Here is some of the knowledge I gained over the past 15 years of making bikepolo products. Hope this is useful to others who want to supply good stuff to the community. To be extended…
Rotor guards
MPC rotor guards were always laser cut from 5 to 6 mm EN AW-5754 H111 aluminium. This provides the neccessary strength, I wouldn’t recommend using thinner or thicker plates. With some lightweight structures my guards always weighed in at around 200 g, which is light enough IMHO.
I see no real benefit in using higher strength aluminium alloys because of the following reasons:
- On a big impact, something has do deform. If it’s not the guard, then it might be the disc tab on your fork, which you want to avoid of course. Better bend back the guard after a big impact than having to swap your fork.
- Higher strength alloys are usually not available as a standard option at most laser cutting contractors. Using them anyways will drive up the cost significantly.
- Most high strength aluminium alloys lose a big part of their strenght after welding. In those cases it actually only makes sense to use such materials if you also go for an expensive post-weld heat treatment.
I have no experience with making steel guards, but a key part of the logic above still applies: the guard should deform instead of the fork.
Forks
Between 2010 and 2015 I tried finding reliable manufacturers for steel forks in Europe. Companies that could provide what I needed for MPC (strong steel forks, 25-150 pcs per batch) were hard to find. Those who offered reasonable prices failed to deliver forks that were strong enough for bike polo. I ended up looking for a manufacturer in Taiwan and never looked back. The quality was always top notch, I only had one cracked fork returned to me since 2015. Anyway, if you’re looking to manufacture bikepolo steel forks yourself:
- Absolutely don’t weld cable guides onto the front half of the fork legs, as they will act as stress risers. The front side of the fork will be under tensional stress during braking, so cracks will definitely develop over time.
- The best material choice is probably 25CrMo4 (=4130). There are other similar candidates, for example 15CDV6, but you’ll probably struggle to find the right sizes at reasonable cost. In any case, use tubes with a tensile strength in the range of 750-1100 MPa. Below that, your fork will likely bend. Above that strength, the risk of fatigue cracks increases.
- If you can’t find your tubes in the right heat treatment state, you can have the material heat treated yourself. Ideally after welding the entire fork – in this case make sure that the forks are stacked vertically in the heat treatment oven, otherwise they will bend during quenching! You should insist on having the strength checked by Vickers or Rockwell hardness measurement (not Brinell, this will yield wrong results for thin tubes). Ask for your parts to be processed with materials of similar carbon content, otherwise relevant properties of your tubes might change.
- Fork leg tube dimensions should be at least Ø 28 x 1 mm straight section. However, I would recommend 1,2 mm wall thickness if you can find that. I always went for Ø 28,6 x 1.2-0.9-0.7 mm tubes, they worked extremely well.
Frames
Most of my frames were made from straight wall EN AW-7020 T6 tubing with no additional post weld heat treatment. In some instances I used Columbus and Dedacciai mountainbike tube sets from 7005 aluminium, but they were not strong enough even after post-weld heat treatment (mostly resulting in bent down tubes on minor impacts). Cold bending of seat- and chainstays never caused any problems. Dropouts and yokes were CNC’d or water cut from EN AW-6082. Again, some things I had to learn the hard way:
- Many design iterations lead to these tube dimensions: top tube Ø 35 x 2 mm, down tube Ø 40 x 2 mm (ends ovalized), seat tube Ø 35 x 1,5 mm, seatstays Ø 19 x 1,5 mm, chainstays Ø 25 x 2 mm (ovalized to 30/20). If you decide to go below these dimensions, you should really know what you’re doing.
- Be VERY careful with dents on the chainstay to increase tire clearance, they will be a weak spot.
- Look twice if you get a custom frame made at a too-good-to-be-true super cheap price. You might end up with a very different geometry than what you ordered. Precise manufacturing has its price.
- Some thoughts on wheel sizes:
- Does the wheel size define the handling of your bike? No. It’s the combination of rim and tyre size that counts. 26″ rims with 42 mm tyres will roughly add up to 643 mm wheel diameter (559+2×42), almost exactly the same as for 650b rims with 30 mm tyres (644 mm). Changing tyre size from 25 to 35 mm will increase the wheel diameter by 20 mm. Among other things, the wheel diameter defines trail and bottom bracket clearance, which heavily impact steering and riding behaviour of your bike. So if you just specify a wheel size for the frame, you’re leaving out a lot of important details. That’s why there are recommended rim/tyre combinations for these frames, making sure that the bike you build has the optimal riding properties.
- Assumption 1: There is an ideal weight distribution between front and rear wheel. Assumption 2: A comfortable riding position depends on your body height. Assumption 3: You don’t want toe-overlap.
Keeping the wheel base of your bike fixed at a certain value (ideal wheight distribution and riding position) while increasing the wheel diameter will inevitably lead to toe overlap at some point. If you want to avoid toe overlap, there is a maximum wheel diameter for a given frame length which you should not exceed. In other words: The shorter the frame, the less sense it makes to use 700c wheels, because there simply is not enough space. The longer a frame gets, the more space you will have for the wheels. While you don’t neccessarily have to increase wheel diameter with frame size, experience shows that bigger frames handle nicely with bigger wheels. That’s why the recommended wheel sizes for the V6 frames scale with the frame size.