Adding Disc Brakes to a Nomad Mk2

[George Hetrick]

I recently had my LBS add disc brakes to my Nomad (and add a suss fork) in preparation for a summer trip.

I have the Thorn Expedition rack, and was surprised that the rack wound up not clearing the disc caliper without adding a wide spacer to move out the rack.
I don't think this sort of spacer is doable in a real touring situation with a heavy load on the rack.

So, my question is how to fix this?
  - is there a lower profile disc caliper that I should use instead?
  - is there a suitable spacer that is higher strength?

I'm sure Thorn (and most likely, Danneaux) has thought this through, but haven't seen the answer in a cursory search.

Thanks, all.

[JimK]

Yeah that is really ugly. I see that Topeak has racks with spacers like that integrated with the rack. That would seem to be stronger.

How does something like this fail? Seems like no matter what, the load on the rack puts a shear stress on the bolt. The spacer doesn't make that better or worse.

Definitely the spacer does create a tension on the bolt. As the rack load presses down, the spacers can turn and stretch the bolt. Seems to me that increasing the outside diameter of the spacers is going to reduce that tendency to transform shear to tension. I think a stack of wide washers should work fine, the biggest that fit. Probably it is the smallest washer that is the weak link, so no point in mixing sizes. The one closest to the dropout will probably be the limiting size.

[JimK]

Or maybe machine one or two of these that give a lot more offset:

http://www.sjscycles.co.uk/thorn-for-rohloff-cast-end-2-x-m5-to-1-x-m6-stainless-dropout-adaptors-prod11703/?geoc=us

I would thicken the part that the 2 5mm bolts go through, the part that is thin on the existing part. Just use long 5 mm bolts. That 5 mm part is like a nice wide stack of washers already.

Maybe you could just augment the exist part by machining some washers with the 2 5mm holes to line up with this. Just make the washer match the diameter of the existing part. Ha, unless somebody already makes washers with 2 holes and the right spacing! Though actually a short bit of tubing might work... outer diameter the same as the outer diameter of this part, inner diameter so the 5mm bolts just clear.

[George Hetrick]

Jim, that 2xm5 to m6 converter is already on there; the LBS added additional spacers. The additional bolt length worries me, since it's a lot easier to bend a long lever than a short lever. At least it's an m6, though.

[JimK]

Ha, it's the third cup of tea that does it! How about this:

Just use the exist adapter but get longer 5mm bolts. Get some kind of block of metal. Width = distance between the 5mm bolts: the block will fit between them. Height = outside diameter of the adapter. Depth = that stack of spacers you need.

If you can just find some strips of metal whose width is the distance between the 5 mm bolts. Cut the strips into short pieces, whose length is the o.d. of the adapter. Then stack them up to make up the space you need.

[George Hetrick]

Ahh. That sounds like the ticket. Thanks. I'd still love to hear whether there are calipers that don't create the situation.

[JimK]

I am just an amateur but it might work. Yeah, easier just to avoid the whole problem, for sure!

[David Simpson]

Instead of a stack of spacers between the M5 bolts, you could use a single large spacer of the width of the Thorn adapter with holes drilled through it for the M5 bolts.  That would give a larger bearing surface, so less change of it wanting to "tilt".

As for the width of the brakes, I have a Magura MT2 hydraulic disc brake on the rear of my Nomad.  From the center-line of the disc to the outermost part of the brake is about 18mm.  I'm not sure if you wanted to stay with the mechanical disc brakes, but at least my brake gives you some point of reference as to the width.  I am guessing that hydraulic brakes would generally be thinner than mechanical brakes, simply because mechanical brakes need some width for the mechanical part.

By the way, if you decide to go with hydraulic brakes, I like Magura.  They use mineral oil for brake fluid, which lasts forever.  On my previous bike (2002 Cannondale F800 mountain bike), I have gone for 12 years without needing to bleed or change the brake fluid.  Other manufacturers of hydraulic brakes usually use "regular" brake fluid (similar to or identical to car brake fluid), which absorbs water, and should be changed every few years.  Regular brake fluid has a higher boiling temperature than mineral oil, but that doesn't make a difference on a bike, unless you are doing some serious braking.

- Dave

[Andybg]

My first sugggestion would be to speak to SJS. It is their frame and their rack so I would hope they have looked at this problem before.

Failing that solution

For long term durability I would look at gettting a new m5/m6 adaptor made of the right thinkness. One offs can be expensive but if you look around (advirtise) in your local area you will find someone with a laithe at home who will knock you one up for the cost of the stock and a beer.

I think it will be the best solution especially if you are looking at serious loads off road.

[Danneaux]

Hi George!

I'm a bit late to the party here, but I do have a question before proceeding further:

What caliper brand/model and rotor size are you running?

My first reaction is to second Andy on this one and ask Thorn/SJSC (Oh, Dave...!) as they have surely run into this before. As I recall, the Avid BB7 should fill the bill with fewer clearance issues unless that's what you're already running.

No, I don't like the idea of spacing the rack out beyond Thorn's very well-conceived adapter. Extending the spacing increases the leverage and shear forces on the bolt just as Jim said, and I wouldn't wish to do so unless absolutely necessary. When I used my Miyata 1000LT as my expedition bike, I turned 12.5mm slugs of 6061 aluminum and used those as spacers to better distribute the stress against the dropouts for the bottom mount of the Thorn Low-Loader MkV front racks (my first Thorn purchase that was later transferred to Sherpa and now the Nomad). It worked well, but I kept thickness to the absolute minimum. The larger diameter and single-piece design kept the stack from collapsing and eased the load on the mounting bolt as well.

Now I want to hear about your summer's tour as well, George.

All the best,

Dan.

[Dave Whittle Thorn Workshop]

Hi,

The spacer was made to let the Nomad take hydraulic calipiers like Hope fit with our rack, sadly there isn't a way with the current Thorn Expedition rack to use an Avid BB7, hence there is no option for it on the bike from new. 

Dave.

[Danneaux]

Very helpful information, Dave; thanks!

Best,

Dan.

[George Hetrick]

Thanks, Dave -- that exactly what I had to hear.

I had the LBS put BB7s on, so I'll ask them to look into getting the Hope hydraulics.

Dan, I'm going to be taking the Adventure Cycling http://www.adventurecycling.org/guided-tours/educational-tours/2014-introduction-to-dirt-touring/, which does a very short section of the Great Divide trail. I have extensive credit card road touring experience, but no experience off-road, so figure I'll put a toe in the water this way, and see how I like it.

[Danneaux]

Wonderful tour ahead of you, George; you should have a grand time. Stay safe and remember to have fun. Looking forward to hearing all about it on hour return.

<nods> Yep, given Dave's information, I'd also go with the Hope caliper if it means avoiding the rack-spacing issue. If for some reason that doesn't work out, then I'd go with as beefy a spacer as possible and try to keep any mounting bolts as short as possible. However, given the rough ground you'll be traversing, I'd go with the brake swap if possible. If you do require rack spacers, then I'd use cinch straps to secure the bags to the racks and prevent load-lofting and second-order vibrations and impacts and so ease the strain on the extended mounting bolts.

You're in for a wonderful time, dirt-road touring. It is similar in many ways to paved touring and as different as night-and-day in other ways. A lot of fun to be had there; it is some of my own favorite touring.

Please let us know how you come out with the Hopes. [EDIT: Just to add: 'Sure sorry you ran into troubles as a result of the disc addition; I hope it resolves soon and favorably for you with the change of brakes. Will the shop take the Avids back in trade on the Hopes?]

All the best,

Dan.

[Andre Jute]

I solved a somewhat different but related mounting problem in a way that has worked now for three years, though I don't pretend it is ideal.

The huge, heavy battery for my bike's electric motor is shaped like an oversized water bottle, and mounts to the water bottle sockets. That limits the screw diameter. Furtermore, the size and shape of the bottle requires standoffs if it is not to foul the pipework at the bottom end. I therefore mounted the bracket for the battery on long bolts through an aluminum standoff, the kind common for standing off electronics boards. This works well, without rocking, because the bike has flat-top inserts for the water bottle sockets, and the battery bracket has a flat surface too at the mount, so in essence the standoff quadruple the shear resistance of the fixing bolts.

I'm not suggesting this as a solution for fitting a rack to be used for carrying a heavy load over rough terrain. For a start, off-the shelf spacers will have to be steel, not ali, and will have to be machined to the curvature of the bicycle tube and rack part they are to brace; above all, they would have to be very short because the lever arm effect would be working over quite a distance.

In the specific case, I agree with George. Better to swap out the Avid brakes for the Hope, and use the proven Thorn part.