Lowering BB & Tightening Head Angle

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stinkytodamax

Likes Dirt
Hey Burners

I wasn't great at geometry at school so I'm looking for some advice to seek an even better handling bike.

Bike- Santa Cruz Blur XCc converted to 650b. 100mm rear and 120mm fox float front fork. BB height 13.3 inches.

Issue - i'd like to lower the BB by 0.3 of an inch or thereabouts by using an offset kit from Burgtec or someone similar but keep the head angle in check by reducing the 120mm fork to 100mm.

What I'm thinking - Stock head angle is 69.5 degrees and making allowances for the 650b wheels and the 120mm fork I'm guessing its 67.5ish and I'd like to keep it there or thereabouts. Will fitting a 10mm fox spacer into the fork actually lower the fork and will an offset kit lower the BB height?

Current BB height is 13.3 inches so just a bit to high for some of the tighter XC trails we have here in Canberra. I like the 650b wheels so I want to keep them as the bike is my trail and race machine for endurance and 24 hour team efforts. The current setup really is a good mix of comfort and speed its just the handling I want to get sorted a little better.

2 kids now so a new Solo or Tallboy type frame is out of the question and I'm now addicted to the light weight of the Blur so I can't justify getting an Anthem or cheaper 650b bike.

Tell me if I'm retarded with the above. Most of my research has been people just seeking to slacken and lower their rigs so lowering the fork sounds like a good way to keep the head angle in check.
 

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mitchy_

mitchy_

Llama calmer
650b wheels wont change the geometry angle wise, only the BB height.

reducing the fork length will lower the BB and steepen the head angle. (presumably back to 69.5 degree if it was originally specced with a 100mm fork)

have you measured the BB height? stock is listed as 12.8" with 26" wheels and 100mm fork. 27.5" wheels and 100mm fork will be about 13.3" and 27.5" wheels with 120mm fork even higher still. you wont get to 13" with your current wheels.
 
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moorey

moorey

call me Mia
650b wheels both end wont have changed your geo, and 20mm of fork (depending on axle to crown), will have made less than 1 deg difference.

Edit: Snap :doh:
 
mitchy_

mitchy_

Llama calmer
calc says your current BB height is ~13.6" and head angle is 68.5°

with 100mm fork it will go down to 13.3 and 69.5° again.
 
S

stinkytodamax

Likes Dirt
Thanks guys failed to account for 120mm fork in relation to BB. So 13.6 would be right. No wonder it feels a bit on the high side.

I think even 13.3 would be an improvement but I'm not sure howuch of a difference an offset kit would make.
 
moorey

moorey

call me Mia
mitchy_ said:
calc says your current BB height is ~13.6" and head angle is 68.5°

with 100mm fork it will go down to 13.3 and 69.5° again.
Click to expand...
I reckon you're spot on there, assuming the 20mm longer travel fork is actually if actually 20mm taller. I have an A to C list somewhere I'll chase up.
 
S

stinkytodamax

Likes Dirt
Just measured its is 490mm and my BB is 13.6.

measured my XTC 27.5 and no wonder it feels like its on rails 12inchs at BB.

So I'm clear will simply lowering the fork decrease my BB height?

also a big thanks to the brains trust for the help so far.
 
moorey

moorey

call me Mia
stinkytodamax said:
Just measured its is 490mm and my BB is 13.6.

measured my XTC 27.5 and no wonder it feels like its on rails 12inchs at BB.

So I'm clear will simply lowering the fork decrease my BB height?

also a big thanks to the brains trust for the help so far.
Click to expand...
But a HT can have a lower BB then a Duallie, of course....
 
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