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| No. 06 DECEMBER 03, 2001 | |||||
Bottom Bracket Overhaul/Install
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Bottom
brackets are one of four main bearing components on a bike. In addition to
being splashed with mud and water in the wet seasons, they also need to resist bending and twisting from the rider's pedaling force. They are also one of the most neglected parts on a bike. This month we'll take a look at how to overhaul and install both Shimano and after market bottom brackets. Before I get into the tool list, I want to mention a few details. We’re going to assume that the BB currently spinning in your bike is the correct size. This may not be the case, so, to find out, you'll need to investigate a few things. Chain line is the distance from the center of the seat tube to the center of the middle chainring. For most drivetrains to shift well, this measurement needs to be between 47.6 and 50mm. If it is more than 50mm, you might want to buy a BB with a shorter spindle to correct shifting problems. If it is shorter the 47.6mm, you might want to buy a BB with a longer spindle. Spindle lengths usually range from 103mm to 127mm. The most common lengths are 107, 110, 112, 113, and 116mm. The BB shell width is the width of the shell on the bicycle's frame where the BB is installed. This measurement will either be 68mm or 73mm. The BBs dimensions change anytime you change the brand of the crank. In some instances you won't need to change BBs, but be prepared. The need to change BB dimensions is caused by the placement of the chainrings in relation to the cranks themselves. Also, on most after market BBs, the driveside cup has no flange (figure 1) so small adjustments in chain line can be made up to a certain point. Too far to either side and riders run the risk of breaking a spindle, wearing out their bearing prematurely, or damaging the threads on the bike's BB shell. Finally, here are a few ways to find out if you need a new BB. Shimano BBs such as the UN-72 and the ES-70, usually last about 3 years without any problems. If you feel any grinding when you turn the spindle by hand or if you feel play in the spindle, chuck it out, can get a new one for about $25. If you have an after market with cartridge bearings, the best time to replace the bearings is at the first sign of roughness when the spindle is turned by hand. If you do feel a bit of roughness, don't bother cleaning them out or repacking them with grease because the dirt has already done its damage. If the trackways and raceways are already pitted and you repack, you'll only ensure a more spectacular demise once the bearings "throw in the towel". The basic tools. -8mm hex wrench--removes the crank bolt. -Crank puller--pulls the cranks off the spindle -bottom bracket tool--loosens the cups of the BB from the BB shell *make sure you have the right tool for the right type of spindle. You’ll have either square or splined. (Figure 2 and 3) also, some companies like Real Design use a traditional BB cup so you'll need a traditional BB tool. (Figure 4). -Socket wrench or large adjustable wrenches for use with the BB tool. -WD-40 or degreaser to clean the BB and shell. -Plastic bristle brush or a tooth brush to thoroughly clean the threads. -Ti prep and grease--ti prep for the cups and grease for the bearing carts on aftermarket BBs or the inside of non-driveside cups on Shimano BBs. -finally, washing the bike, if it's dirty, is a good idea and will make the clean up a little easier. Overhaul -start by using the 8mm hex key to loosen and remove the crank bolts from both cranks. -Clean the bolt's threads. -Use the crank puller to remove both cranks. Make sure to fully engage all the threads of the crank. If you only grab a few, you'll damage the crank threads and end up having to remove the cranks with a large hammer and screwdriver. Not a pretty sight. -Clean the crank hole. The narrow head of a toothbrush helps here. I usually trim the bristles on a brush to half their length so that it cleans more aggressively and to help the brush fit in tight spots. -Using the bottom bracket tool and socket wrench or adjustable wrench mate up the splines from the tool to the BB's non-drive cup and turn counter clockwise to loosen the cup. -Repeat the process for the driveside, remembering that the threads on the driveside are reversed, so to loosen the driveside, turn clockwise. -Clean the cup threads and the threads of the BB shell (the modified toothbrush works wonders on threaded parts). You should be able to drag a toothpick between the threads and not have any dirt get pulled up. -If you have an aftermarket BB with bearing carts, gently pry off the seals with a sharpened spoke. If you only have access to one seal for each cart, don't worry. -Flush out the bearings with WD-40 or other degreaser. -Repack with grease and seal up. Install -begin by applying Ti Prep or grease to the threads of the bottom bracket shell. Don’t skimp here. Make sure you apply a generous amount. -If you have a Shimano BB then apply grease to the inside of the non-driveside cup (driveside is always fixed). -If you have an aftermarket BB then you'll need to apply grease to the inside of both cups. -Begin to thread the driveside cup into the driveside of the bottom bracket shell, remembering that the driveside is reversed, so turn counterclockwise to tighten. If you mix them up and cross the threads you'll be out one bike frame, so make sure you have the right side. Also, thread the cups in by hand. Take your time. -Thread the non-driveside cup clockwise to tighten. -Make a final check to make sure the cups are properly tightened to 50 Newton meters or two grunts. -If you're installing an aftermarket BB make sure to have an equal amount of threads showing on either side of the BB shell. This will give you a starting point from which to make adjustments to the chainline. -Adjustments are made by moving the BB from drive to non-drive side to achieve the correct chainline. -Apply a very, very, very, thin film of grease to the areas of the spindle where the crank will sit. These are called the spindle flats because they are the flat parts of a round spindle. -Slip on one crank arm -apply a liberal amount of grease to both the crank bolt threads and the underside of the bolt head. -Thread the bolts in and tighten 35-50 Newton meters or two grunts. -Repeat for the other side. -Check and retighten cups AND crank bolts after the first 20 hours of riding for the first 60 hours. Failure to do so can ruin a frame, cranks, and BB. Kvestions? Confusion?
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