Brakes for bikes with low seatstays, and front disc question

[Andrew Drummond]

I'm planning a frame for my (almost) nine year old daughter, and while she only weighs 62 lbs, I'm considering discs because of ankle clearance. The frame's going to have low seat stays (making about a 35 degree angle with the chain stays), and will probably end up looking something like these Vanillas:

vanilla01.jpg

vanilla03.jpg

I think the green one was built first, and maybe Sacha went to discs on subsequent bikes because of ankle clearance? Sidepulls or V-brakes could be an option too, but I want as much clearance for fenders and a good-sized tire as I can get. Assuming I go with discs (Avid mechanical BB7 Road with 160mm front/140mm rear rotors), what's the preferred front dropout treatment to keep the axle from disengaging on braking? I've seen lawyer lips and angled dropouts used to address this issue; any other thoughts? Are discs completely ridiculous for a bike/rider combination that won't break 100lbs (for a year or two, anyway)?

thanks,
Andrew

[Jamie Medeiros]

I need more information. What size wheel and tire? What kind of riding is your daughter going to do? I have built children's bikes around 20" (ISO 451) and 24"(ISO 520) as well as 26" (ISO 559) wheels with no problems with the brakes. I feel what makes a good children's bike is keeping it light and simple. I would recommend long reach caliper brakes over disk. The clearance is good, they are easy to set up and install, and they are much lighter. Disk brakes seem like overkill on a bike with fenders like this. Here is a bicycle I did for Corner Cycles, 26" (ISO559) Wheels, long reach calipers, SRAM red (yellow).
225068_10150191825052867_8215694_n.jpg

[Andrew Drummond]

Thanks for the reply, James. 26x1.5-ish tires, commute-type riding, farmer's market trips, short weekend rides. Maybe the occasional camping trip on a rails-to-trails path, so some gravel and dirt. It'll have Porteur bars, and some kind of rack. A super-long reach caliper may be the way to go; I'll have to take some measurements and see what fits.

Andrew

[Andrew Drummond]

James, I forgot to say that's one lucky kid! Nice bike!

Andrew

[Willie1]

On the other hand, not necessarily disagreeing, but with an alternate viewpoint, I use discs on everything because of the modulation and reliability. They are not affected by weather either. The weight penalty is really minimal.

[Andrew Drummond]

Yeah, weight is not an issue - I'm just worried about my daughter throwing herself over the bars the first time she stops! Of course, I'd also like her to be able to stop on a dime if she needs to... it's just going to take experience on her part.

Andrew

[Dave Anderson]

I am with James on this one. I've built a lot of kids bikes and while I will grant that they've mostly been of the "road bike" variety...one thing has always been constant...I've built them as light and as simple as I could (within reason). In general, I like to use caliper brakes on bikes with drop bars and v-brakes on bikes with flat bars. Both are simple and have plenty of stopping power.....but what's very important, from my perspective, is weight..... Kids fatigue easily and they aren't always as able to pace themselves or moderate their efforts as we adults learn to do. Therefore, they get a lot more enjoyment out of biking, and can go a lot farther comfortably, if they don't have a lot of weight to drag around. (Consider that many (most) "kids" bikes are heavier than their parent's bikes!)

I am not saying that you plan on building a heavy bike, or that disc brakes are that much heavier (although a properly built disc equipped steel fork will be significantly heavier than your typical "road" fork IMO), I've just biked a lot with kids and know that it makes a difference and its something that I am always mindful of, etc.

Having said the above....just build what trips your trigger...they'll all make the bike stop, and if you have a personal preference, whatever the reason, that's really all of the justification that's needed.

With respect to your drop out question...either method will work...I personally prefer forward angled drops.

Just my $.02

Dave

[Willie1]

We tend to do a lot of DH and all mountain riding in my circle. We have always used the discs, and have seen caliper and V brakes fail too frequently in our area. It seems this lucky young lady will be mostly on road/gravel so the calipers would likely be fine. If you ride in the rain/mud much, I would go for the discs though. There are a lot of lightweight sub 400g disc specific rims that will offset some of the weight of the discs (most rim brake compatible rims are 400+g.) and the loss is at the outside of the gyroscope. DT revolution 2x lacing or even 2x/radial with aluminum nipples, lightweight tubes or tubeless makes a difference for kids, and with iso559, tubeless works great.

[jon_norstog]

Disk brakes on low-angle seatstays work pretty good - you can set the caliper more-or-less parallel to the slot on a HZ dropout, and use a SS or internal geared hub, no sliders necessary. OTOH, and this is an issue I took up with Bill Ryan of SuperCross BMX - what he calls the meat slicer issue." Kids **will** get their fingers into everything, and that could easily include therotating disk. So it comes down to the coordination and physical maturity of the child involved.

remember Murphy's Law. Other than that I guess it's your call.

jn

"Thursday"

[Willie1]

Being 265lbs, I used a DJ frame for BMX in the old fogies class. (no weight restrictions unlike most BMX frames) I couldn't believe the anti disc protests I experienced. After 4 years and no DNF's due to a smashed disc, or ripped off hydraulic hose (but a lot of great late braking squared off passes BTW) people still were anti disc. Sorry, just an aside.

On the other hand, there are rotating sprockets on the crank, and rotating spokes in the wheels which pose the same hazards. We don't suggest kids should not use sprockets or spoked wheels ;)