How to use your brakes with a 26 inches wheel on a 28 inches frame ?
Hello everybody. This is my first instructable (at least achieved one)
A few weeks ago I bought an e-bike conversion kit. I ordered a kit with a 26 inches motorized rear wheel, because I had in mind to put it on my "tribolt" trike, once it is done (other instructable to come)
I was however thinking I could meanwhile try and enjoy my purchase with another 26 bike bike I had in a garage.
It did not fit my size (and more important, nor dit the pedal assist sensor had enough room between the bottom bracket and the pedal) so I decided to use my 26 motorized wheel with my 28" bike.
However, you may guess there is a big issue with safety here, if you don't have disc brakes. As you replace your wheel, the brake position will no longer be in contact with the rim of your smaller wheel. As the rear wheel hub and the battery make the rear part heavier, you can maybe rely on your front break, without flipping frontward. However it may not be enough, or you may slide, it is risky not to brake with both wheels. We don't want to play with safety. So I wanted to adapt my rear brake to the new wheel's size. I made various attempts. It is now working. Here is the how-to.
Step 1: Gather Material and Measure Your Needs
Adapting your brake to the new rim size is not so easy as it looks, even though not so complex. You will have to consider various questions :
- where to attach your new caliper, at which distance from the former fixations ?
-will you use a mudguard/fender ? if you keep it, where is going to be your brake cable ? Just above or under ?
-which angle do your adapter have to the wheel ? How close will the brake thus be from the rim ?
Take time to look at your brakes, your caliper, the size of your wheel and tire, and assess the best way for you. I will show you what I did, may be you way should be somehow different.
But for me the idea was the following : using some square wooden bars or metallic tube on a symmetric way. One for each side. A hole in it allows to attach to the former caliper attach on the frame. Another hole closer to the center of the wheel (2 inches downward in my case) will be used for the the new caliper axis location. Close to this hole there will be a tiny hole for the caliper spring, you will need to measure their distance from one another too.
Your caliper may have some friction against the tube, or its movement may simply be blocked. You may have to cut a part of the tube to let the caliper turn around its axis.
Use rather steel than aluminium, if you want an effective break. It may be heavier, but less flexible as well. I used aluminium and it does not brake so well as it could, part of the enegy transmitted by the brake lever is distributed along the flexion of the tube instead of the brake pad pressure against the rim.
At last you will have to attach your two metallic tubes, so that they remain at the same distance in spite of the repeated pressure of the caliper tubing and getting back. You can drill holes on the top part of each tube and pass a threaded rod through them, with nuts on each side. Measure your position, the size of your tire and the possible highness of your mudguard to see at which distance from the other holes you will drill this ones. This will influence the total length of tubes too.
To sum up you will have to design :
- front side of the tube, in its middle part, one hole for fixation to your frame
-back side of the tube, in the lower part (more or less 2 inches lower depending on your angles and geometry of the rear fork of the frame) another one for the the caliper axis
-close to this one, some tiny holes to fix the tension of the caliper springs
-lateral sides of the tube, upper part, holes for the threaded rod to give a solidarity between both tubes
Measure the necessary distance between, and the diameter of each hole. Note how much you need to remove the metal to allow the rotation of the caliper.
Gather your material :
-square aluminum tubing (mine is 3cm, 12 cm long each one)
-one thin threaded rod and fitting nuts
-screws , for the caliper axis (8mm maybe) and fitting nuts
-some washers and nuts for your frame/former-caliper-attach to prevent any loose situation and movement. You will use the former screws with them to attach your device to the frame.
Step 2: Cut Your Tube Into Two Tubes and Drill
Cut and get two equal tubes
Drill all the holes you have designed.
Remember your two tubes are symmetrical, not identical. It is so easy to make some mistakes in drilling. Don't get confused between the sides of your square tube either.
If you need to remove some parts of the metal for the caliper to turn easily, then drill as wheel some fat holes centered on the cutting lines. This will allow you ton pass the saw inside and start cutting parallel to the tube, which is otherwise impossible.
Step 3: Cut
Cut the parts that hinder your calipers movement.
Step 4: Install Your Caliper and Brakes Pad on the Caliper.
Now that your pieces are crafted, you can assemble your caliper on the tubes, not forgetting to pass the spring end meet one of tiny holes you have done. Screw more or less loosely for the rotation to remain easy.
Screw your brake pad on the caliper. Ensure even with the pad the caliper rotates freely around its axis, and the spring does more or less its job.
Step 5: Screw Your Device
Attach each tube independently, on each side of the frame. Use some nuts and washer to stiffen enough the tube position.
Then you can pass your threaded rod through each tube, to give solidarity between each side of the device. Use two nuts on each side of each tubing's wall, which means 8 nuts in total. One nut will then be used for setting, the other to lock the first one.
Step 6: Set Everything and Adjust
Adjust everything so that it brakes well :
First make sure your pads ave a big coverage over the rim, give them the the best orientation to this end.
Then you can put back the brake cable and adjust its tension (just liken whatever bike), along with the setting of the nuts along the treaded rod. You will have to set the tension of the springs as well, to adjust quickly your other settings
Then it is done, you can enjoy your new bike geometry and have a smaller wheel !