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Lets talk bottom bracket drop.

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#21 ·
LaXCarp said:
I dont think BB drop matters much honestly, what matters is absolute BB height. BB drop is irrelevant as it relates to the center of gravity, which is much higher on the bike and is the dominating force.
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Agreed, saddle height is usually set by the extended leg on the lowest point of the pedal stroke. Lower the BB by 5mm, and your saddle will be 5mm lower too. That'll put your center of gravity nearly 5mm closer to the roll axis, which is why the handling changes. (You'd need to also lower the stem 5mm to get all 5mm drop in COG.) Since the roll axis is the line between tire patches, this is determined by absolute BB height. Changing the height of the gyroscopic effect of the wheels (by changing wheel sizes) will have some effect on handling too, but that is independent of BB height. BB drop may reflect both changes in BB Height and wheel size, but tells you nothing much about handling changes.

It's interesting to read what Bike Radar had to say about it in their Ultimate Guide to Bike Geometry and Handling:
The bottom-bracket drop itself is less important than some people have supposed. The distance by which the bottom bracket hangs below the wheel axles is seen by some to directly determine the stability of the bike in turns, as if the bike’s roll-axis (the line about which it turns when leaning into a corner) was at the height of the axles. This argument was used in the marketing of 29in wheels, claiming that because the bottom bracket sat slightly below (not above) the axles, the bike was far more stable.

In fact, the roll axis is – roughly speaking – the line connecting the tyre contact patches. The important measurement for cornering is the height of the centre of mass above this line, and not the height of the bottom bracket relative to the axles.

Fitting smaller wheels reduces the bottom-bracket height but doesn’t affect the bottom-bracket drop. This makes a bike significantly quicker to change direction of lean because the centre of mass of the bike and rider is lower.

Interestingly, some bikes (such as Pivot’s Switchblade) feature height-adjustable ‘chips’, which compensate for different wheel sizes. Using these, the bottom-bracket height remains similar with the smaller wheel size, but the bottom-bracket drop changes. This results in a much smaller change in the bike’s handling, suggesting bottom-bracket height is important, not bottom-bracket drop.

Bottom-bracket drop is still a useful measurement, though. Bottom-bracket height is affected by not only wheel size but also tyre choice – comparing bottom-bracket drop between bikes of a given wheel size removes this variable.
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#22 ·
My last few frames BB drop did not match manufacturer specifications. They all had less drop which was probably a good thing. I recently purchased a Pole Evolink. The current v1.4 has a -3mm drop, my v1.3 has 20mm. Deciding to get the older model was solidly based on the lower BB height of the older model. I do get strikes more often than not and wonder if the move to the taller BB height would have been a positive move.
 
#23 ·
{ too high, just right, too low} in this order
Road bike (28mm tire): { 67mm, 74mm, 81mm}
Gravel bike (42mm tire): { 73mm, 79mm, 85mm } -- gravel bike have much taller tire than road bike, so it deserve some more bb drop.
Short travel Mountain bike (29" wheel with 2.4" tire): { 31mm, 42mm, 53mm } -- relatively tall to compensate for suspension movement and uneven terrain.
 
#24 ·
I have a 29er and 27.5 bikes. I run the high setting for my suspension. On my 27.5, I tested the low setting and I did have more pedal strikes. No BB strikes though. I can't say there was a major difference between a high and low BB when you adjust it. It may feel different for bikes that have low BB built in like Whyte bikes.

In the past, I built an Intense Recluse. Great bike but wow...the BB sat so high. I felt like I was riding too high! At least I had ground clearance.
 
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#26 ·
I think most modern bikes are too low. Easy!

Maybe they're designed for people that ride groomed trails, with stiff suspension, and mostly downhill so they don't need to pedal and worry about pedal strikes. But for riding on pedaly trails, with camber, and roots, higher is better.

plummet said:
What do you think Is too low, just right and too high?

Add your wheel size, travel and riding style.

I'm of the opinion that 15-20 mm drop is optimal for enduro bike/technical riding.
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#27 ·
my Hard tails
Nordest bardino 2 LTD has a 55mm drop
Octane one Prone 27.5 has 45mm drop

my 2018 Scott genius 940 is apparently 20mm but I cannot find an exact figure. the bike is a work in progress for me
 
#29 ·
345mm bb height for 27.5 bikes with 160 rear travel worked well for me and my 29er now is 170 travel and 351mm bb height, running ~30% sag for each size. So not much bb drop, 10mm and 21mm if the geo charts are correct? Newer 2.5 and 2.4 tires on the 29er give a few more mm ground clearance than the 2.3 tires I ran on the 27.5 bikes. New England riding with a ton of pedal catchers everywhere and always running a well used bash guard too.
 
#31 ·
genixia said:
Agreed, saddle height is usually set by the extended leg on the lowest point of the pedal stroke. Lower the BB by 5mm, and your saddle will be 5mm lower too. That'll put your center of gravity nearly 5mm closer to the roll axis, which is why the handling changes. (You'd need to also lower the stem 5mm to get all 5mm drop in COG.) Since the roll axis is the line between tire patches, this is determined by absolute BB height. Changing the height of the gyroscopic effect of the wheels (by changing wheel sizes) will have some effect on handling too, but that is independent of BB height. BB drop may reflect both changes in BB Height and wheel size, but tells you nothing much about handling changes.

It's interesting to read what Bike Radar had to say about it in their Ultimate Guide to Bike Geometry and Handling:
Click to expand...
Yup. Thats the core issue with 'bb drop': Its a measurement that CAN be made of a frame. You can't know chainring clearance or actual pedal min height without the actual bike setup, so its what is in frame geo charts.

But just because its A measure doesn't mean its useful.

We all want lowest roll center. What is negotiable is how much pedal and chainring clearance you can tolerate.

Funny in the discussion folks complain about hitting pedals. Reason I care about bb height is because I've got tech features I can't send that risk hosing my cr against rocks. Risk to $50 chainring is pretty much the only reason I'll get off and walk.
 
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#32 · (Edited)
angieri918 said:
The new Pole bikes have zero BB drop or BB height. Can’t remember which. Leo says it helps you change direction quicker. I’d definitely like to try one of those bikes just to see how it would ride
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www.bikeradar.com

Pole Voima review | BikeRadar

Machined from billet aluminium, Pole’s first ebike is as groundbreaking as it looks
www.bikeradar.com www.bikeradar.com

The idea seems to be that with less BB drop, the bike is more agile. Getting extra ground clearance to pedal without pedal strikes is a welcome benefit too. That and long wheelbase bikes have more ground clearance issues.

Image


The ground clearance seems to work in this scenario, to clear this small ledge thingee.
 
Discussion starter · #33 ·
LaXCarp said:
I dont think BB drop matters much honestly, what matters is absolute BB height. BB drop is irrelevant as it relates to the center of gravity, which is much higher on the bike and is the dominating force.
Click to expand...
It is a interesting measurement when comparing frame geometries. All other factors being equal bb drop determines how much lower or higher the bb will actually be comparing brands.

Thus the discussion.

I'm trying to get a handle on idea on what is too much drop.

So... please respond with your bikes bb drop, travel and whether you consider it to low, about right or too high.

That will help with this discussion.
 
Discussion starter · #34 ·
ocnLogan said:
The bikes travel makes a difference too, because 30% of 180mm is more than 30% of 100mm.

Most long travel 29’ers are between 29-35mm of bb drop. But some have over 40mm of drop (stumpy evo in low).

I’d be really interested to try lower bbs with shorter cranks, and see how that felt.
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What's the bb drop of the bikes you are riding now?
 
#35 ·
plummet said:
What's the bb drop of the bikes you are riding now?
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My Banshee Titan is running a taller axle to crown fork (Mezzer) at 170mm (was designed around a Fox36). So it’s 25mm of standard BB drop is about 23mm. I’m also running 165mm cranks.

My old Kona was 29mm of BB drop. And 175mm cranks. I had a huge pedal strike on that bike at speed (shattered toe in 5 places and broken ribs), but I think it was due to higher speed and lower shock pressure.

I’d be willing to try a lower BB for around here (PNW).
 
#37 ·
Mostly all mountain/enduro with lots of natural rocky stuff and loamy off-piste.

My V1 Megatower has a 29mm drop in its "neutral" configuration (160mm fork and flip chip in high position). That was too low with 175mm cranks and I was catching pedals everywhere.

I swapped in some 170mm cranks (which is what SC specs on their completes) but it still wasn't quite enough, so I increased fork travel by 10mm and it's been fine ever since.

So the perfect BB drop for me on this bike is roughly 26mm as it turns out, 3mm higher than stock thanks to overforking it, but I could maybe get there with thinner pedals or 165mm cranks without affecting the geo. It doesn't feel any different to me though.

It's not something I considered when I bought the frame. It will be moving forwards (along with stack, but that's another topic).
 
#38 ·
Always find this Paul Aston interview interesting when discussing BB height and drop (both of which I have no idea what mine are):

The BB is currently about 375mm. Blasphemy! I'm not a super fan of low bottom brackets, especially in Finale where there are plenty of things to catch a pedal on. My theory is that low bottom brackets were great to improve stability on tiny bikes with small wheels a few years ago, nowadays we mostly use bigger wheels so we instantly have higher axles and more BB drop, plus this bike is so inherently stable that a higher BB lets me ride through rocks and ruts with much more confidence.

I find it also switches direction easily with my style of riding: I pump the bike a lot and unweight it between corners, at full extension I am standing above the axles and so it tips over easily. This also keeps the bike leant over in turns better: if you are standing far below the axles your weight pushing through the BB is trying to stand the bike up.
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#40 ·
Pisgah said:
I though BB drop was an alternative measurement used by manufacturers to disguise ridiculously low BB heights?
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It's nothing more than a measurement which exists beneath the span of the axles on the frame. No accounting for sag and/or sags, no accounting for rear-axle movement during articulation.
It is a wannabe measurement.
 
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