20/24" fork dropouts vs 16T high torque Mac motor?

I'm planning to build a cargo bike frame from scratch. The bike will be a long-john style cargo bike with the cargo in front of the rider, low to the ground with a 26" rear wheel and a 20" or 24" front wheel. Kinda like this:



Currently I have a 16T Mac motor in a 26" wheel on the front of a Yuba Mundo. Don't like how the Mundo handles with cargo on the rear deck (which is where I almost always have to haul my heavy, bulky stuff), so I'm building a whole new bike. The motor will get laced into the new front wheel which will be either 20" or less likely, 24". I need to have disc brakes because of the speed/torque of the motor as well as all-weather performance, and to stop the loaded e-assisted bike better than rim brakes. Problem is, there is very little in the way of 20" forks with disc tabs here in the U.S. You have to go overseas to UK/Europe websites to look for trials bike forks basically. I could just take a BMX fork and weld a disc tab on, or a kids bike fork, but I'd rather have a fork that's already built to take serious punishment and that means a trials bike fork. Who'd have thought that here in the U.S., land of "hold my beer and watch me do something stupid," that there would be absolutely NOWHERE here to buy a trials fork?! So, I already bought a fork from a UK seller but it's a P.O.S. (came with crappy welds and a bent dropout), so I'm in negotiations to trade up in an exchange to a higher dollar fork. Only problem is those higher dollar forks look to have dropouts that might not play nice with a geared hub motor. Right now I use two Grin Tech torque arms on both sides, but I've considered the next option being those torque arm dropout thingies that you actually epoxy permanently to the fork. Either way, what do you think of the forks linked below? I'm wary of the dropout that have a hole in them above where the wheel axle goes, and I'm also somewhat wary of the dropout design where the fork leg bottom is hollow. However, we're talking about forks that are made to withstand some serious force from jumping off of high objects or bouncing up onto things where the front wheel takes a direct, high amount of force which obviously heavily loads the fork/dropout area.

kiltedcelt said:

I need to have disc brakes because of the speed/torque of the motor as well as all-weather performance, and to stop the loaded e-assisted bike better than rim brakes.

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Most disc brakes are absolutely not more potent than linear pull brakes with good pads and a booster arch fitted.

Also, consider that most of the time, a front loader has less weight on the front wheel, and less forward weight shift under hard braking, than a normal bike. It is entirely possible to skid the front wheel and wipe out, if you have brakes that come on strong with poor sensitivity, like most hydraulic discs.

Because the cable routing of a front loading bike is long and indirect, hydraulic brakes have a rare advantage in this case. But you should look at Magura hydraulic rim brakes, because they have better feel and sensitivity than discs. They fit on ordinary cantilever brake studs.

The main potential problem I see with the forks in the picture is that they appear to have inadequate offset for a cargo bike with normal head angle. Regardless what kind of brake you use, a fork like this is likely to result in better steering geometry and superior handling.



The disc tab can be used to fasten a torque arm.

I was more concerned about the fork legs and dropouts being strong enough to handle the torque of a motor in a 20" wheel versus in a 26" wheel. Interesting thoughts about hydros versus mechanicals. On my Mundo, the current brake setup is TRP Spyke mechanical disc brakes which have dual pistons and because that, they have MUCH better modulation than any Avid product. Performance in all weather is also a reason I really prefer disc over rim brakes, that and not having to worry if a rim gets a little out of true. Although these days, the bomber type rims I build wheels with I seldom ever have to worry about a wheel going out of true. However, hydraulic disc brakes were what I was likely going to install on this bike, specifically the TRP HY/RD which is a self-contained hydraulic caliper actuated by a cable. The one thing I hate about the Spykes on my Mundo is that with the current softer pad compound they require frequent adjustment of the barrel adjuster to account for pad wear, which is much worse on a heavy e-bike than on something much lighter like my Kona Wo 29+ fatbike. The self-adjusting pads on a hydraulic caliper are much more to my liking for a project like this. As to the angle of the fork, I'm not sure it would make much difference since I'm welding the frame up, I can set the angle to reflect whatever sort of trail and handling characteristics I want. More to the point though - do you think the dropouts with the hole through the middle as with the first fork would be inherently weaker in a geared hub application? The welding of the dropouts on the other two forks looks to be more secure design, and in the case of the third fork which has both canti posts and a disc tab, I could (with a dual control lever), theoretically have two braking systems on the front wheel, but as the bike will be lightly loaded a lot of the time that seems pointless overkill. I was thinking of designing the frame to accommodate the battery case right behind the fork just ahead of the cargo area in order to concentrate some weight on the front end of the bike for improved handling and traction when unloaded.

Motor torque, and therefore stress on the torque anchor, is the same regardless of wheel diameter. With a smaller wheel, you get more thrust from the same torque.

I recommend against using cable-hydraulic brakes on a front loader. In that case you combine the shortcomings of the hydraulic brake with regard to sensitivity, with the shortcomings of the cable brake in an installation with long indirect routing.

Unlike a normal bike, on a long john the rear brake is more important. Choose the front brake for best feel and feedback, because most of your available braking power will be in back.

On a small wheel, the braking response of a disc brake is magnified, whereas a rim brake's response remains the same independent of wheel size. The long tortuous cable routing suggests hydraulics, but the small diameter, lightly loaded front wheel suggests a rim brake. It's for these reasons I recommended a Magura hydraulic rim brake.

A modern disc brake with radial master cylinder like e.g. Formula RX, RO or T1 has enough power AND modulation for your purpose.
I advise against Shimano Servo Wave or Hope brakes as the few I've ridden had terrible modulation, albeit good braking power.
Get a small brake rotor, you will not need more than 160mm on a 20 inch wheel, and the rotor will stay true over longer period.

Chalo / miuan - thanks for the input. The one thing I *can* say with certainty is that the Mac motor pretty much requires a 203mm rotor for a mechanical disc brake, simply because no other caliper would work with a smaller rotor due to freakishly tight tolerances. Another vote for Magura hydraulic rim brakes on the front wheel then. I think I'll just do a 24" front fork. The seller I got the cheapie fork from agreed that it's not up to their standards and are letting me keep it to experiment with for welding purposes and such, and will offer me a full refund as credit towards another fork. I think I'll go with that 24" fork as that middle one is apparently an all-aluminum fork, so completely unsuitable for this purpose. The other fork (24"), of course has rim brake bosses and a solid dropout design. They have some other forks that have a different trials type of rim brake mount that sits on the backside of the fork, and ostensibly is designed to maximize the braking force typically used in trials. So, I think I've got all the answers I needed. Rim brake hydraulics up front and for sake of not having to fiddle with constant pad adjustments, hydraulic disc in the rear. Thanks guys!

A decent steel fork should be fine,, just get it from a good BMX,, not a 50 buck one.

Easy to weld on a disc mount, if you do weld.

the 16t motor will not have more torque than an 8t. In fact, it will pull lower wattage under load. But if you mean to rotate slow under heavy loads, then the lower rpm motor will be running more efficient, and might in fact, feel like more torque. In any case, it will run cooler at low rpm under heavy loads than a faster motor.

Even better IMO, a lower than usual rpm DD. Not a 16t though, 9 t is plenty for a DD in 20" rim. That is what I used on a bike designed particularly for carrying a heavy load of battery, and towing a trailer for heavy cargo. In 20" rim, it ran pretty efficient at speeds as low as 8 mph.

Better still, if you are buying new shit,, that bike needs a mid drive. I only went with the low rpm hub motor because I had 3 of em laying around the garage.