Saturday, August 28, 2010

Q&A: Straightening Bent Chainrings & Finessing Your Shifting

Q: Hi Jim,.
I have a Raleigh road bike, and I've been told I need a new front chainring (the largest one), because it has been bent due to “hard shifting.” I would prefer not to have to buy a new chainring and I noticed your recommendation that shifting not be done with much force.

What shifting strategy is used when you are dealing with coasting into dips on a pathway that has a very sudden and maybe longer rise (the kind you may find in city bike paths that have valleys, rivers nearby, and the like)? Does one enter the dip with the highest gear (highest speed), then very quickly and successively downshift with the rear gears as one is traveling upward again? Do you leave the front shifting alone?

Sometimes this causes the “crunching” noise of the chain as one travels out of the dip, indicating too much force is being used, or that the chain may also be skipping sprockets during the shift, and ultimately bending the chainring?

I’m reluctant to continue an enjoyable non-stop trip on these paths as it’s expensive to replace the chainring. On the other hand, not having to stop and walk the little way out of each dip is the goal of a good ride.

Look forward to your advice,
Karl
Canada

A: Sorry to hear about your bent chainring, Karl. I'll talk about your options there first and then talk about the shifting issue.

Truing/Straightening Bent Chainrings (also called "chainwheels")
The first thing I’d ask is how badly the chainring is bent? I ask because in most cases you can bend them straight again. Not if they’re bent so badly they will break if you try to bend them back, but as long as the metal is only bent a little out of true or a few teeth got knocked out of alignment, you can almost always bend them straight again and just keep riding them. Steel is easier to bend back but aluminum isn’t too hard either. I’ve fixed many bent chainrings and rarely have to replace them.

Before trying to straighten a bent ring, check that the chainring bolts are tight because loose bolts will let a ring flex and it can appear bent when it's only the loose bolts causing the issue. Also, check that there's no play in the bottom bracket bearings, which an also create the illusion of wobbly rings.

To straighten chainrings try using an adjustable wrench with the jaws adjusted to just slip over the chainring. With the chainring bend between the jaws you can use the wrench handle to flex and bend the chainring back into shape. There is also a tool made just for this by Bicycle Research (photo).

To gauge your progress, spin the crankset by hand and sight the ring where it passes the front derailleur cage to see if it's nice and round and true with no side-to-side wobble.

Another way to do it is to strike it with a rubber mallet, or a hammer and a screwdriver. The mallet is easy to use, just striking the ring on whichever side needs moving over. To use the screwdriver, which is good for reaching hard-to-reach places, hold the tip of the screwdriver on the bend and strike the handle of the screwdriver with a hammer.

To get the hang of straightening with the hammer or hammer/screwdriver technique, tap gently at first and then increase the impact as you learn how hard to strike it to true the chainring. You can then fine-tune it until it is using your adjustable wrench or screwdriver and hammer. You can also use a screwdriver as a prying tool to true rings and the adjustable to bend the spider of the right crankarm (what the chainrings are bolted to), which is sometimes the part that gets bent.

Notes: Someone might tell you that bending or hitting the chainring will effect/impact the crankset bearings, but they can handle anything you can do with your tools and it's nothing to worry about. Also, in my experience if a mechanic doesn’t know that you can bend them back (because he hasn’t had a lot of experience or because he taught himself how to fix bikes), he may say you have to replace bent chainrings – and that’s not true except for seriously bent/damaged ones – as I mentioned).

And for some chainring fun, be sure to visit Bike Cult's amazing Chainring Archive and the incredible Chainring Tattoo Project.

Learning to Shift With Light Pedal Pressure
Next, on the shifting, you can actually make any shift you want at almost any time if you can just remember to ease the pressure off the pedals when you’re doing it. The chain needs to be flexible sideways in order for the derailleurs to be able to push it sideways onto another gear or chainring. Also, it has to be flexible so it can release from the teeth and move sideways. When there’s pressure on the pedals the chain becomes a solid bar of steel and it will not want to move sideways and that causes all the problems.

What you might do is practice shifting over and over and over in a traffic-free parking lot until you get really good at shifting while soft pedaling with no pressure at all on the pedals. You only turn the pedals to complete the shift - you put not pressure on them. Then you’d have the technique down and you could shift anytime you want.

Or, you could just limit your shifting and choose your gears before you need them. This is like driving a underpowered standard-shift car. You see a hill in the distance and you know your car can’t make it up the hill in 4th gear, so you shift well before the hill rather than stalling out trying to shift halfway up the hill when the car doesn’t have the gearing/power to make it up any further.

On your bike, you would shift before the dip (on the downhill) and then shift back once you got up to the top again and could take the pressure off the pedals.

To smooth shifting and chainrings that stay straight,
Jim Langley

1 comment:

Myles Francis Barrell said...

Precisely the guide and encouragement I was looking for after being told my slightly bent and misaligned chainring needed replacing. Thanks!