Frame bosses (bottle / fork cargo cages)

[highandwild]

Is there a move towards having 4mm bosses for bottle and cargo cages instead of the traditional 5mm?

I've just bought a cargo cage and it only takes 4mm bolts (slots are too narrow) and only two 4mm bolts have been two provided.

Also no washers.

[jameso]

Odd. Bottle mounts and rack/guard eyelets are M5.

[highandwild]

I always thought that but Sonder Ti Gravel frames have 4mm bosses.

I do wonder if this is the case on all of their frames.

I am quite sure that someone has bought a cargo rack before for a frame with 5mm bosses and told them about it.

[jameso]

It's a bit of a break from convention if so but I suppose you could go smaller for bottle mounts, for racks or cargo cages I'd rather use M5. M6 for rack foots wouldn't be a bad idea.

[highandwild]

It is probably OK but to sell a cargo cage that does not take 5mm bolts is a bit odd.

[PaulE]

Just to check, are you meaning the thread size/diameter or the allen key size?

As above, I thought all bottle mounts are M5, but with either a 3, 4 or (I think) 5mm allen key head. M5 also handy as it's the same size as a brake rotor bolt in case of loss

[jameso]

Good point.. and SPD cleat bolts.

[fatbikephil]

I've actually wondered if manus might start speccing M6 for rack mounts as M5's are pretty weedy. M4 might be strong enough for bottles but they will be less idiot proof for the likes of me who tend to crank things up to the point the allen key starts flexing....

5mm drill bit?

[highandwild]

Just to check, are you meaning the thread size/diameter or the allen key size?

As above, I thought all bottle mounts are M5, but with either a 3, 4 or (I think) 5mm allen key head. M5 also handy as it's the same size as a brake rotor bolt in case of loss

They are 4mm thread size and the allen key head is not 5mm, I assume that it is 4mm.

No other bolts on any of my six bikes are 4mm.

This is unconventional and I just wonder how much strengh one loses by using 4mm instead of 5mm
if any.

[jameso]

This is unconventional and I just wonder how much strengh one loses by using 4mm instead of 5mm
if any.

Very roughly 1/3 shear strength increase each step between M4-M5-M6. I'm not sure why M6 wasn't the default to begin with for rack foots - at least for actual touring bikes that have separate eyelets for racks and guards. But I wouldn't worry about 2 or 3 M4s holding a cargo cage with a couple of kgs or less in it. Use threadlock and large washers with the right ID, it'll be fine.

[Lazarus]

Aren't stem bolts all 4 mm as standard ?( Unless you just mean water bottle bosses in which case ignore )

Strength well it's smaller so weaker but metal is incredibly strong
You can read the difference here. For cycle applications and holding a bottle or rack a M4 will be fine probably but bigger will be stronger . I assume at some point the thickness of the tube will also be a factor - small hole may be stronger as a big hole may weaken tube ( probably matter more on seat stays which can be thin)
https://www.engineeringtoolbox.com/amp/ ... _2026.html

[Bearbonesnorm]

Obviously there are many different grades of bolt. If in doubt, buy better / higher spec bolts.

I'm tempted to say that with bottle bosses, the weakest link won't be the bolt but more likely the integrity of the rivnut.

[thenorthwind]

When I was fitting the racks to my new commuter recently, it also occurred to me that M5 seems worryingly inadequate. Most of the mounting parts are made of 3mm plate, which is reassuringly chunky, but then ultimately the majority of the load goes through two (one on each side) M5 bolts. I haven't looked up the shear strength, and I'm sure it would cover the rated 25kg load easily, but that doesn't tell the whole story (static for a start).

[Zippy]

I'm tempted to say that with bottle bosses, the weakest link won't be the bolt but more likely the integrity of the rivnut.

I concur entirely.

Generally speaking, I'd expect M5 to be fine on a bicycle for racks and bottles and stuff unless you're into cargo bike territory. I'd happily sit on a bicycle rack whilst it's going along held in with a couple of M5 bolts... and I think I read Steve Peat to make is DH bike lighter used to run 3 bolts on his discs...which is 3 No. M5 bolts doing downhill bike things! M6 i car stuff territory and using you're 10mm spanner.

You've also got to think about the direction the forces are acting, and how the system works. Normally with a bike rack, the bolt will be in shear against the rack, but some of the force may actually be imparted through the frictional force of clamping the rack tight up against the bike frame. Your car wheel for example is actually in place due to the friction of the wheel against the brake disc due to the clamping force of your M12/M14 wheel bolts/studs (although in M5 i beleive they use shear studs, and I'm not sure how the porsche centerlock system works...but generally speaking this is correct!).

Bottle mounts on the other hand depending on their location may well be utilisng the tensile capacity of the bolts.

And like norm said, you can get different grade bolts. I've used aluminium bolts to hold in bottle cages before with no issues - a proper graded 8.8 M4 bolt will likely be stronger than that.

[Alpinum]

As so often; circle

viewtopic.php?f=7&t=15662&hilit=M4+rack

Little reminder - I rode across rough surfaces for many weeks carrying food for 36 days, 15 L water and gear to stay warm at >6500 m eg a set of lightweight mountaineering boots. End of March I rode across Iceland with the very same rack and bolts, food for 13 days + winter gear.
Still waiting for the tiny, weak bolts to fail

and I think I read Steve Peat to make is DH bike lighter used to run 3 bolts on his discs...

Yep. Saw it live, he was doing it for many years and is still alive. I run 2 x 6 alloy bolts to hold rotors on one of my mountainbikes, one dedicated to only ride steep stuff in the Alps. No issues.

"Don't pack your fears" and such...

[jameso]

One reason I''d be in favour of M6s in some places for racks etc is that so many people fitting them and using the products don't have the mechanical nous to run 3 disc rotor bolts etc safely. And racks are firstly a heavy load tourer or daily commuter product. They tend to be the most used and worn-out (in?) bikes you see. And it's pretty common that people can't hear the difference between things like a loose rack foot mount and the pan rattling in a bag, or don't care if they hear it at all, people with no real mechanical sympathy. Still, you can't make everything foolproof. RTFM etc and there's sense in standardising fittings where you can.

[Alpinum]

One reason I''d be in favour of M6s in some places for racks etc is that so many people fitting them and using the products don't have the mechanical nous to run 3 disc rotor bolts etc safely.

Quite understandable from a product developer's point of view.