Normally, grocery shopping is not a highlight of my week. Occasionally, however, I stumble upon a bargain that makes it altogether more pleasurable. Today I found some button mushrooms going for about a third of their usual price, as they were approaching there sell-by date (but still looked in pretty good condition). Needless to say, these came home with me and were cooked up for my tea with garlic, olive oil, butter and a bit of salt and pepper. Very tasty and a great way to show I can write short blog posts if I put my mind to it. 🙂
All posts for the month March, 2016
Posted by Magnus on 2016/03/23
For quite a long time now, I’ve had at least a theoretical grasp of the importance of correct torque settings for bike mechanics. Recently, though, I had practical confirmation of this — and of why my recent purchase of a torque wrench was money well spent. In the process I also discovered a handy alternative to my previously limited repertoire of ways of dealing with a stripped screw head.
Torque is essentially a measure of turning force and in this context it amounts to how tightly you do up your nuts (stop sniggering at the back there!). If you want to know more about the gory details, Google can furnish you with many happy hours of reading material.
The reason that torque is important for bike mechanics (and I’m sure also for many other engineering-related disciplines but I’ll try to stay focused here) is that if you don’t tighten up nuts and bolts sufficiently, parts tend to drop off the bike at inopportune moments, while if you overtighten them you not only make them harder than necessary to remove when the time comes for maintenance but run the risk of doing damage to various components. Unfortunately, while it’s generally fairly obvious which bits need to be tightened a lot and which bits can and should be left looser (though there may be a few surprises in store), it is pretty difficult – even with quite a lot of experience – to correctly judge torque settings by feel alone. As a rule of thumb, which seems to be borne out by my own experience, the tendency is to overtighten parts which shouldn’t be too tight and to insufficiently tighten ones which should be good and tight.
Having had a few bad experiences in both directions, I finally got round to getting myself a torque wrench about 3 years ago. I went for a traditional beam-type one since they are relatively cheap and robust. I already had a socket set (probably one of the single most useful tool purchases I’ve ever made, especially for non-specific bike tools) so this, along with a handy chart of torque settings found in the back of my bike repair handbook, enabled me to get at least into the right ballpark with my torque settings. The downside of the beam-type wrench is that it can be quite difficult to accurately read the scale as you need to be able to look at it head on to avoid parallax errors; this is sometimes virtually impossible to achieve while you’re actually using it (or maybe it’s just my poor technique?). Even if you do manage to get it at a good angle, this particular wrench doesn’t have a particularly finely grained scale so the results are necessarily fairly approximate.
I therefore decided a few weeks ago to invest a little bit more in a click-type wrench. This is basically a ratchet driver which is set up so you can set it to a desired torque setting and it will “click out” when you reach that level of torque. They are somewhat more delicate than the beam-type, and may need occasional recalibration, but as long as it’s well looked after a click-type wrench should work well for a good long time. It certainly makes it much easier to achieve a specific torque setting as you can feel rather than having to see when you get there.
The main disadvantage of click-type wrenches is that they tend to have a relatively limited operating range of possible torque settings, and it’s just about impossible to find a single wrench that will cover all the settings needed on a typical bike. I decided to start by getting one that would do the low-torque settings (mine goes up to 24Nm) since I’ve usually found I’m more likely to overtighten than undertighten things and my beam wrench also seems to work a bit better for giving it quite a bit of welly (to use the technical term). At some point I may also get a click-type wrench to cover the higher torque range, but it’s not a very high priority at the moment.
When my new torque wrench arrived the other day, I was enthusiastic to start straight away on checking that all the various fittings on my bike (or at least the low-torque ones) were tightened to appropriate levels. This went well until I got to one of the bolts holding the brake arms on to the frame. The bolts for these particular brakes (which I put on as replacements for the original front brakes a couple of years ago) appear to be made of aluminium and I discovered that last time I took them off for maintenance I overtightened one of them to the point that the recess had started to deform and the hex key would no longer grip sufficiently to be able to undo it. Looking on the bright side, it’s better to discover the problem now rather than waiting until the brake had seized up and needed imminent maintenance while I was stranded on a roadside miles away from home.
My usual trick when faced with a screw or bolt with a damaged head that will no longer take its usual driver is to use a hacksaw to cut a slot so that I can attempt to use a flathead screwdriver on it instead. Sometimes this works and often it doesn’t (not to mention being a non-starter when the entire bolt head is recessed and you can’t get in to cut it. On this occasion I tried my usual trick and it failed.
Since the brake is a pretty important component to be able to keep in good working order, I didn’t really have the option of giving up on it. I didn’t fancy trying to drill the bolt out in the hope that I could somehow cut a new thread and save the frame to be able attach the brake again. Nor was I quite ready to admit defeat and take the wretched thing to the bike shop, so I used the time-honoured research method known as Google to explore alternative ways of fixing the problem.
This turned up two possible solutions I’d previously been unaware of. One is a little gizmo known as a screw extractor — basically a drill bit that you use to drill into the stuck screw in such a way that it basically unscrews itself. The other is a little bottle of stuff which isn’t quite glue but seems to have the effect of providing a better grip so that your tool can get enough purchase on the damaged screw to be able to turn it. There appear to be several brands, pretty much all of them of American origin, which may or may not be the same stuff under the different labels (it seemed to be pretty vague about what’s actually in it); the cheapest I could find was one called Screw Medic (costing about £3 for a small bottle, but it’s supposed to have a pretty unlimited shelf-life and only require a couple of drops per application, so it should last a fair while). This latter option seems a bit more straightforward to use than the screw extractor but only work on screws with fairly light damage, while the extractor should get pretty much any screw out.
Undecided as to which option to go for, impatient to wait and try one before ordering the other if the first didn’t work, and figuring that knackered screwheads seem to crop up fairly often (perhaps due to my previous lack of care and attention to appropriate torque levels — or making sure I’m not using a Pozidriv screwdriver with Phillips screws or vice versa), I decided to order both. The Screw Medic was the first to arrive and was awaiting me when I got home from work this evening. I’m delighted to say that it worked wonderfully and I was easily able to unscrew the damaged bolt and replace it with the one that I’d saved from the old brake (evidently a steel bolt and much sturdier than the other, though I still took care to tighten it carefully to only the required 6Nm with my new torque wrench).
So when the screw extractor set (there are actually 4 or 5 different bits for different sizes of screw) arrives, hopefully in the next few days, it may be a while before I need to use it. And if I make good use of my torque wrench (and my Philips and Pozidriv screwdrivers) I may never need to use it. Which would be fine by me.
Posted by Magnus on 2016/03/21