Crown-to-axle measurement

[chansen]

I'm putting together a 26er for my 10-year-old daughter who is 5' tall and all legs. That way I get to use my old Easton wheelset.

I bought a Manitou M30 fork already. It was a crazy price on AliExpress, and came in less than a week. I can't share the listing because they are sold out, but there are more on there.

The fork has a crown-to-axle measurement of 476mm. I've been looking at three different frames. One has a crown-to-axle of 490mm, one 474, and one 450.

Is this measurement critical? Seems to me it would be, for clearance at least, if not geometry. Or am I just being too picky?

490mm:
https://www.aliexpress.com/item/Cheap-price-stock-frame-2014-cube-frame-mtb-bike-frame-cube-mountian-bike-frame-size-14/32249756705.html?spm=a2g0s.9042311.0.0.CsF0bw

475mm:
https://www.aliexpress.com/item/Full-carbon-mtb-frame-26er-16-or-14-inch-26-carbon-fiber-mountain-bike-frame-with/32610714078.html?spm=2114.search0104.3.76.7a5a87b00pc4hD&ws_ab_test=searchweb0_0,searchweb201602_3_10152_10151_10065_10344_10068_10130_10342_10547_10343_10340_10548_10341_10084_10083_10618_10630_10139_10307_10313_10059_5711218_10534_100031_5722318_10103_10627_10626_5711318_10624_10623_10622_10621_10620_10142,searchweb201603_25,ppcSwitch_5&algo_expid=340646f2-cfb2-4f16-aee9-5be5d4e49e54-13&algo_pvid=340646f2-cfb2-4f16-aee9-5be5d4e49e54&transAbTest=ae803_3&priceBeautifyAB=0

450mm:
https://www.aliexpress.com/item/High-Quality-Full-Carbon-Fiber-UD-Matt-26ER-Mountain-Bike-MTB-BSA-BB30-Frame-FLX-FR/32557323827.html?spm=a2g0s.13010208.99999999.277.pKHkzk

[mechDad]

I am doing the same thing for my fast growing 10 year old son. He already has a 14" 26er with a fork a2c of 450mm.
He will outgrow it in the next year I reckon. Would like to reuse everything on a new 16" frame.


No expert here, but this is what I think.....

Ideally you would match the frame to your existing fork length.
20mm fork difference works out around a 1 degree change in head angle.


So the frame with a 490mm a2c I would avoid. It already has a steepish 71 head angle, and the shorter fork would make it even steeper. Might make the handling too quick.
plus the shorter fork will drop the bottom bracket clearance as well.

The frame with the 450 a2c has a 70.5 head angle, so this would end up somewhere around 69 degrees with your fork.
Bottom bracket would be raised a bit. This would make for a slower handling bike that what is was designed for, but a lot of newer bikes have slack head angles, and many people prefer that.

The 475mm a2c bike matched your existing fork. I have seen others build this bike up, and it looks great.


First post for me, so apologies if I got this wrong.

[bxcc]

So generally speaking, a 100mm 26" suspension fork will have an a2c in the 470 to 475 range. Going 20mm in either direction won't be too drastic of a change but do be aware of the BB drop when going shorter. Keeping the cranks at 165mm will help with this and the terrain she will be riding plays a part too. If it's relatively low key stuff, i wouldn't sweat it with 165mm cranks. At 5' tall, you probably don't want to go longer than that anyway.

Like Mechdad mentioned, 20mm of travel equates to about 1 degree of head angle change.

These rough a2c numbers are based off of 100mm travel forks
505 = 29"
485 = 27.5"
475 = 26"

So by those numbers, a 29" fork with 80mm of travel is only 10mm longer than a 100mm travel 26" fork. My daughter ran that setup for 2 seasons on as XS GT Avalanche and it worked great for her. I swapped the stock 6 pound coil fork for a 1600 gram Sid that a local rider sold me for cheap.

All this info really just means that if she isn't hucking big drops and riding crazy single track, she should be good with either frame. Just remember that a longer fork is usually better than a shorter fork. Especially since most of the low cost 26" carbon frames have really steep head angles.

[chansen]

Okay, went with the first one, with a 490mm specified fork length, and got a 27.5 fork to match (495mm). I'll sell the 26" fork locally.

The frame is very well finished. It's a 71 degree head angle, but I don't consider the 1 degree difference to be a dealbreaker. I like the internal cable routing for the gears. I'll keep the brake line external. This frame, I'm sure, could accept a 27.5 wheel. In fact, they sell an almost identical 27.5 frame, and the only difference in the listed geometry is 5mm longer chainstays. I measured mine, and they are 425mm, as listed for the 26" frame.

All in all, this is coming out nice.

[chansen]

Part of me is also thinking if I put at 27.5 hoop on the front wheel, that also slackens the head angle.

[carbonazza]

Part of me is also thinking if I put at 27.5 hoop on the front wheel, that also slackens the head angle.

A smaller wheel at the rear to slacken the head tube, isn't it ?

[chansen]

A larger one at the front does the same thing.

[carbonazza]

A larger one at the front does the same thing.

Oops... we were not in the 29er section