M6 Rack Bolts - Overkill?

[slowster]

It was a full head height as it was the best screw I had. I reckon I needed only a millimetre or perhaps two extra but it wasn't there.

I think the standard height of the head of an M6 button head screw is 3.3mm, compared with 6mm for a standard socket head. A standard thickness (form A) washer underneath the head would add 1.6mm.

Although inserting the bolt from the inside is preferable, I doubt it is something which the designers of drop outs/frames and frame manufacturers incorporate intentionally in their designs. I suspect it will only be a minority of drop outs that have a rack eyelet positioned sufficiently far from the axle that a bolt could be inserted from the inside without fouling the chain on the smallest sprocket (or when shifting to/from it).

It is possible to insert the M6 rack bolt from the inside on the Spa Wayfarer, but it uses disc specific drop outs, the material of which extends well above the axle and so allows the rack eyelet to be positioned quite high.



Rather than focusing on how some designs might be preferable to what you have, maybe a better perspective would be how much worse some designs are than what you have:

[Brucey]

Rather than focusing on how some designs might be preferable to what you have, maybe a better perspective would be how much worse some designs are than what you have:

that cinelli design deserves a special prize. Quite exceptional!

cheers

[horizon]

It was a full head height as it was the best screw I had. I reckon I needed only a millimetre or perhaps two extra but it wasn't there.

I think the standard height of the head of an M6 button head screw is 3.3mm, compared with 6mm for a standard socket head. A standard thickness (form A) washer underneath the head would add 1.6mm.

I looked again today and it is in fact slimmer than I thought - about 2 mm maybe. It still catches the chain though - on the up change back to 12T sprocket - and jams solid. It does actually fall onto the 11T OK. I've put the limit screw on the two smallest sprockets now so that is beginning to be a tangible loss of gears but I'm going to leave it for now. I also did what I should have done at the beginning - checked whether it was for a 700c wheel or not. As it is for a 26" wheel it's going to go onto another bike at some point. This rack has actually been wrong in every way possible!

Rather than focusing on how some designs might be preferable to what you have, maybe a better perspective would be how much worse some designs are than what you have:

Despite the inordinate faff with this rack, the bike is a joy of simplicity, from square tapers and V brakes to bar end shifters, external cables and a nine speed set up. The failure to check the rack spec is totally mine.

[alexnharvey]

Would a 1mm head give you sufficient clearance if you were to file the head down?

[PH]

If you use a full-threaded set screw from the inside of the dropout (clearance for head permitting, reduced head size is a good idea) such that it becomes a captive stud, and then use a good washer and nylock on the outside, this arrangement

1) is immune from b) above; at worst the stud bends once and once only, since it doesn't turn
2) the nut won't be lost
3) the threads in the dropout don't see a turning bolt under load so are unlikely to be damaged
4) the threads in the dropout are not loaded (the fastener head sees the load) and are therefore unlikely to be damaged
5) the loaded length of the bolt is made longer so there is more 'stretch' in the bolt and the tension is less likely to be lost
6) should the bolt ever break the remains can always be unscrewed from the inside.

So I went for this solution in the end rather than mess around with taking a rack off another bike. The head of the bolt fouled the 11T sprocket so I set the limit screw to avoid that. I won't miss the 11T but it isn't a perfect solution.

I might consider using a grub screw, with a suitable thread lock. I can't see why it wouldn't cover 1 - 5 the same, though it would compromise 6.

[horizon]

horizon wrote: So I went for this solution in the end rather than mess around with taking a rack off another bike. The head of the bolt fouled the 11T sprocket so I set the limit screw to avoid that. I won't miss the 11T but it isn't a perfect solution.

I might consider using a grub screw, with a suitable thread lock. I can't see why it wouldn't cover 1 - 5 the same, though it would compromise 6.

I'm trying to visualise this but AIUI, because the frame hole is M6 and the rack hole is M5, there must be a head in order to secure the bolt (and a nut of course). The bolt (of any type) can't be threaded into the frame hole. An M5 bolt (using a nut) can be inserted (not threaded) from either side, the advantage from the inside being that only the bolt head protrudes, not the nut and remaining length of bolt.

[Brucey]

perhaps an M5 CSK bolt in a M6 threaded hole can have the head trimmed until it is almost flush? A slot can be cut in the head to accept a flat-blade screwdriver.

cheers

[reohn2]

Rather than focusing on how some designs might be preferable to what you have, maybe a better perspective would be how much worse some designs are than what you have:

that cinelli design deserves a special prize. Quite exceptional!

cheers

The inappropriately placed and not very strong brazeons special prize would be my guess

[PH]

I'm trying to visualise this but AIUI, because the frame hole is M6 and the rack hole is M5, there must be a head i

Sorry. my bad. I haven't quite grasped the concept of not being able to enlarge the hole in the rack, probably because I don't think I've ever had a rack where I couldn't (Not that I've needed to)