Likely to damage crank or BB with overtightening?

[peterh11]

Hi,

Hope someone has some insight here. I wonder how likely it is that I have damaged my crank or bottom bracket axle. Story below ...

I've just fitted some Spa TD-2 triple square taper cranks along with a First Components DX-30 113mm sealed cartridge bottom bracket, as recommended by Spa. I'm using them with a Shimano Tiagra groupset - ie everything else is Shimano's 5703 groupset (I wanted smaller chainrings, and have had good experience of their XD-2 on another bike).

The crank bolts are now done up as tight as I can reasonably do them with a 200mm hex wrench. The right hand one seems to be rather close to the centreline of the bike, I estimate 1-2mm closer than as specified in a Shimano datasheet linked to an earlier post in this forum, by measuring from the inside of the inner chainring (between 34 and 35mm as compared to spec of 36mm).

The gearing all works fine (at least on the work stand - will test on the road tomorrow) so it's within the tolerance of the 5703 FD.

However, the inner chainring is very close to the frame (splays out to give clearance for big tyres), and the inside edge of the crank where it goes on to the axle is very close to the bottom bracket cup. I am not sure that I have done them up tight enough really (can't feel any play with hands or standing on the pedals, will check again when I have ridden a few miles) so I am considering getting a 116mm bottom bracket to move it out a bit plus allow a bit of tolerance in case they need to be tighter.

BUT ... when I first put the right hand crank on and fitted the bolt, I was using a new torque wrench set to 40Nm, and either it didn't click or I didn't notice. I stopped when I realised I had missed the click. By then the crank was touching the outer edge of the bottom bracket - oops. From the force I was exerting on the wrench, I estimate I had put somewhere in the range 60-80Nm or torque on. I hurriedly loosened it again. Clearly a beginner's mistake with a torque wrench, I will need to have a test to make sure it works and that I know what to listen/feel for.

So ... is it likely I have damaged either crank or BB axle? If I would not have damaged the axle, then a 116mm BB would be appropriate. If so, then I'd need a new 113mm one. Thoughts?

Peter H

[Marcus Aurelius]

Yes. You can damage both, by over tightening. Get a decent torque wrench and spanner head attachment.

[thatsnotmyname]

Highly unlikely you will have damaged anything, due to the nature of square taper. It's possible you may still need a wider BB axle though, but you may have a better idea tomorrow after a test ride..

[ElCani]

Highly unlikely you will have damaged anything, due to the nature of square taper. It's possible you may still need a wider BB axle though, but you may have a better idea tomorrow after a test ride..

I agree it unlikely that you’ve done any harm, you’d have to have really beasted it to damage the crank. But it does sound like you need a longer BB spindle.

[Brucey]

I would say that you could dress the back of the crank to avoid the clash and if necessary use a spacer between the RH BB cup and the frame to 'fine tune' the chainline with the present BB. But as a rule I like to set my triples up on road bikes to give the smallest possible chainline measurement (*); in practice this is limited by the chainring hitting the frame, the cranks hitting the RH cup, and/or the FD running out of travel.

(*) this improves the chainline in the 'cruising gears' (big ring) and when climbing in the small ring, using the lowest gears. The chainline is worse in other ratios of course but these are ones I don't use often or are duplicated in better form anyway.

BTW I would examine the inside of the crank taper and check for cracks but I doubt that you will have done any real damage.

cheers

[David9694]

The place you can do damage is when removing the cranks - the outside of the remover is screwed into alu threads, don’t forget.

[Samuel D]

There’s no chance of having damaged the steel taper or the crank’s square hole. The bolt wrings off before that. I’ve done that a couple of times.

Jobst Brandt has tackled this worry of crank damage somewhere. If you’re interested I might be able to dig up the post I have in mind.

I find this the most difficult bolt on the bicycle to set without a torque wrench. The torque doesn’t kick up in the usual way but smoothly and evenly creeps up with no hint of when to stop. The fear of wrecking a crank on the first ride can easily lead to a broken bolt.

If your crank hit the bottom bracket hard enough, there might be localised damage there. Have a look? Unlikely to be more than superficial unless you drove the crank around against the cutting edge of the bottom bracket.

I can vouch for this spacer for fine-tuning chainline or crank clearance.

[mattsccm]

Can't comment on the likely damage, you'll have to inspect for that. Can't even comment on optimum chain line as that may vary according to your needs . (although I do what Brucey does). However I do have a thought about the chain line and its measurements. Surely you can't go by Shimano guidance if you are not using their cranks and BB?

[Samuel D]

Couldn’t find the Brandt comment I wanted, but here’s one to allay fears of over-tightening crank bolts. That’s from a quarter of a century ago when most road bicycles had square-taper cranks.

[peterh11]

Thanks all.

I had a look this morning, and I can see no visible damage to anything. I did realise immediately what had happened and took the crank off again, so that will have helped avoid damage. I also checked in case the shop had sent me a shorter BB than specc'ed - sorry to doubt you Spa it is the correct one. Everything feels OK when I ride it up and down the street but it seems to me that I cannot do the crank up even as tight as I could get it with a 200mm hex wrench (which has been my standard practice for many years), before either the crank rear face or the inner chainring touches the frame.

I will take the bike out for a ride tomorrow, with said 200m hex wrench in my bag, and stop part way round to check if anything is coming loose. But I am inclined to get the 3mm longer BB anyway since I don't like having that small chainring so close to the frame (less then 1mm clearance!) and I have some room to move my shoe cleats out a bit (therefore shoes in to the crank) if it makes the pedal stance too wide for me.

I have a nice shiny new torque wrench which I bought specifically to deal with jobs like this. Not sure what happened. I will set up a test rig to practise with it and check it is not faulty.

By the way, the Tiagra 5703 gear system performed perfectly with the Spa TD-2 cranks (24/34/46 chainrings and 11-32 cassette) on my little test ride. I just moved the front derailleur down to match the 46 tooth outer (was standard Tiagra with 50 tooth), adjusted the cable and bingo! I reckon once I have sorted out the BB axle I am ready for some real hills. Looking forward to being able to travel again and use those low gears!

Peter H

[peterh11]

Thanks all.

I had a look this morning, and I can see no visible damage to anything. I did realise immediately what had happened and took the crank off again, so that will have helped avoid damage. I also checked in case the shop had sent me a shorter BB than specc'ed - sorry to doubt you Spa it is the correct one. Everything feels OK when I ride it up and down the street but it seems to me that I cannot do the crank up even as tight as I could get it with a 200mm hex wrench (which has been my standard practice for many years), before either the crank rear face or the inner chainring touches the frame.

I will take the bike out for a ride tomorrow, with said 200m hex wrench in my bag, and stop part way round to check if anything is coming loose. But I am inclined to get the 3mm longer BB anyway since I don't like having that small chainring so close to the frame (less then 1mm clearance!) and I have some room to move my shoe cleats out a bit (therefore shoes in to the crank) if it makes the pedal stance too wide for me.

I have a nice shiny new torque wrench which I bought specifically to deal with jobs like this. Not sure what happened. I will set up a test rig to practise with it and check it is not faulty.

By the way, the Tiagra 5703 gear system performed perfectly with the Spa TD-2 cranks (24/34/46 chainrings and 11-32 cassette) on my little test ride. I just moved the front derailleur down to match the 46 tooth outer (was standard Tiagra with 50 tooth), adjusted the cable and bingo! I reckon once I have sorted out the BB axle I am ready for some real hills. Looking forward to being able to travel again and use those low gears!

Peter H

On second thoughts, I will not take this bike for a long or high intensity ride till I have fitted a longer BB. I will use a different bike tomorrow. There may be no safety issues with one ride, but better not to risk it.

[Samuel D]

Everything feels OK when I ride it up and down the street but it seems to me that I cannot do the crank up even as tight as I could get it with a 200mm hex wrench (which has been my standard practice for many years), before either the crank rear face or the inner chainring touches the frame.

Only you will know how tight you mean by that, but I have broken crank bolts with an Allen key shorter than that and I’m not a powerful man.

The John Deere book, Fundamentals of Service: Fasteners, says “The maximum pull that the average person can exert on a wrench with safety is about 100 pounds (445 N)” although a man’s full exertion goes higher. Even allowing for the impossibility of applying all force at the very end of the 200 mm wrench, I think your method would result in more than 40 Nm at the bolt. Maybe much more. But that’s why you got the torque wrench.

But I am inclined to get the 3mm longer BB anyway since I don't like having that small chainring so close to the frame (less then 1mm clearance!)

That does sound too close. I’d be happy with a couple of millimetres and prefer the chainline to err below the Shimano spec rather than over.

My last Spa frame had a little too much milled off the ends of the bottom bracket shell. Did you measure the width of yours? Maybe that accounts for their recommended bottom bracket being a little short.

I have a nice shiny new torque wrench which I bought specifically to deal with jobs like this. Not sure what happened. I will set up a test rig to practise with it and check it is not faulty.

Maybe exercise it a few times from the bottom of the gauge upward, making sure not to dip below the minimum lest the parts reassemble in a slightly different position the next time, conceivably throwing out the calibration.

[peterh11]

Everything feels OK when I ride it up and down the street but it seems to me that I cannot do the crank up even as tight as I could get it with a 200mm hex wrench (which has been my standard practice for many years), before either the crank rear face or the inner chainring touches the frame.

Only you will know how tight you mean by that, but I have broken crank bolts with an Allen key shorter than that and I’m not a powerful man.

The John Deere book, Fundamentals of Service: Fasteners, says “The maximum pull that the average person can exert on a wrench with safety is about 100 pounds (445 N)” although a man’s full exertion goes higher. Even allowing for the impossibility of applying all force at the very end of the 200 mm wrench, I think your method would result in more than 40 Nm at the bolt. Maybe much more. But that’s why you got the torque wrench.

But I am inclined to get the 3mm longer BB anyway since I don't like having that small chainring so close to the frame (less then 1mm clearance!)

That does sound too close. I’d be happy with a couple of millimetres and prefer the chainline to err below the Shimano spec rather than over.

My last Spa frame had a little too much milled off the ends of the bottom bracket shell. Did you measure the width of yours? Maybe that accounts for their recommended bottom bracket being a little short.

I have a nice shiny new torque wrench which I bought specifically to deal with jobs like this. Not sure what happened. I will set up a test rig to practise with it and check it is not faulty.

Maybe exercise it a few times from the bottom of the gauge upward, making sure not to dip below the minimum lest the parts reassemble in a slightly different position the next time, conceivably throwing out the calibration.

Thanks. I did a bit more research on the torque wrench and I reckon I bought one which is too big/long/powerful (28 to 210 Nm), so 40Nm is near the bottom of the range. I found several sites including a supplier suggesting that the "click is less noticeable at low torques, and you shouldn't rely on it at less than 20% of the maximum - which is where I was. So I ordered a smaller one (maximum 107Nm) and will experiment with that. Oh well, I learned something, and the bigger one might be useful when doing things to the car, or will make a good present for someone.

I am quite small. Holding a 200mm hex wrench with my hand fully on it, I reckon I am applying force at about 150mm from the axis of rotation, so to get 40Nm I need to apply say 260N or thereabouts, or 26-odd kg force. That is 40% of my body weight. Anyway, it seems to have worked on several bikes over several decades, and that right hand crank just doesn't seem tight enough for safety. I am actually just as concerned about the bottom bracket itself, which is why I decided to get a torque wrench, and maybe I will get a surprise once I get a proper reading!

My BB shell is genuinely 68mm across, I checked that.

PeterH

[cycleruk]

A few years back I have fitted a couple of bikes with spacers to offset the cranks to stop exactly what you are experiencing.
Fitted between shell and right hand BB cup. Worked a treat until I got the correct BB length. In fact I think one may still be on a bike?
I'm not sure but seem to remember using a spacer from a cassette.
Chainset chainline for a triple is middle of frame downtube to teeth top of middle ring.

[Brucey]

There’s no chance of having damaged …...the crank’s square hole. The bolt wrings off before that....

You are unlikely to split the crank asunder in one go. However what can happen is that the crank is damaged through being overtightened such that it fails at a later date. Hence my suggestion that you inspect it for cracks; if you have made even a small crack then this will cause the crank to break at some point.



FWIW some are better than others, but I do not think there is a brand of crank that cannot be broken.

cheers