Author Topic: Drawbacks of a high bottom bracket  (Read 1093 times)

mattc

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    • Didcot Audaxes
Drawbacks of a high bottom bracket
« on: December 16, 2018, 06:27:02 pm »
In particular, if using a CX frameset for more general duties.

Apart from the (rather obvious!) issue of getting your toe on the floor at traffic lights, what possible issues does this create? Does it have other geometry/fit/comfort side-affects?

[I now own a cross bike, but never ridden it for more than an hour. It has too many differences from my other bikes to make solid comparisons at this stage. But I can confirm I've clipped the pedals on the ground while racing! ]
Has never ridden RAAM
---------
No.11  Because of the great host of those who dislike the least appearance of "swank " when they travel the roads and lanes. - From Kuklos' 39 Articles

LittleWheelsandBig

  • Whimsy Rider
Re: Drawbacks of a high bottom bracket
« Reply #1 on: December 16, 2018, 06:31:55 pm »
It is generally a little slower to manoeuvre the bike through quick back-to-back hairpins, assuming everything else is the same, otherwise not much handling difference. CX bikes often have more laidback head angles with more trail than road bikes, so the steering will normally be a little different anyway.

Your bars and saddle are higher to match the higher BB. You are less likely to get pedal strike on rocky trails (which was more of a problem with hanging toeclips). It'll be slightly harder to swing your leg over the saddlebag and saddle.
Wheel meet again, don't know where, don't know when...

Re: Drawbacks of a high bottom bracket
« Reply #2 on: December 16, 2018, 06:37:44 pm »
all things being equal you are that bit higher up (which has implications for visibility) and you will catch the breeze a tiny bit more;


- when drafting it will be that much better for everyone else and that much worse for you and

- there is a wind velocity gradient above the ground; even if your frontal area is about the same, any headwind you are pushing through is a little bit stronger.

cheers

Re: Drawbacks of a high bottom bracket
« Reply #3 on: December 16, 2018, 09:47:46 pm »
Those of you who watched the Kellogg’s city centre racing will remember Phil Thomas. He could corner like no other - Gerry Knetterman told me my man was crazy!
His bike had a lower bracket - against all ideas about crank clearance
. Maintained that the lower centre of gravity gave an advantage - and the corners weren’t pedalable anyway.
His records hard to dispute.
The logical opposite of a high bracket would raise the centre of gravity and would lose stability.
.

LittleWheelsandBig

  • Whimsy Rider
Re: Drawbacks of a high bottom bracket
« Reply #4 on: December 16, 2018, 09:53:29 pm »
'Stability' is a matter for discussion with regards to bike handling. An object with a high centre of gravity (e.g. broomstick on end) is easier to balance than one with a low centre of gravity (e.g. pencil on end). Conversely, a low BB racing bike certainly is easier to transition into and out of corners and 'feels more secure' but I'm not certain why.
Wheel meet again, don't know where, don't know when...

Basil

  • Um....err......oh bugger!
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Re: Drawbacks of a high bottom bracket
« Reply #5 on: December 16, 2018, 09:55:21 pm »
Probably could have done with a higher bb on my fixed in Brum.  I was constantly bouncing on corners as the inside pedal grounded at speed.
I got quite good at expecting it and controlling the bounce.
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Re: Drawbacks of a high bottom bracket
« Reply #6 on: December 16, 2018, 10:16:52 pm »
I do think there is a modern trend to higher. Small frames with 700c wheels always look particularly badly proportioned.

Kim

  • 2nd in the world
Re: Drawbacks of a high bottom bracket
« Reply #7 on: December 16, 2018, 10:18:27 pm »
'Stability' is a matter for discussion with regards to bike handling. An object with a high centre of gravity (e.g. broomstick on end) is easier to balance than one with a low centre of gravity (e.g. pencil on end). Conversely, a low BB racing bike certainly is easier to transition into and out of corners and 'feels more secure' but I'm not certain why.

I reckon low centre of mass is more relevant to wheeling the bike than riding it.  (Which is a reasonable enough consideration for eg. a loaded tourer, where keeping the COM low down means less sideways force is needed to balance the bike as you hold it.)

When riding the pendulum principle applies.  Low-racer recumbents have much higher frequency balance-wobble than ordinaries, and a higher bike is generally easier to balance, up to the point where mounting and dismounting require advanced skills.  This isn't the whole story of stability though, as there's all that steering geometry stuff that affects the tendency to self-stabilise.

Cornering behaviour is voodoo that I don't really understand, but it seems to be that bikes that corner well at speed are more difficult to low-speed balance, and vice-versa.
To ride the Windcheetah, first, you must embrace the cantilever...

LittleWheelsandBig

  • Whimsy Rider
Re: Drawbacks of a high bottom bracket
« Reply #8 on: December 16, 2018, 10:20:20 pm »
I do think there is a modern trend to higher. Small frames with 700c wheels always look particularly badly proportioned.

It is easier to design and build a small 700C bike with a high BB. Otherwise the down tube overlaps the front wheel if the head and seat tubes are at sensible angles and the top tube is a sensible length, particularly as fatter tubing, wider tyres and mudguards are more fashionable now.

Edit: similarly with seat tube and rear wheel for shortish chainstays.
Wheel meet again, don't know where, don't know when...

Blodwyn Pig

  • what a nice chap
Re: Drawbacks of a high bottom bracket
« Reply #9 on: December 17, 2018, 07:31:32 am »
I have , in states of unusedness, 2 mtbs. An Orange P7, with an avg 90's bb height, great for everything, and a full sus Rocky Mountain, 'free ride' ( not downhill, but a bike designed and hand built in Canada, to deal with logging trails, trees, and drop offs) . This bike has a seemingly very high BB, so much so, that for general rough stuff, it's quite terrifying, as you cannot touch the ground with your toes at all, and I'm 6'2".

Re: Drawbacks of a high bottom bracket
« Reply #10 on: December 17, 2018, 07:56:12 am »
Drawbacks of a high bottom bracket..... But I can confirm I've clipped the pedals on the ground while racing!

Shirley, this suggests your BB is, in fact, lower ? (or longer wider cranks) Either that or the handling is better (?) allowing greater banking on the turn.

mattc

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Re: Drawbacks of a high bottom bracket
« Reply #11 on: December 17, 2018, 10:18:18 am »
Drawbacks of a high bottom bracket..... But I can confirm I've clipped the pedals on the ground while racing!

Shirley, this suggests your BB is, in fact, lower ? (or longer wider cranks) Either that or the handling is better (?) allowing greater banking on the turn.
Ah no - I missed out a key detail: this only happened when cresting banks.  I mentioned it to show that having a higher BB probably IS quite valuable during cross races!
Has never ridden RAAM
---------
No.11  Because of the great host of those who dislike the least appearance of "swank " when they travel the roads and lanes. - From Kuklos' 39 Articles

Re: Drawbacks of a high bottom bracket
« Reply #12 on: December 17, 2018, 06:12:25 pm »
In particular, if using a CX frameset for more general duties.

Apart from the (rather obvious!) issue of getting your toe on the floor at traffic lights, what possible issues does this create? Does it have other geometry/fit/comfort side-affects?

[I now own a cross bike, but never ridden it for more than an hour. It has two many differences from my other bikes to make solid comparisons at this stage. But I can confirm I've clipped the pedals on the ground while racing! ]
 
As I understand it, the reason why CX bikes got high bottom brackets was because it could give a fairly good race advantage to bunny hop UCI 40 cm obstacles instead of dismounting and running.
   
Years ago I bought a CX frame with race geometry like short top tube and high bottom bracket. I never liked it, it just felt strange to sit so high up and forward compared to my other bikes, so I gave it to a friend who likes it very much. He only uses it for short commuting rides though. 
 
Some like the CX race position, and it is probably very good for going all hammer and tongs when out riding, but I didn't like it. I much prefer a more relaxed audax-like frame geometry and riding position. 
 
So while riding positions and frame geometry ultimately are personal preferences, I do think that most people probably shouldn't buy a CX bike with racing geometry if what they really want really is a commuting road  bike that can take fat tyres. These days there are several bike frame types  that are a better fit, like ATB/Adventure/Gravel bike frames.
--
Regards

LittleWheelsandBig

  • Whimsy Rider
Re: Drawbacks of a high bottom bracket
« Reply #13 on: December 17, 2018, 06:20:25 pm »
BB height makes no difference to bunny hop height.
Wheel meet again, don't know where, don't know when...

PaulF

  • "World's Scariest Barman"
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Re: Drawbacks of a high bottom bracket
« Reply #14 on: December 17, 2018, 07:04:54 pm »
BB height makes no difference to bunny hop height.

I’m guessing what was meant was “manualling” the front wheel over the obstacle and then having the rear roll over it. But a 40cm high obstacle makes me not so sure ???

Re: Drawbacks of a high bottom bracket
« Reply #15 on: December 17, 2018, 07:19:23 pm »
mattc, you're not riding it fixed, are you?  There is a real advantage to a high BB on fixed, where you HAVE to pedal through the corners.

I have bikes with all sorts of BB heights; the lower ones are nice in stop-start traffic but I have occasionally been caught out with pedal strike (turning right at a humped mini-roundabout is a favourite place).  Bikes converted from 27" to 700c and run on race tyres can have BBs slightly too low for comfort.  It's not just 4mm - the swap from 1 1/4" tyres to 23mm adds twice that.  The highest ones are the MTBs, as you'd expect; I always have to slip off the saddle at traffic lights on those.

The optimum for a fixie is about 10.75" - high enough to avoid grounding issues except in extreme situations, but low enough to get a foot down without dismounting every time.  This is with 165mm cranks.  Track standard BB height is 11" and mandated by some indoor velodromes, but I've never known it actually be measured if you take your own bike along.
Never tell me the odds.

Re: Drawbacks of a high bottom bracket
« Reply #16 on: December 17, 2018, 11:55:14 pm »
BB height makes no difference to bunny hop height.

It isn't about height of the jump, but about the clearance between the top of the UCI barrier and the bottom bracket. Hitting the barrier with the BB mid-air may lead to a nasty crash, so clearance matter. The UCI have tweaked the technical obstacle rules countless times (again this year too I see), seemingly unable to decide whether bunny hopping was to be discouraged or not.

--
Regards

LittleWheelsandBig

  • Whimsy Rider
Re: Drawbacks of a high bottom bracket
« Reply #17 on: December 18, 2018, 05:57:28 am »
As long as the chain is on the outermost chainring (or the only chainring), even gross differences in BB height don't really matter as the chainring will behave like a MTB bashguard and slide (or fold if too weak). Regardless of the chainring, the rear wheel still has to get over the obstacle without hanging up, so 10mm or so variation at the chainring makes basically no difference in how easy it is to bunnyhop obstacles.

Cyclocross BB heights noticeably dropped when clipless pedals became popular because hanging toeclips weren't going to catch debris and lower BBs made it easier to remount. Passing fashions have meant BB heights have wavered a little since then.
Wheel meet again, don't know where, don't know when...

mattc

  • n.b. have grown beard since photo taken
    • Didcot Audaxes
Re: Drawbacks of a high bottom bracket
« Reply #18 on: December 27, 2018, 11:53:28 am »
So, thanks for the replies. I've been digesting them gradually ... and I finally got round to some measuring in the stable yesterday.

FOR ME, there are 2 reasons for asking this:
1 - Do higher BBs *generally* create problems? [for use in assessing future purchases]

I think the answer has to be "Not necessarily!". I hadn't considered the extra wind-resistance, but I suppose it is a genuine-if-small drawback.
Mr interested made a good point, that CX bikes aren't generally the ideal purchase for general road use. But I think there are many different scenarios; for example, there are lots of old CX bikes on the 2nd-hand market to suit some folks' budgets, and there are some wierdos who don't want d**k brakes, and that rules out 95% of new so-called adventure/gravel bikes.

2 - Should I try using my Kinesis FiveT on the road (200km+ rides in particular)?

I almost certainly will after the main cross season, say Februrary-ish. The bike was probably designed as a compromise (e.g. has 2xbottle mounts!), so you'd expect it to work better as a road-bike than pure race-machines would. Now I don't fully understand frame geometry* but I have measured my Airborne Valkyrie against the Kinesis. [I've done many multi-300km-day rides on the Airborne.] The position of saddle, bars, head-tube etc is only a few mm different. in fact even the BB-height turns out to be closer than I thought [that's even on 33mm tyres vs 25!]
So I think as a negligible-cost experiment, it's a goer  :thumbsup:


*well I doooo ... but the combinations of factors are something I haven't totally grasped ::-)
Has never ridden RAAM
---------
No.11  Because of the great host of those who dislike the least appearance of "swank " when they travel the roads and lanes. - From Kuklos' 39 Articles

Morat

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Re: Drawbacks of a high bottom bracket
« Reply #19 on: December 29, 2018, 11:40:49 pm »
I like my CX bike on the road. I did find it felt high and a bit "floppy" at first, in that it felt it would flop into a corner if you were turning at low speed but I've got used to it now.
What I really like about it is the tyre clearance front and rear which lets me run nice comfy tyres.
Tandem Stoker, CX bike abuser (slicks and tarmac) and owner of a sadly neglected MTB.