015, 05:29 PM Thread Starter
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What was your measurement on the 919 wheel? Did you find a difference left vs right?

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post #5 of 12 Old 01-01-2015, 05:36 PM Thread Starter
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Your 4.5mm equals 0.177"... I am getting pretty close to that on my right side (0.1695)

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post #6 of 12 Old 01-01-2015, 05:39 PM
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I started from the premise that the wheel would be centred when mounted in the fork, and that the disks were already centred on the wheel.

The 919 disks were 142mm apart, the F3 135mm, so I needed to bring each caliper in-board by 3.5mm. [My 9mm recollection was an overestimate, sorry]

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post #7 of 12 Old 01-01-2015, 05:40 PM Thread Starter
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Wow, found this in a another thread by cloroxbleach;
"Next you will have to mock up your calipers and shim them with spacers to get them centered over the F3 rotors. I am using CBR954RR calipers and had to shim the L/H side 3.45mm and the right 4.15mm, you can use some stainless washers or have some spacers turned on a lathe."

That is 0.163" Right and 0.136" left.
Pretty damn close to what I am getting by calculating.

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post #8 of 12 Old 01-01-2015, 05:42 PM
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I then fussed with the axle spacers to get the disk-to-fork leg clearance measurement the same on both sides.

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post #9 of 12 Old 01-01-2015, 05:52 PM Thread Starter
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I think the use of off-the-shelf spacers gets it to within 0.045" or so of being centered. I wonder if the overall length of the stack is correct for the 919 or if taking some off one of the spacers could get it more correct? Anyone measure the exact needed stack of spacers/bearings on the 919 axle?

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post #10 of 12 Old 01-01-2015, 06:09 PM
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I ended up trimming [grinding] a RHS spacer in order to get it all together, and have two spacers on the right and one [a big flanged one] on the left.

I had a full set of spacers for both wheels, but couldn't just mix and match them, as others have reported.

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post #11 of 12 Old 01-01-2015, 09:04 PM
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I had a spare spacer for my setup and also shaved it down until it was spaced evenly and there was no drag or extra space in the rotor/caliper clearances. That's all you really need to get.

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post #12 of 12 Old 01-02-2015, 06:29 AM
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Its been awhile since I ran this setup .... but here is a pic of my spacer stack ups and orientation. CB919 with F4i forks, F3 front wheel & CBR954RR calipers.

[/QUOTE]

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