Mid fork rack mounts

[Freddie]

I have some mid fork rack bosses (described as Blackburn lowrider bosses) on an early 90s touring frame. I was looking to use a Tubus rack on the fork, but it seems the Tubus requires the bosses to have threading, while mine seem to be threadless.

Have lowrider bosses changed over time and would it be simple enough to get threads tapped, so that I may run a Tubus rack, if needs be.

[pwa]

I have some mid fork rack bosses (described as Blackburn lowrider bosses) on an early 90s touring frame. I was looking to use a Tubus rack on the fork, but it seems the Tubus requires the bosses to have threading, while mine seem to be threadless.

Have lowrider bosses changed over time and would it be simple enough to get threads tapped, so that I may run a Tubus rack, if needs be.

Are they the sort where you could just use a longer bolt and put a nut on the other side? If so, that would be my approach.

[Freddie]

Come to think of it, I don't see why not. Mightn't the bolt be a little loose inside the tube (laterally speaking) and would this compromise load holding ability?

[andrew_s]

If you are to use a long bolt and nut right through the fork blade, there needs to be a tube right through, connecting the inside and outside bosses.
Otherwise, tightening the nut will squash the fork blade.

[Freddie]

It does have said tube just sans threading.

[Gattonero]

A passing-through boss is actually a better option, if brazing is correctly done (and a steel bike from the '90s is most likely so) it acts as reinforcement so it's better.
You are to use a long bolt, and use a class 10.9 bolt (or Stainless 304 is acceptable) for improved strength, and long enough to use all the washers (large "penny" washers are better to spread the load) and spacers that you may need. And Nylock nuts. IIRC, the Tubus racks come with good fastenings anyway, but if you need a longer bolt you're probably looking to a 60+ mm length.

[alexnharvey]

What is the inside diameter of the tube? Will an M5 bolt pass through? If it does you obviously could not thread at M5 and would need to go to M6. An M6 bolt might not pass through the holes on the rack though.

A passing-through boss is actually a better option, if brazing is correctly done (and a steel bike from the '90s is most likely so) it acts as reinforcement so it's better.
You are to use a long bolt, and use a class 10.9 bolt (or Stainless 304 is acceptable) for improved strength, and long enough to use all the washers (large "penny" washers are better to spread the load) and spacers that you may need. And Nylock nuts. IIRC, the Tubus racks come with good fastenings anyway, but if you need a longer bolt you're probably looking to a 60+ mm length.

I agree with you on the first point about passing through being preferable and the nyloc nuts but I have some queries on some of your other instructions, which you supply with your typical authority

Why would large penny washers be needed when they will be bearing against the end of the tube, probably about 1mm wall thickness? A normal washer or even a narrow form washer should suffice.

Can you explain how you concluded that a 10.9 bolt is required and why 8.8 (for example, or 12.9) would not be sufficient for the load that would be carried on a front rack, given that the bolt is supported along it's length by the tube? Especially puzzling because an A2/304 bolt would be weaker in tensle strength than an 8.8 one.

I would guess that the bolt length is more likely to be around 30-40mm, it should be possible to bolt it on to a tourer without many spacers and so on.

[Gattonero]

What is the inside diameter of the tube? Will an M5 bolt pass through? If it does you obviously could not thread at M5 and would need to go to M6. An M6 bolt might not pass through the holes on the rack though.

A passing-through boss is actually a better option, if brazing is correctly done (and a steel bike from the '90s is most likely so) it acts as reinforcement so it's better.
You are to use a long bolt, and use a class 10.9 bolt (or Stainless 304 is acceptable) for improved strength, and long enough to use all the washers (large "penny" washers are better to spread the load) and spacers that you may need. And Nylock nuts. IIRC, the Tubus racks come with good fastenings anyway, but if you need a longer bolt you're probably looking to a 60+ mm length.

I agree with you on the first point about passing through being preferable and the nyloc nuts but I have some queries on some of your other instructions, which you supply with your typical authority

Why would large penny washers be needed when they will be bearing against the end of the tube, probably about 1mm wall thickness? A normal washer or even a narrow form washer should suffice.

Can you explain how you concluded that a 10.9 bolt is required and why 8.8 (for example, or 12.9) would not be sufficient for the load that would be carried on a front rack, given that the bolt is supported along it's length by the tube? Especially puzzling because an A2/304 bolt would be weaker in tensle strength than an 8.8 one.

I would guess that the bolt length is more likely to be around 30-40mm, it should be possible to bolt it on to a tourer without many spacers and so on.

Hey, I may apologize for my bad use of the english language that makes you feel my comment been done with "authority", but I'm not sure where you get this from.
Truth is that most people speak for having fitted a couple of racks in their life, while I've fitted several bicycle racks only in the last week...
But let's move on, brazed bosses on bicycle are almost always for M5 bolts. For example, older Colnago's were known to use M4 bosses for their bottle cage mounts, while some city/work bikes from north Europe would use M6 threaded bosses for heavy-duty rack mounts. Your forks are most likely to be either threaded or non threaded, but for an M5 bolt.
The class of bolt is related to the load you are going to carry and how safe you want to be: some people like me do not like to carry a lot of weight on their bike (I have a trike than can carry over 80kg ) but other may need to carry some relatively heavy stuff on their bikes. A class 8.8 bolt can be sufficient and the zinc-plated ones won't get corrosion too easy, the A2 bolts are tempting for most but if you want a rustproof bolt a 304 is a better choice, stainless fasteners react to stress in a different way from carbon-steel ones, with less chances to fracture (they don;t really have a yeld point).
A class 10.9 or above is more sensitive to rust so it needs some form of protection (there's several products that can be sourced from the automotive industry, i.e. the ACF50).

[alexnharvey]

I'm sorry, I was taking advantage of your use of English. "you are to use..." reads as a command rather than a suggestion.

I don't think that the load the bolt sees when bolted through the unthreaded, close fitting tube is very different to it going into a threaded hole, but I am not certain and willing to be corrected. On that basis I don't think a special strength bolt is needed and so I wondered why you recommended it.

What grade are the bolts that come with racks from tubus and other manufacturers?

Anyway, I'd use an a2 stainless bolt too.