floating rotor bobbins and bolts questions

[mikey]

Hi all,
My bobbins and bolts are separate pieces. the bobbins fit the rotor slots and are 0.004" thicker than the slot. what is a normal amount of rotor float? is 0.004" in the range of normal?

Also, I am planning to replace the bolts and jet nuts. I am unclear what type of bolts these are or what I should use. Grade 8, AN, NAS or something else? They are 1/4-28 thread and I need a minimum grip (shank) length of 0.335" or a maximum of 0.69". Overall length (excluding the head) should be 1.00", or within a few hundredths.

I found AN4-10A on Pegasus or NAS1104-11 on Skybolt or NAS6204-11 on Aircraft Spruce. Aircraft Spruce calls them NAS shear bolts?

Any recommended assembly lubrication or anti-seize?

Thanks for any input / guidance,
Mike

[Rick Kirchner]

spruce should have the same AN 4-10A as pegasus. NAS shear bolts have tighter tolerance in the unthreaded portion and a thinner head with a dimple in it. either the AN or the NAS ought to work

[Garey Guzman]

Hopefully I'm understanding your terminology but I would use the "shear" bolts, assuming the forces are in acting in shear and these bolts aren't being loaded "clamping", other than nut torque for retention.

[Fred Michael]

NAS bolts with stainless nuts is what is commonly supplied with bobbin kits. Often 12pt bolts with proper washers and nuts
No lube or loctite is needed

[tlracer]

There is some good advice on disc mounting, hardware, correct float, etc. on p. 42/43 of the AP catalogue, as linked previously.

Just in case anyone wonders...the only reason I keep referring to the AP documentation is that their engineers have done all the work already on what's necessary

[mikey]

Tim,
SMH! I didn't think to look further through the AP catalog - thanks for the pointer, that info helps confirm what I needed to know and also gave me a very important insight! The bobbin can be turned around to create a spacer the width of it's flanged head! This completely solves my odd disc offset issue that I described in a different thread and negates the need to have new front hats made with the different offset!

Garey,
The description of "shear" on NAS bolts was confusing to me, as I've also seen bolts that are described and designed to shear as a fail-safe to protect more expensive equipment (like a PTO on a tractor - they use low strength "shear" bolts on the blades in case the mower hits something). In both our brake application and the tractor example, the bolts are in a shear environment. But obviously we don't wish for them to shear off in brakes!
I thought I read somewhere else that NAS bolts have higher shear strength than AN bolts and are thus desirable for applications in shear.

Fred,
Thanks!

Cheers,
Mike

[mikey]

According to the AP catalog, normal float is 0.2-0.4mm, which is 2x-4x the amount of float I have. My disc still freely "rattles" although movement is 0.004" or 0.1mm

[Garey Guzman]

...Garey,
The description of "shear" on NAS bolts was confusing to me, as I've also seen bolts that are described and designed to shear as a fail-safe to protect more expensive equipment (like a PTO on a tractor - they use low strength "shear" bolts on the blades in case the mower hits something). In both our brake application and the tractor example, the bolts are in a shear environment. But obviously we don't wish for them to shear off in brakes!
I thought I read somewhere else that NAS bolts have higher shear strength than AN bolts and are thus desirable for applications in shear....

That may be true for commercial bolts but for AN/NAS/MS, it means they are designed for shear force application. While they are typically the same or similar in tesional stregnth, they don't have the same tensile capacity due to smaller head. I think someone else mentioned the closer-tolerance shank for shear capicity with the same size.

According to the AP catalog, normal float is 0.2-0.4mm, which is 2x-4x the amount of float I have. My disc still freely "rattles" although movement is 0.004" or 0.1mm

Personally, I'd want the designed amout. The alum hat will expand the most, possibly taking out the "float" and perhaps even binding the assy, maybe even binding inconsistantly around the hat. Alternatively, the lack of "float" may just be due to a burr or slight deformation of the har, which can be cleaned up.

[mikey]

Looks like the bobbins I have are a mismatch for rotor flanges. I’m ordering new rotors anyway so I can either have the flanges made 0.2mm thinner or get a new set of bobbins from AP. The ones I have I think were made by Lola originally, but the AP ones should work nicely.

[mark elder]

Looks like the bobbins I have are a mismatch for rotor flanges. I’m ordering new rotors anyway so I can either have the flanges made 0.2mm thinner or get a new set of bobbins from AP. The ones I have I think were made by Lola originally, but the AP ones should work nicely.

Buy new rotors and new bobbins all from AP. Don't machine the rotors because you will get caught down the line when you change rotors again (either urgently at the track between races or at home; the replacements almost certainly will not have been machined the same; ask me how I know).

Worst senario you can machine the bobbins trackside to get the disc to float (with a hand file and a vice or vice grips... yes, grimace) but you cannot machine the rotors trackside easily unless you bring a big lathe or have a deft touch with a grinder (grimace again).

Keep the old bobbins in your trailer. Not at home in the workshop. Use new nuts and have a spare set too.

Float cold and float hot depend on what the hat is made of. New discs need to be bedded in with new pads. After the first run, check the float hot and check the nuts for tight.

Just my lock down 2 cents worth.

[mikey]

solid advice Mark. My rotors are custom though, a standard 280mm AP disc doesn't fit - have to be made to 276mm. Having that done and will always carry a spare set of rotors with me in the trailer. Actually will have spare hats and bobbins as well. I don't leave anything at home if it fits in the trailer for sure.

good point to check float hot.

[mikey]

New rotors and center hats have arrived - Coleman racing made them for me. Measured and determined I need bobbins made to my new spec. AP standard won't fit.

[fkennette]

Will the Coleman bobbins work?:
http://www.colemanracing.com/Drive-B...Kit-P3603.aspx

Francis

[R. Pare]

Anyone with a CNC mill can make these for you. However, a custom small batch like this will be way more expensive than a set that was machined in a batch of a few hundred.

My suggestion would be to rework the rotor slots to accept existing bobbins that are readily available.

[tlracer]

As per the other thread on this, originally I felt new bobbins was the way forward. However on consideration the small dimensional differences led me to feel the logical solution is to have Coleman machine the slot width and flange thickness to match the AP spec, so standard bobbins can be used.

AP flange = 6.30-6.35mm
Slot width = 9.40-9.45mm (clearance)

Those dimensions are clearly fine on strength/safety as AP discs are to the same sizes.

[mikey]

Anyone with a CNC mill can make these for you. However, a custom small batch like this will be way more expensive than a set that was machined in a batch of a few hundred.

My suggestion would be to rework the rotor slots to accept existing bobbins that are readily available.

Spot on - thanks. Got a quote from a very capable machine shop to make bobbins and it was prohibitively expensive. Could have bought a whole set of new rotors and bobbins for the same price. Getting my rotor flanges and slots re-machined is much more reasonable cost, even with the two-way shipping. Then I will be able to use the standard bobbins.

Attempting this route now, provided that we can get the AP bobbins!

I'll be sure to update once mission accomplished.

[mikey]

Update:
Well the new AP bobbins arrived and somehow they all fit the slots of my Coleman made rotors. The Coleman sheet says the slot width is 0.370" (9.40mm) which would be exactly the width of the AP bobbin. But the bobbins slide in seemingly fine. Difficult to measure with my calipers, but now I do believe the slot width is actually 9.50mm and that coincidentally is what AP discs use for slot width also. One machine shop I had talked to had claimed I needed a bobbin to be 0.015" narrower than the slot, but I don't believe this is the case now after reviewing the AP catalog data.

Bottom line, I guess I can use my new rotors as-is with the new AP bobbins that are the correct length for 0.4mm float.

Happy ending to this story with some learning along the way.

Cheers,
Mike

[tlracer]

Old saying: "Measure twice, cut once"

I'm pleased it all worked out and, in a way, not at all surprised the Coleman discs actually had the correct dimensions and clearances to be directly-compatible with AP mounting hardware.