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I recently just purchased my first 9 zero 7 last week. This is my first fat tire and I had a question about the disc brakes. Notice when breaking hard, the torque causes the fork to flex in under the bike. There seems to be a large amount of stress on the frame and fork. There is no play in the headset or anywhere at rest but feel a shift or play in the front when braking and popping over stumps. Basically, is this normal? Is there any concerns? Is there anything I should look out for? I am used to my suspension eating up the brakes and haven't rode a rigid frame in years. The brakes are avid BB5s with 6 inch rotors. Nate's and toobs, rolling Darryl, 907 headset, 907 hubs. Thanks!
 

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I call this brake squirm.

It's basically because the axles we are using on fatbikes were originally designed for skinny tyred road bikes. We have heaps more grip so there's got to be more feedback and my belief (no proof) is that the axles are flexing. When you think of it, our fat wheels are putting much the same loads into the frame under heavy braking as a small motorbike.

I've tested 100mm and 135mm forks and the best was the symmetrical 100mm, followed by the offset 135mm (brake is closer to the wheel line) and worst the 135mm symmetrical.

Hubs with nuts seem to be better than QRs which is why one reason I prefer them. I have never understood why we haven't adopted the 14mm axles used in some BMX hubs.

The answer is simple - larger diameter through axles like on the new Surly Ice Cream Truck. Then the fork and wheel will operate as a unit whereas with the current setup the legs are probably moving independently fore and aft*.


Edit:
*I tried to check this during a race on a fast corrugated downhill. Stuck my head over the front end while braking to have a look (not a good idea). Apart from nearly killing myself all I saw was that the forks ends were a blur, and there was no way to tell if they were moving together or alternately.
 

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Fat & Single
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When you think of it, our fat wheels are putting much the same loads into the frame under heavy braking as a small motorbike.
I imagined the same thing when building my current bike, i went with Hope M4's (4 pot) with 183mm rotors front and rear. These brakes will stop the bike on a 2c piece and awesome for steep rocky mountain decents and techy terrain but for snow and sand these are a massive overkill.

Saying that, the slight weight penalty over 160mm rotors and lighter brakes isn't a lot compared to running BB7's.

The weight of the wheels, the diameter of the tyres and the additional grip of the tyres is definitely a big factor on additional stress. Most fattys come with 44m headtubes or normal so the near 31" diameter of 4.8" tyres has increased the torque on the fork/frame over regular bikes.

All my bikes are fully rigid so while i know the difference in braking a niner fork on a 29er over a carver fork on my fat bike, its the above factors that make the difference IMO.

+1 on the screwed hubs, i run Halo hex key skewers on all my bikes including a 135mm on on the front of my fatbike, still got a 170 Salsa on the rear though.
 

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This is very interesting. I have not ridden a fatty with a rigid factory fork as both bikes I have I equipped with Lefties during the builds. I ride a lot of rugged single track and do a lot of hard braking. I have never noticed any decreased performance, wheel movement or fork wiggle, probably because the front wheel is built on a regular Lefty hub. Of course I do notice the extra weight.
 

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Nemophilist
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Ha;

Front end flex... ya think? I've ALWAYS noticed it on all my bikes, but it is pronounced on the Fatty. They all have 8" front rotors (yes, I'm big and ride Fatty hard!). Feels pretty strange at first, worrisome even, but you get used to it. No folding forks or other adverse effects.... so far.
 
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