Measuring BB drop?

String tied to rear axle and to front axle – measure how far below the line the bb center is?

yes but why do want to do this?

yes

is that with you sitting on the bike or not?

not

Just realize that the BB drop will depend on the fork’s axle to crown length. A longer fork will result in less drop.

Why do you want to know? It’s not a brilliant measure for the frame – useful for me to know when setting my jig to build a frame, but BB height is more useful when taking about how the bike will ride.

Drop is dependent on wheel size, BB height isn’t.

Just realize that the BB drop will depend on the fork’s axle to crown length. A longer fork will result in less drop.

As will anything that moves the BB relative to the axles. Sag, offset hardware etc.

To those who say it isn’t an important measurement, you’re on crack.

Drop is dependent on wheel size, BB height isn’t.

Do you mean drop is a pre-fixed frame measurement (relative to fork) and wheel / tyre size then affects BB height?

I think he has it the wrong way round.

Like Al says BB height depends on wheel size, BB drop doesn’t (well except that BB drop can’t be bigger than the wheel radius)

Nope. Imagine you’re designing a bicycle. The important measurements are:

– bottom bracket to saddle
– Saddle to handlebars (x and y)
– seat tube angle
– head tube angle
– bottom bracket height
Etc etc

Really, imagine the bike doesn’t exist, you’re just looking at the riding position – the rider is at a certain distance above the ground, that distance determined by things like how easy it is to reach the ground (penny farthings are harder than recumbents) and angle of lean (the ISO standards specify an angle the bike can lean to before the pedals hit the ground).

So imagine a rider sitting in a certain position, with the centre of their foot rotation (ie the bottom bracket) say 12cms above the ground. Where in that measurement is wheel size specified?

Another way of looking at it – go find a picture of a rider on a conventional racing bike, and one of a rider on a Moulton. The riders will be in basically identical positions, down to the bottom bracket height, but the bottom bracket drop will be very different – in fact on something with small wheels like a Moulton or Brompton, the drop is negative – the bottom bracket is above the rear axle.

So that’s what I mean when I say that BB drop isn’t a very useful measurement – it’s a secondary measurement that depends on the bottom bracket height and the wheel size.

I know what you mean Ben. Wheels and position etc drive BB drop when putting a frame geometry together, in that sense we’d look at it differently to a rider measuring their bike to compare to others, where drop is basically fixed (ed, the frame is fixed geo) so BB height changes with tyres or fork fitted.

Measure height of rear axle centre (tyre radius) and measure height of crank axle centre, get calculator out then Bob’s your uncle

So that’s what I mean when I say that BB drop isn’t a very useful measurement – it’s a secondary measurement that depends on the bottom bracket height and the wheel size.

Eh? So you’re saying varying BB drop has no influence?

Ok Ben so we are looking at it from other end of the manufacturing spectrum.

Neither of us wins…meaning YOU LOSE!

Just wondering if the pedal strikes on my el guapo are a bb drop issue or it’s normal

IMO measuring drop is more important than height because it is a constant – adding fatter or thinner tyres changes height, but not drop – so at least with drop you can make comparison’s between frames on something of a level basis. If you are looking at height you need to work out what size tyres are being used for measurement, and that’s before you start talking about fork length, with or without sag etc.

Then there the fact that differing degrees of drop will give differing effects depending on wheel size. i.e. 60mm of drop will feel quite different on a 29″ wheel than a 26″ wheel – all else being equal. For an interesting take on the impact of BB drop check out Grant Petersen’s (of Rivendell) article ‘stilt step factor’.

Well, that’s kind-of my point. If you take the same frame and fit 29″ wheels instead of 26″, the drop remains the same but the BB height increases – raising your centre of gravity and altering the handling.

Whereas, if you have two bikes – one with 29″ wheels and one with 26″ wheels, but with the same BB height – then the drop will be different but the centre of gravity will be the same so handling will be unaffected*.

So drop is not a useful measure of how a bike will ride, height is. Unless you’re comparing bikes with the same size wheels, which means you don’t get to have fun arguments about which wheel size is better 😉

*Unaffected by CG position – the bigger wheels will have other effects on the ride and handling.

Whereas, if you have two bikes – one with 29″ wheels and one with 26″ wheels, but with the same BB height – then the drop will be different but the centre of gravity will be the same so handling will be unaffected*.

So you’re saying mass location relative to axles has no effect on handling?

I would have another think about that.

but the centre of gravity will be the same

will it? the C of G of the larger wheels will be higher

anyone answer my point??

Q2 ‘is it normal’, dunno anything about that particular bike sorry ) 11.5-13″ seems a fair range for sagged BB height. So if it’s below that and has long travel, you’ll get pedal strikes even on just rooty woodland stuff. I got a fair few on my low-BB 29er at first (11.6″ I think with 2.3s) but I did get used to it and adjust, now I’d say it’s a good trade off as long as you’re not riding rocky, trials-y stuff.

So you’re saying mass location relative to axles has no effect on handling?

I would have another think about that.

Because the gyroscopic axis will be higher? That might have a small effect I suppose. Not one I’ve ever been able to detect when riding.

will it? the C of G of the larger wheels will be higher

The CG of the wheel itself, yes – but the CG of the rider will be the same.

Just wondering if the pedal strikes on my el guapo are a bb drop issue or it’s normal

The definitive thread on this is on another forum – http://forums.mtbr.com/titus/why-bottom-bracket-height-my-new-eg-v-3-so-low-813147.html

rob jackson – Member
Just wondering if the pedal strikes on my el guapo are a bb drop issue or it’s normal

Ooh I sense a new frame coming on! I think I said March time? *goes off to check*

😛

Houns – Member
I’ll give it ’til March 24th

POSTED 1 MONTH AGO #

😀