Do PF30 type PAS sensors exist?

Manc44

Pedelecer
Jun 21, 2021
126
16
I hit a dead end planning to fit my Hollowtech 2 PAS sensor to a carbon MTB frame that has a PF30 bottom bracket shell.

This frame has a Praxis Works Shimano based bottom bracket and while that Praxis thing works really well under normal usage, it is too wide for the PAS sensor ring to fit around it.

It would require a PAS sensor ring that's got an ID of 42.1mm (the Praxis is 42mm).

My PAS sensor ring is designed for a standard Hollowtech 2 bottom bracket cup, that's got a diameter of 35mm. My PAS sensor ring is 35.1mm.

This means the PAS sensor ring is 7mm too small on the inside, so it would need 3.5mm filing off around it, leaving it just 1.5mm thick around the edge!

Even if the PAS sensor ring was filed down, it's leaving hardly any material left to it and I'm not sure it would be feasible due to my BB shell bulging out as much as it does.

Praxis Works BB cup: 42 mm
PAS sensor ring: 32.1 mm
PAS ring width: 5 mm

I am sure the Chinese don't make PF30 specific PAS sensors?

This is going to rule out using this frame for me and I'd have to go with my original plan - buy an old aluminium hardtail MTB frame that has a normal 35mm wide BB shell with an IS mount at the rear brake. I spent 2 days sawing, drilling and filing metal plates to use as torque arms on this carbon frame all to find out the sodding PAS can't be fitted!

I have got a throttle on it, but never use it because the PAS is so much more convenient.
 

Nealh

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West Sx RH

Nealh

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Manc44

Pedelecer
Jun 21, 2021
126
16
No Nealh, I have already got one of those, as explained in the original post when I said "This frame has a Praxis Works Shimano based bottom bracket".

The PAS sensor metal ring has an internal diameter that's too small to fit the Praxis Works BB cup (that's 42mm when the PAS is only 35mm).

It's not a bottom bracket converter I need, it's a PF30 specific PAS sensor.

I'll have to get a sheet of 1mm steel and make it myself if they don't exist, I know I've never seen one on sale anywhere - they are always for either square taper, or Hollowtech 2.

What might work is some sort of 1mm thick penny washer with an ID of 42mm I can file off the excess, leaving me with a square platform I can bend back and up. Then the problem is the sensor is probably too high up for the magnet disc (can't fix that) and set back too far as well (can fix it by making a little rectangular shim and getting longer screws exactly the same width and thread as the ones in it). Depending on how much higher it would be (my guess is at least 5mm) it could possibly be angled down and still work.

It would be a right faff!

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RogerA

Finding my (electric) wheels
Jun 21, 2021
20
6
Could you use something like this...
It cable ties to the frame. You should be able to trim the magnet ring to fit and epoxy glue it to the chainring.
 
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Manc44

Pedelecer
Jun 21, 2021
126
16
You should be able to trim the magnet ring to fit and epoxy glue it to the chainring.
The magnet disc is solved already and cable tied to a 24t inner chainring.

I'm using the kit right now, just on a different bike.

It [sensor] cable ties to the frame.
Cheers RogerA, that inspired me on how to solve it :) maybe...

Like this, with the sheet of 11cm x 4cm corrugated plastic being 1mm stainless steel, with the sellotape holding it being cable ties, with it possibly bent here and there to get the sensor in the right place.

Not sure if this is approximately where it needs to end up. The BB cup on these pics isn't screwed all the way in - but if it was then the the sensor would be about flush with the outer surface of the BB cup...

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EDIT: Actually the sensor outer edge needs to be about flush with the BB shell of the bike frame so this is sticking out too far.

Maybe the KT-D12L down the seat tube is the only option, if it can be put low enough.

This frame has bulges just about everywhere around the BB shell (for example the down tube has a circumference of 19cm) and the seat tube is about the only place where it's recessed back enough to accommodate a sensor.
 
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Manc44

Pedelecer
Jun 21, 2021
126
16
After pushing my crankset on to check this, there's no way it's going to work.

Has anyone ever fitted a pedal assist sensor to a PF30 (press fit) bottom bracket?

Do such things exist?

The only ones I have seen are for square taper, or Hollowtech 2.

I am just about ready to give up with it and accept that I can only use a throttle!

The reason I can't fit one is, the bottom bracket is carbon and starts bulging out immediately after the shell face. This makes it really difficult to get a PAS thin enough (that will also needs a little wedge of wood or something under it so it sits flat).

The latest one I tried is the KT-D12-R but, it can't be fitted without making my chainline about 8mm too big.

I went back and got a standard Hollowtech 2 one (in the pictures from the post above) and unscrewed it from the metal ring and rectangular plastic box, now this one is thin enough, but there's no way to attach it securely. I considered drilling 2 holes in a 1mm carbon strip and screwing the PAS on that way but then I am stuck again with this 8.2mm plastic box around the circuit board, making it too tall.

I need a PAS that's incredibly thin, with some sort of thin strip (about 100mm long) coming off it at 90° so I can cable tie that along the down tube.

I thought about smothering the bare circuit board of the PAS in epoxy, not to glue it but to seal it from water - but even if I do that, it's probably going to be too tall again, once I have somehow attached that to the 1mm carbon strip. It's such a tight fit down there, even the 1mm carbon strip is a problem, pushing any PAS I put on it another 1mm out.

I reckon this isn't possible. It's the only thing stopping me doing the build. :rolleyes:

PAS was always one of the best features to me about having a motor on.
 

Nellie

Pedelecer
Jun 15, 2020
45
11
Could u use something like Sugru to stick it on . I used it on my tandem and it’s held for over 2 years or am I misunderstanding your problem
 

Manc44

Pedelecer
Jun 21, 2021
126
16
It could be glued on but what happens when the glue cracks? I keep trying to find some way to cable tie it, but the tubing tapers around there.
 

guerney

Esteemed Pedelecer
Sep 7, 2021
1,310
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Glued on, plus a few layers of epoxy and small sections of fibreglass?
 

Woosh

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May 19, 2012
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Has anyone ever fitted a pedal assist sensor to a PF30 (press fit) bottom bracket?
yes, you can glue the magnet disc to the chainring with double-sided adhesive pads and make yourself a seat tube adapter. I make the adapter in 3 different heights to suit different bikes because of the location of the front derailleur.

https://wooshbikes.co.uk/?hubkits#rhpas
 
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Manc44

Pedelecer
Jun 21, 2021
126
16
It's so limited on space down there. I need some sort of bare circuit board PAS that's as flat as possible, like 1mm or something. The circuit board I have now has a sensor chip poking out 3mm + 1mm circuit board +1mm carbon fiber + wires, it's taking up 6.5mm when I have probably 1mm or 2mm to play with.
 

Woosh

Trade Member
May 19, 2012
16,400
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wooshbikes.co.uk
It's so limited on space down there.
Did you follow the link I gave you?
https://wooshbikes.co.uk/?hubkits#rhpas
The design is very simple and should be easy to copy using a small amount of 2mm aluminium sheet (5cm W x 4cm H).

Cut a piece of cardboard 5cm W x 4cm H to see if you can slide it in between the seat tube and the chain ring. Trim the cardboard until it fits.
Then draw on the cardboard the sensor's footprint, 4cm x 1cm.
Then work out the placement of the bolt that holds the assembly.
Lastly, workout if you need one or two pieces of spacers.

 

Manc44

Pedelecer
Jun 21, 2021
126
16
This is where the sensor ends up, with mock up 2mm plastic under it.

When a chainset is fitted, the magnet disc (on a 24t inner chainring) overlaps about 3/4 of the way over the notched part of the BB cup - leaving practically no space there for anything to fit between.

The magnet disc probably ends up about where the center of this sensor is.

The BB shell doesn't have a lip like on a metal frame, where the sensor can just be set back (like on the Hollowtech 2 PAS sensors) and instead, my BB shell tapers away from the flat circle at an angle.

This (8mm wide) sensor would need to be 1/4 or 1/5 of the width it is to work.

This shows it along the down tube but the problem is the same 360° around the BB cup, because it has no recess on the frame at the distance the sensor needs to be to line up with the center of the magnets on the disc.

The only way I can think is to use a bigger inner chainring like 30t that adds 12.3mm to the radius, then I'd need a magnet disc that size too and maybe it could be done that way.

Honestly it's easier to just quit trying to make it work and buy a metal bike frame with a standard BB shell where none of this is a problem.

 

Woosh

Trade Member
May 19, 2012
16,400
14,108
Southend on Sea
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stick the sensor directly onto the seat tube with a bit of sticky pad to see the difference compared to your photo.
you should give it a try.
 

Nellie

Pedelecer
Jun 15, 2020
45
11
Sugru doesn’t crack it’s a silicone rubbery compound that can be removed without too much difficulty. But it sticks well

 

Manc44

Pedelecer
Jun 21, 2021
126
16
I managed to shave a few millimeters off the plastic box for the sensor.

This gets the gap from the edge of the sensor to the edge of the BB cup to 4.1mm.

I'm still not convinced this is enough, because I think the magnet disc on my chainset (when I tried it) was overlapping about half way into the BB cup. Putting the sensor at the seat tube makes no difference, because the center of it has to be 11mm from the edge of the BB cup regardless of which tube it ends up on, and I have that bulging BB shell to contend with all the way around it.

I'll leave this sellotaped on and tomorrow, I'll take my chainset off again and push it up against the carbon frame to check what's what. I think with no spindle spacers I get a 50mm chainline which is correct, but I'm pretty sure that's still going to be touching this sensor.

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Here's why it won't work, look at how much my other bike's inner chainring overlaps with the BB cup and this is without the addition of a magnet disc, that would probably end up about in line with the BB shell face:

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I know I shouldn't be looking at another bike to work it out but, this is just showing how much the inner chainring overlaps - half way into the BB cup.

My modified PAS sensor is roughly at that same point.

The above chainset is the exact same one I'd be using on the carbon frame.

I'd need a completely different solution like how a cadence sensor works on a crank maybe, although the lag would be horrendous lol, if it were even possible to do it.

I can't fit a left sided PAS because I have no gap anywhere on the spindle on that side, plus of course the BB bulges out just the same as it does on the drive side, anyway.

The problem is the amount I need to space out the drive side crank, to make the gap to the sensor 2mm, I'd probably need about 6mm to 8mm worth of 24mm ID spindle spacers - I do have them, but I'd have a chainline at about 56mm to 58mm and it's just too much, it's meant to be 50mm.
 
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Manc44

Pedelecer
Jun 21, 2021
126
16
Ditch the small chain ring to create some space?
That's a good idea but I only have a 13-23t cassette on and want to keep it rideable if not running the motor. Ditching it would make my lowest gear 36x23t which would make it tough up steep hills with no motor on. I'd rather not use a PAS than do that.
 

Nellie

Pedelecer
Jun 15, 2020
45
11
Maybe you could 3D print a larger diameter disc for the magnets not sure if you would need to reposition the magnets to account for speed difference