fietsbob

or as said heat treated steel bolts.. the steel in the bolts holding the cylinder head on the engine are a different steel

than the ones holding the licence plates on.

DropBarFan

Thanks for the ACE tip. I always grease threads but it's good to hear that superficial rust isn't a big problem.

DropBarFan

I agree, too many makers cheap out on little stuff like bolts/mounting hardware. Today was tightening 2.5 mm-head bolt on friend's cheapo SRAM grip shifter: high temp/humidity caused hand to slip & rounded out the allen surfaces. 2.5 is just crazy, even 3 mm is stupid IMO. Back when I had Campy road bikes I admired how their stuff had indestructible bolts, often 1 mm larger than competition.

DropBarFan

BTW what ever happened to chrome-plated spokes? Supposedly more fragile than stainless but I never had a break. After cleaning the wheels it was a nice reward to see chrome-plated spokes glinting in the sunlight.

elcruxio

I've done a bit of research on this and here are my findings. However not going to cite stuff since I'm in the law field and I can only properly cite legal stuff and higher engineering mechanics are just too complicated. So I think I have a very Basic understanding of this, but if you want clarification then someone in the engineering field is going to have to come and weigh in a bit

So as number one

1) tensile strength is a good indicator of bolt strenght also in shear since the shear strenght is typically about 60% of miminum tensile strength. Essentially this will give us the maximum load of about 600kg per bolt with 8.8 grade M5 bolt so a total of 1200kg of capacity. However as that's the maximum load that is actually when the bolt snaps so loading the rack to a fraction of that is advisable (big bumps in the road can raise the point load a bolt suffers to alarming levels with a heavy rack). A 12.9 grade M5 bolt gives us about 900kg of load capacity so 1800kg for the whole rack.

But that's not the whole story. grade 8.8 bolts and 12.9 bolts have pretty much the same fatigue strenght and this is also very important since a rack bolt is going to be suffering fatigue stress due to the constant bouncing around. But the 12.9 bolt is also harder, ie. more brittle which means that even though its absolute strength is higher, it also bends less before snapping. A lower grade bolt is softer and more ductile so it can take more 'bounce' before damage occurs and can actually bend before snapping.

Also a weird thing, but using the smaller M5 bolts may not be a bad idea. The funny thing is that smaller diameter bolts actually have better fatigue strength than thicker bolts. So while with a M6 you get a LOT of absolute strenght you get a bit less fatigue strength. Here's where it goes full layman, but I suspect it's due to the fact that a smaller diameter bolt is more elastic, so it can bend more without permanent damage. Or at least a thicker bolt will require a longer lever distance to get the same amount of bounce or spring. However going smaller than M5 may again not be optimal since then the absolute shear strength suffers dramatically (strength actually drops exponentially with diameter drop so an M5 is not 20% weaker than an M6 but rather 32% weaker)

But I'm still against the use of stainless bolts as I believe those have a 1/3rd of the shear strength of a carbon steel bolt. Also when using zinc plated bolts and threadlocking compound one has a double defence against corrosion welding for example in aluminum frames as one has the zinc as sacrificial metal and the threadlocker blocking contact.

Which brings me to
2) It's actually a really good idea to space the rack out from the frame a little. Tubus racks always come with a lot of spacers and they do recommend spacing a rack out rather than force bending it against the frame.

So why does it work? A longer exposed bolt has again, more spring to it than a bolt that has the shear concentrated on the very small area directly between the frame and the fork. Again going full layman as I don't completely understand how this works, but a bolt with no spacing will snap when the fatigue limit is overcome and that fatigue is concentrated on a very small area. Steel as a material doesn't really like high point loads. Steel wants to flex. So By spacing the rack out a bit we allow for that flex so a bolt head can travel a longer distance horizontally before the plastic stage and eventually fatigue limit of a bolt is exceeded.

If we think about this in bolt size, a smaller bolt requires less spacing than a longer bolt. In this regard I think the M5 actually a pretty good compromize.

Of course one can just brute force it and use a bolt so tough and big that it's just not going to break in any case, but I think that's going to need to be bigger than an M6. Although M5's with good bolt grades don't go that often so maybe the brute force method is an M6... I dunno, I'm gonna go with what I got

So to summarize
Using a mid strength bolt rather than a very high strength bolt is a good idea because of
- more ductility
- same fatigue strength
- 'weaker' threads (not really weak) can give a better tension preload rather than the very hard threads of a high strength bolt which just caps the frame threads easier (this is a total guess)
Spacing is good because
- Allows for more bolt flex thus raising the fatigue damage threshold
Using an M5 bolt is good because
- requires less spacing to allow for bolt flex. A bigger bolt flexes less at the same length of bolt so requires more spacing
- Absolute strength is still sufficient for most rack needs
- fatigue strength is better than with a bigger bolt.

Now keep in mind, this all only works if good fastening methods have been applied so
- bolt has been properly tensioned
- a thread locking compound has been used
- if not, an alternative vibration loosening prevention method has been used (Nord-Lock spacer or Nyloc nut)
- Don't grease bolts to prevent corrosion damage, use copper paste. It insulates better, stays in the threads better and is also sacrificial in terms of corrosion
- Best to use a thread locker compound. Although I'm going to use a Nord-Lock spacer and loctite 243 compound

Some of you may think I'm over thinking this but I'm essentially an engineer at heart and looking up all kinds of metal properties etc is hobby of mine (when I get the money / house I'm going to start my own hobby bladesmithy/bicycleframesmithy). I just could never cut it at math so I went into law instead. Oh well, at least I can put people in jail :d

Now if some real engineer could come and shoot all of this down it would be much appreciated

imi

Good job elcruxio!

DropBarFan

Elcruxio, interesting though counterintuitive ideas there. I wonder about the longer bolt concept. If a rack is mounted on the end of a longer bolt wouldn't give more leverage to potentially break bolts? Seems that Tubus' rec to not bend racks to fit is to ensure rack integrity & not specifically intended to to use longer bolts due to better fatigue resistance. After all, most racks have traditionally been designed for short bolts; seems unlikely that mfgs' engineers would have overlooked an obvious shortcoming.

BTW, I find it interesting that disc brake makers (Avid at least) use Torx-head bolts for rotors. Link below agrees w/me that Torx is a good step for smaller bolt heads (ie 2.5 & 3 mm esp). Also, FWIW, the sturdiest racks may be ones that are welded or/brazed onto the frame. But in the US where 99% of bikes are for recreation/fitness only we don't see those much.

Bolt Tech - Will Torx Win? - Slowtwitch.com

elcruxio

BYou're actually right with the spacing thing. I got it wrong. Unfortunately there's nothing that can usually be done about it.

DropBarFan

I wonder how much of rack design is due to tradition & market limitations vs the ideal? Vertical rack stays seem to rely on compression--isn't tensile strength higher than compression? IE touring bikes often have a braze-on mount for rear rack upper stays, maybe they could have a 2nd mount a bit lower on the seat stay? Might help clean up the area around hub etc. Also, the thin tubes of racks don't seem to make much sense for heavy loads. Wouldn't 1 or 1.5 cm rack tubes (hollow not solid) be a lot stronger?

elcruxio

I don't think actual good quality steel racks failing is an issue. At least tubus racks are seriously heavy duty and I'm sure they would cope very well in even rough offroad use with heavy loads.
It's usually the bolts that is the issue, but by using better bolts than stainless most problems are solved apparently. I've only read bolt snapping with manufacturer provided bolts which are usually stainless. Then there are people who use stainless bolts even in offroad touring and do fine.

Also, a rack doesn't really get close to its weight limitations ever, so it's not really about tensile vs compression. A good quality steel rack can take probably tonnes before it goes and that is very hard to achieve in bicycling context.

And actually when I think about it, Tubus does use 1-1.5cm cromo tubes so there's that.

Tourist in MSN

Tradition often has a design basis behind it. If it works don't change it. And often the village blacksmith was a pretty good self taught engineer who learned from poor designs what does not work.

If you mounted it midpoint on the seat stay instead of at the dropout, you are then applying side force to a seatstay that was designed mostly for compression and some side force from rim brake operation. On many (most?) bikes the seat stay is more robust where the rim brakes are attached, perhaps the seatstays were designed with extra strength for side forces from the rim brakes?