New to cross, how does Geo work?

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mattyNor
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by mattyNor

I've done a lot of road and mountain biking and am looking into my first cross build. My mountain background says long toptube, short stem and wide bars. My preferred road setup is 44c-c bars and whatever toptube gives me a 120mm stem. I'm a 54 square all day but the frame I'm looking at comes in either a 52 or 55 toptube.

stormur
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by stormur

cx doesn't work that way :)

There's few critical measures which determine cx bike characteristics : Chain stay length, BB drop and HT angle/ fork rake, wheel base.

Angles/ rake is fairly easiest : slacker angle more stability, less agility. Golden standard here is 45mm of rake and 72.5* of HT angle. Rarely some frames vary from that ( HT is + - half degree at most ) - new "American" brands start to use more rake lately, but…

Chainstay length : most underappreciated dimension. 420mm is absolute minimum to not have "bambi on ice" -425 is kind of "rule of thumb"- this length comes in "racing geometry" , 430 , 435 in more relaxed "gravel grinders" .

BB drop : 60 -65-70 ( +- 1 ) : this determine how bike behave : higher BB: better cornering, mud/ obstacles clearance, accelerating, better climbing more "agil" and nervous ( racy )characteristic. Lower BB - more sluggish feel , but better stability. I had 69mm, have now 60mm and difference is monstrual. Won't come back to 69, will buy 64-5.

Weight balance is very important - so "amateurs" use about 100-110mm stems, pro riders 120mm ( didn't saw longer, but didn't digg for it ) , saddle seatback is around 5-8cm .

You have to find own "balance" on the bike : not to much weight on front to not loose rear control and not loose steering ability, but not to much on back to have some steering at all :) - for me works best when bar covers hub, some have bit shorter stem, but never heard about longer.

In CX ( real CX ) cadence is way lower than at any other discipline.. forget 85, welcome 65 ;) - that determine geo too.

TT length : about 1cm shorter than on road bike ( slacker HT angle ) .

Wheelbase : depends on size, but 101cm on 54cm TT is good starting point.

Very important : main triangle size : bigger - better : and forcing by some brands slopping geometry won't change facts .

If I could build "perfect" cyclocrosser it will look like this ( i'm 180cm ) :

TT : 540mm
ST C_C : 540mm
Rake : 45mm
HT Angle 73degr
ST angle 72 degr
chainstay length : 430mm
BB drop : 64mm
seatpost : 15mm seatback
stem : 110mm
bar 44cm ON HOODS
frame material : CF
brakes : canti
cabling : traditional route , hidden in TT, rest outside, NOT full housing.


The biggest mistake is to small frame : bike will be unstable, nervous and that cause you'll be significally slower ( and in danger time to time ) .

IMHO : Look at wheelbase of these to frames ( 52 & 55 ) : if difference is 1cm and below : take smaller, of it's more than 1 cm - bigger, and then shorter stem .
Go to Heaven for the climate, Hell for the company.
Mark Twain


I can be wrong, and have plenty of examples for that ;)

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TwiggyTN
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by TwiggyTN

This is s great post with so much good info. I don't even do CX, but I just had to say thanks anyway.

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kman
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by kman

Agreed, good post. Bb height is one area in particular where there are varying ideas about what is best. Mine is pretty low, maybe 69 or so. Id like to go higher.

Also, another reason to avoid a frame that is sloping and small is it will be harder to swing up on your shoulder.
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stormur
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by stormur

With BB height is easy to choose "purpose" of the bike ( there's no good nor bad- just different ) : as gravel grinder , winter bike 67+ will be better than 60mm. For real CX training-racing, technical courses and so, 60-61 will be way better. Sweet spot is (IMO) around 64. Still not to low and sluggish, still high enough for obstacles and fast acceleration. Something like Ferrari Combi Turbo Diesel :mrgreen:

Next thing is cable routing : I HATE it routed as road bike ( under DT ) and under chainstay (RD cable) - yes Cannondale , this is sick :evil: / you designed such a nicely made SuperX with impressive accuracy and build quality and same way screw up cable routing…what for ? to cut fingers ? to collect mud and snow there ? ARGH! / - TT is the place for cables ( best if hidden there ) , everything coming out from there or can "breath" or sealed . Full housing is A. heavy ( +110g on 55cm TT frame) , and B. cause more friction than non sealed "naked" cables. To not mention C. dirt coming and staying in frame thru holes for housing ( look at KFC Crockett: MLC just covered all with silicone… ) .

Next important thing is BB system : Cx bike require often change/ clean+ re grease of bearings . I would avoid at any price BB30 ( lifespan is 3-4 t km ) and if possible any press fit ( even if - adapter and then BSA ) . BB bearings ( huge part of decision ) pushed me to Campagnolo Ultra Torque ( even if it has to be crank only ) - ease of BB serivce is awesome :) . There's good reason why Colnago still in both their CX frames make BSA BB… .

Just wouldn't feel comfy knocking every 3 months nice CF frame… nothing never happened, but…. thread is thread :)

or Ceramic Speed bearings : flush, regrease and no need to knock-out/ press in new ones.
Go to Heaven for the climate, Hell for the company.
Mark Twain


I can be wrong, and have plenty of examples for that ;)

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mattyNor
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by mattyNor

Thanks for all the info guys!!! @srotmur, only thing I'd change about that is full length housing :)

mattr
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by mattr

Nah, cables need replacing so often anyway that full length just isn't needed. The endless cleaning gets the mud and cack in at the rear mech end, then you need to replace the whole run.

External and naked runs, easy to clean, easy to replace. Can usually do gears, brakes and setting up in well under an hour.

TurboKoo
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by TurboKoo

Cables? Nah, go for Di2 the Gods gift to cyclocross.
Mine SuperX is close to perfection with Di2.
Cannondale SuperSix
Shimano 9270

stormur
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by stormur

If you can manage on bumpy terrain with Di2 - sure, absolutely: Nice system which doesn't require "temperature adjustment" :) . I found buttons on levers way too small for my fingers in (double) gloves… Sram double tap pads are in good size, but don't like brake levers shape - they're created to brake on drops, I'm on hoods 99.9% of time - can't feel safe on drops in CX :(

Just like undoubtful feedback from shifting levers what I did or not… . So for me ( but it's just me ) Campagnolo works best: I feel and hear shifting, plus brake levers have for my hands best shape ever made.
Go to Heaven for the climate, Hell for the company.
Mark Twain


I can be wrong, and have plenty of examples for that ;)

mattr
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by mattr

Never seen Di2 brakes, where do you get them from?
;)

OJ
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by OJ

Cannondale would be perfect with a bit higher BB. With current BB drop the pedals scrape the ground quite easily. I like under BB cable routing, it works even if it's caked with mud and crap and there is very little housing that could collect water. Never had any issues hurting myself with cables, gloves or no gloves.
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rasmussloth
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by rasmussloth

OJ wrote:Cannondale would be perfect with a bit higher BB. I like under BB cable routing,... Never had any issues hurting myself with cables, gloves or no gloves...


+1

Grabbing the downtube for shouldering has made a lot of scratches on the frame though.

mattr
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by mattr

Mines got the front mech routing on the down tube, everything else on the top tube.

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stormur
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by stormur

rasmussloth wrote:
OJ wrote:Cannondale would be perfect with a bit higher BB. I like under BB cable routing,... Never had any issues hurting myself with cables, gloves or no gloves...


+1

Grabbing the downtube for shouldering has made a lot of scratches on the frame though.



Scratches are marks of experience ;)
Go to Heaven for the climate, Hell for the company.
Mark Twain


I can be wrong, and have plenty of examples for that ;)

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