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Specialized fat 24 q factor and chainline

1085 Views 10 Replies 3 Participants Last post by  rocwandrer, 
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Got my daughter a specialized fat 24. Changed out the factory 160mm cranks for 140mm with a 30t narrow wide. The cranks have widened the q factor from the already pretty wide factory width. There is about 1/2" of clearance from the chain to the tire in the lowest gear, and 1/2" of ring to frame clearance, and way more than 1/2" of crank to frame clearance. That's with the ring mounted inboard. It would shift better with a narrower chainline.

Anybody have a line on a 100mm BB with maybe a ~142-150mm square taper spindle instead of the factory 164 spindle? I found 155 spindles, but it hardly seems worth it for 4.5mm per side.
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brentos
Good question, and I'm interested to see if such a BB exists, I also looked, but couldn't find anything.

I am also trying to replace the crank on my sons Fatboy 24. What cranks did you purchase that widened out the chainline? I just ordered Bombshell Rocket SLR's (145mm) and am awaiting their arrival. It looks like the ring can be mounted on the inside or outside the spider, and the q-factor looks to be quite narrow. I think the outer ring mount position is standard MTB chainline.

crucialbmxshop.com

Bombshell Rocket SLR Crank Arms

Using some of the highest-grade aluminium helped to maintain low weight while increasing strength, stiffness and rider weight limit. These cranks are purposely built for the smallest riders to save maximum weight and minimalize the Q factor for efficient power transfer
crucialbmxshop.com crucialbmxshop.com
I ordered from a different retailer, but these are the best pics I could find. When they get there I'll let you know how they fit up.

I did first try the Suntour XCR Jr. Cranks (152). But the q-factor was almost 20mm wider than the already stock cranks. The inner (64mm) ring location looked great for chainlink though and would have allowed a 26t, or even smaller chainring. Unfortunately, I've decided to send it back because of the wide q-factor.
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Plenty of Neco fat bike BBS at 100 x 151mm for sale in UK on eBay and online bike shops. Not seen less.
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brentos
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We got his bike set up with the Bombshell Rocket SLR Cranks (145mm). A few things to note.

Using the stock chainring, it did move it in about 6 mm from the stock chainline.

When first installed, the Left crank arm hit the chain stay. The right crank arm had about 4mm clearance. I put a 2.8mm shim between the drive side BB cup and the BB cartridge to shift the BB to the non-drive side. It now has just under 1mm clearance between the Left crank arm and stay, and just over 1mm clearance between the right crank arm and the stay.

Overall, it's a good change. It has a tighter chainline, a q-factor as tight as can be on this frame, they are now a kid appropriate length, and saved 1/2 lb. Hopefully the square taper arms don't creep inward over time.

I also noticed the gearing was too tall stock on the bike for our trails (32/36), so I updated to a used XT 11 speed setup (32/46) I found on the (super) cheap. We made a couple other changes to the bike too to make it ride better and lighter. Set up tubeless w/ PVC tape, straight seat post, hydraulic brakes from the parts bin, & 40mm stem. It does fit him better now and it's down to 28.5 lb. I don't think it's getting much lighter, there's a lot of weight hiding out in those hubs and spokes.

My son hasn't really jived with this bike, like he has with his Vitus Nucleus. I'm hoping the changes help him out. It is possible he is just too small for it at 4'4".

Bicycle Bicycles--Equipment and supplies Tire Wheel Crankset


Bicycle Wheel Tire Bicycles--Equipment and supplies Bicycle wheel rim


Tire Bicycle tire Bicycle wheel Automotive tire Wheel
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brentos
This integrated bash guard/chainring seems like it would move the chainline in a few mm, it looks to be currently available in 28 or 30 tooth.

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brentos said:
We got his bike set up with the Bombshell Rocket SLR Cranks (145mm). A few things to note.

Using the stock chainring, it did move it in about 6 mm from the stock chainline.

When first installed, the Left crank arm hit the chain stay. The right crank arm had about 4mm clearance. I put a 2.8mm shim between the drive side BB cup and the BB cartridge to shift the BB to the non-drive side. It now has just under 1mm clearance between the Left crank arm and stay, and just over 1mm clearance between the right crank arm and the stay.

Overall, it's a good change. It has a tighter chainline, a q-factor as tight as can be on this frame, they are now a kid appropriate length, and saved 1/2 lb. Hopefully the square taper arms don't creep inward over time.

I also noticed the gearing was too tall stock on the bike for our trails (32/36), so I updated to a used XT 11 speed setup (32/46) I found on the (super) cheap. We made a couple other changes to the bike too to make it ride better and lighter. Set up tubeless w/ PVC tape, straight seat post, hydraulic brakes from the parts bin, & 40mm stem. It does fit him better now and it's down to 28.5 lb. I don't think it's getting much lighter, there's a lot of weight hiding out in those hubs and spokes.

My son hasn't really jived with this bike, like he has with his Vitus Nucleus. I'm hoping the changes help him out. It is possible he is just too small for it at 4'4".
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That's good info. What bottom bracket are you running? Chain still clears the tire in the lowest gear? I switched to a 104bcd 30T narrow wide ring but retained the factory bash guard/pants protector. On her summer bike, the lowest gear she ever uses in 26-33, so I figured 30-40 is fine on this bike. Haven't found a lightweight 9 speed cassette with a 40t low gear yet.
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brentos
It's the factory 164mm bottom bracket. These cranks have very little flare outward. There is still plenty of clearance between the chain and the tire.

Here's a wide range 9 sp cassette. Not super light @ 500 g, but neither is the stock cassette, which I think comes in around 430g.

SunRace | CSM980

SunRace develops and produces bicycle drivetrain and brake components. SunRace products are specified by the world’s leading bicycle manufacturers and are also used extensively in the aftermarket.
www.sunrace.com www.sunrace.com

He does enjoy the bike more with the changes. But I've realized it's not really the bike...he just likes sledding and snowball fights a little more than pedaling around on groomed fat bike trails! He still enjoys riding though and this gets him out with us for our winter rides.
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brentos said:
This integrated bash guard/chainring seems like it would move the chainline in a few mm, it looks to be currently available in 28 or 30 tooth.

Click to expand...
I have one of those in 26T. It only works for single speed, or if you setup the chainline so that the ring teeth are in close to the same plane as the smallest sprocket in the back. Otherwise the chain rubs the bash guard and snags every few hundred pedal strokes in the higher gears.
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R
I went with LDC components. According to Helium, they are very slightly wider Q than Bombshell. $60 less expensive. CNC'd in the US. I'll try to remember to update here after they come in.
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brentos
Nice! Just looked them up, they look like a better option and I think I would have gone that direction had I known they exist.
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LDC 145 cranks. It doesn't look it in the photos (edit: no idea what happened to the other photos), but clearance is MUCH less (therefore q factor less) than the Jet racing 145 cranks they replace. Chain line is about 3mm further inboard as well. I may space that over another 5 mm inboard since low gears get used more than high and there is plenty of chain clearance.

Bicycle Tire Wheel Bicycles--Equipment and supplies Bicycle frame
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