Intro: GT Bicycle IDrive Repair - the Ultimate Fix
Have a GT made bicycle/Mt. Bike with an iDrive 4 rear suspension system? Then you probably have a creaking problem when you pedal. The clever design by GT Bicycles is very innovative, and does a great job at reducing pedal induced bobbing while pedaling. It's a patented design that sold a lot of bikes in the mid to late 2000's. The only problem with it is that once it begins to wear in it will start to creak. And if you are bugged by these types of things it will drive you nuts. I am bugged by these types of things. I went nuts...trying to figure out how to fix it. Then I did. The following instructable details the design, its flaw, and its fix. If you have the mega-tools and knowhow you can do it yourself, otherwise, you can direct a machine shop for a reasonable price to do it for you.
Step 1: GT's IDrive 4 Design and Flaw
The parallelogram design of GT's complex iDrive 4 rear suspension pivot is clever and effective at stopping pedal bob, especially while climbing in the saddle. However, its fatal flaw is the eventual wear of the aluminum to steel connections in their floating bearing design. They use an off-the-shelf 1-1/8" roller bearing from a headset that is suspended between an aluminum split cone washer and the beveled interior shoulder of the swing arm housing. It works well until the steel of the bearing wears the aluminum on both sides. A heavy or aggressive rider will wear it out very quickly allowing a slight movement during pedaling which translates into a pronounced creak or click that can be felt in the pedals. It's just too weak of a design to last under even normal use. Of course, once the swing arm aluminum is worn it will never be right again. Typically, the problem pivot (there are two) is the one closest to the bottom bracket. The one in the swing arm. It carriers the brunt of the pedaling forces.
Step 2: The Fix: Machining and Bearings
To repair the problem permanently requires a redesign. We will start by replacing the floating roller bearing design with two encased needle bearings that fit perfectly, fully supporting the pivot pin. These can be bought on Amazon: Koyo B-1816 Needle Roller Bearing, Full Complement Drawn Cup, Open, Inch, 1-1/8" ID, 1-3/8" OD, 1" Width. The next step is to fit the new bearings into the swing arm housing which requires a milling operation and the creation of a bushing. Any competent machine shop can do this for you. Bring the fully disassembled swing arm, the bearings, and the bottom bracket carrier with you. Ask them to fit the bearings into the swing arm by milling it out and creating the bushing. Critical: the bushing width needs to take into account the deflection of the bottom bracket carrier when tightened. Meaning, it can't be too wide or it will bind. I found that 1.95" worked for me, but manufacturing differences may mean it's different in your case. The machinist should be able to figure out the correct displacement. Bring the pin and pin nut, and tools with you so they can get it just right. There should be .020" clearance when the carrier is compressed by tightening the pin and nut with moderate torque. This is something they can measure at the shop. The bushing needs to have a slight counterbore turned into it on either side that is .045" deep for the O-rings. See the picture. This allows .020" of the O-ring protruding from the bushing which will seal the entire system nicely when the carrier is installed.
Step 3: The Fix: Assembly
Once the machining is done, the bushing and bearings are pressed. Again, you can have the shop do this or attempt it yourself. The pictures above show the process. Once pressed in place so that the bushing and the bearings are perfectly centered. Drill and tap a hole for a zirq fitting exactly as shown in the picture once the bushing is pressed. They are typically 1/4-28 threads. You can get a zirq fitting online or at a good hardware store. This enables you to simply grease the bearings once in awhile (No disassembly) and has the added benefit of holding the bushing in place. Push the pivot pin in place and pump grease into the zirq until oozes out. rotate the pin in both directions while doing so to distribute the grease evenly. Acquire two O-rings 1-7/16" OD x 1/16". They are in the plumbing section of a good hardware store. Carefully install the bottom bracket carrier over these O-rings ensuring that they remain in place as you do so. See the picture. They should have a layer of grease on them. Install the pins, tighten moderately, check for binding, reassemble and ride away. Enjoy the new found silence while pedaling up your favorite climb. (who says you have to toss these bikes once they start to creak?!) Ride on!