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Q-factor and DH bikes (or is a wider BB really better?)

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Pslide

Turbo Monkey
Jan 14, 2002
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Feet on the ground, head in the clouds.
I was having a chuckle at people whining that the Demo is still running an old school 73 mm BB / 135 mm rear end, and it got me thinking about the advantages and disadvantages of wide BBs. My thoughts are below, and it turns out that narrow BBs may actually have some benefits, at least theoretically.

NOTE: For those that don’t know what Q-factor is, it’s the term used for the spacing width between your feet. DH bikes have a wide Q-factor due to 83 mm BB shells and wider cranks, roadies generally run a narrow Q-factor.

Pedaling: Theory has held that narrow Q factor is better for pedaling, but this is primarily for seated pedaling, and therefore not relevant to DH. What is relevant is standing pedaling, and a wider Q-factor will require more body movement (or bike movement) to get your weight over your driving leg. See-sawing the bike back and forth to get more power out of your pedal stroke is a waste of energy. -1 for wide BB.

Stability: A wider stance most likely will feel more directionally stable when not pedaling. ‘nuff said. +1 for wide BB.

Cornering: This one is not so straightforward. Let’s consider flat corners, since this is more critical in racing. Motorcycle road racers keep their weight low and inside on flat corners, and the same applies for mountain bikes. A wider Q-factor places the force driving through your outside leg further from the bike’s centerline. This is not ideal, and the moment generated by the force from your outside leg has to be countered with more inside force at the handlebars.

For the best cornering stability, requiring minimum correcting force at the handlebars, you’d want the force from your outside leg driving right down the centerline of the bike. This is obviously not possible, but a narrower Q-factor does place the force closer to the centerline.

Furthermore, like the motorcycle road racer, the best position for your weight from a dynamics point of view is low and inside, and the narrower Q-factor would be preferred. -1 for wide BB.

So maybe Specialized know what they’re doing with their narrow BB. (Rear wheel strength/stiffness is another issue. As is chain clearance. There are obviously trade-offs.)

I would love to ride two identical bikes, one with a 68 mm BB and narrow cranks (XTR?) and one with an 83 mm BB and wide cranks to see if there was any noticeable difference or if I’m just blowing smoke… Probably the latter, but bike design is getting pretty sophisticated these days, so you can’t blame me for thinking about it…
 
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ebarker9

Monkey
Oct 2, 2007
850
243
Pslide said:
Motorcycle road racers keep their weight low and inside on flat corners, and the same applies for mountain bikes. A wider Q-factor places the force driving through your outside leg further from the bike’s centerline. This is not ideal, and the moment generated by the force from your outside leg has to be countered with more inside force at the handlebars.
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Interesting discussion, though I disagree with this. There aren't a huge number of parallels between motorcycle roadracing and mountain biking technique when it comes to cornering. Have you ever witnessed a DH'er shift their center of gravity toward the inside of a flat turn? Not likely.
 
Dogboy

Dogboy

Turbo Monkey
Apr 12, 2004
3,209
584
Durham, NC
Pslide said:
So maybe Specialized know what they’re doing with their narrow BB.
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The Demo, due to the offset on the rear triangle, actually requires a wide (83mm equivalent) bottom bracket to achieve proper chainline. Lots of people run a standard 50mm chainline crankset (to the detriment of their chainline), but that is not how Specialized specs the bike.

My personal opinion (not that it's worth much) is that Q-factor on a DH bike means fvck all.
 
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slowitdown

Monkey
Mar 30, 2009
553
0
I thought the reasoning for 83mm BBs on DH bikes was (1) to provide room to fit rear suspension linkages and (2) to provide a decent chainline with a 150mm rear hub. I'm pretty sure I got that impression from UBI's framebuilding school.

I didn't think Q-factor ever entered into DH race frame designers' minds.

I don't think most DH racers think about pedaling efficiency. Read James Wilson's training thoughts on pedaling technique and body positioning -- he pretty much disregards the wisdom accumulated by over a century of roadies experience seeking max efficiency and power.

Pslide, I agree with your points about the effect of wide Q-factor though. I would imagine anyone who came to MTBs after a good period of serious road riding would agree too. I think there's a generally small population of former roadies at RideMonkey's DH forum though. Probably not surprising. DH racing is about race times of 2 mins to 10 mins... road racing times are much longer. Efficiency isn't as much a concern for a DH racer.
 
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P

Pslide

Turbo Monkey
Jan 14, 2002
1,373
0
Feet on the ground, head in the clouds.
ebarker - True, I may need to adjust my thinking. I suppose on flat corners in DH the technique is to lean the bike more than the rider and get your knees down, hips to the inside of the turn, and weight over the top of the bike, which is exactly opposite to motorcycle road riding. Now I just have to figure out why from a physics point of view. I'm sure it has to do with the amount of grip available...

Dogboy - Oops, my mistake, thanks for correcting me.

slowitdown - I agree with everything your saying. And yes, I doubt Q-factor enters into a DH frame designers mind. The question is, should it? Maybe Dogboy's last line is correct - maybe not.
 
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slowitdown

Monkey
Mar 30, 2009
553
0
Pslide said:
slowitdown - I agree with everything your saying. And yes, I doubt Q-factor enters into a DH frame designers mind. The question is, should it? Maybe Dogboy's last line is correct - maybe not.
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I think it should. I'd wonder if Dogboy has spent any time playing with Q-factor. I know that for me, a too-wide Q-factor makes my adductors scream and can cause IT band issues.

The point about narrowing the Q-factor = increasing the pressure on the tire/track interface is valid. But there's still a lot of holdover from the 2000 era of really wide tires, I think. People think of bikes like they think of go-karts, needing to be low and wide-tracking at the contact points. The "low" part I agree with, the "wide-tracking" I don't. I run narrower tires than many people I know and don't find it an issue. At some point for some folks, I think people have latent "image" issues that they'd do well to give up... meaning they can't imagine something working because it wouldn't look moto or badazz enough for them.
 
Verskis

Verskis

Monkey
May 14, 2010
458
8
Tampere, Finland
Pslide said:
ebarker - True, I may need to adjust my thinking. I suppose on flat corners in DH the technique is to lean the bike more than the rider and get your knees down, hips to the inside of the turn, and weight over the top of the bike, which is exactly opposite to motorcycle road riding. Now I just have to figure out why from a physics point of view. I'm sure it has to do with the amount of grip available...
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I think road racing motorcyclists do it (lean themselves more than the bike) because they want to be able to open the throttle as soon as possible when exiting the corner, and that requires the bike to be as upright as possible to maximize the rear wheel traction.
Or that's what I've heard, never ridden a motorcycle myself :D
 
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Slater

Monkey
Oct 10, 2007
378
0
Having just put 83mm cranks on my demo after riding it for years with the 73mm cranks, I have to say I like the wider "platform" better. Didn't think it'd matter, but it feels more stable to me.

Cornering any worse as a result? Get cereal.

The amount of available traction dictates body position in DH just as it does with streetbikes. However since available traction is so different between the two, comparing body positions between the two is useless.

Unless of course you've got a streetbike on a gravel road, in which case you end up riding it much like a DH bike. Which it turns out is also a gang of fun!
 
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slowitdown

Monkey
Mar 30, 2009
553
0
Slater said:
Having just put 83mm cranks on my demo after riding it for years with the 73mm cranks, I have to say I like the wider "platform" better. Didn't think it'd matter, but it feels more stable to me.
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Since "feels more stable" is highly subjective, it's usually not the best measure of what actually is more stable. But rider confidence is significant, so what subjectively feels good is valid. Bottom line: there's a difference between what something feels like, and what it actually is... and we're all different.

Slater said:
Cornering any worse as a result? Get cereal.
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Using mockery to derail a semi-serious discussion? Get out of jr high school!

Slater said:
The amount of available traction dictates body position in DH just as it does with streetbikes. However since available traction is so different between the two, comparing body positions between the two is useless.
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The first sentence is accurate, the second one irrelevant. The second sentence is a non-sequitur -- doesn't follow from the first. The second sentence is just your opinion. Personal opinions are like arsewholes and bellybuttons, they are something everyone has but not everyone needs to share with others.:D

I don't know what you "compare" when you consider one bicycle vs another, but I would guess you're either not aware of the similarities, or you haven't earned enough saddle time to see them.

Slater said:
Unless of course you've got a streetbike on a gravel road, in which case you end up riding it much like a DH bike. Which it turns out is also a gang of fun!
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I ride all my bikes the same way. I don't know why you'd ride a "streetbike" different from any other bike. They're all bikes.
 
Dogboy

Dogboy

Turbo Monkey
Apr 12, 2004
3,209
584
Durham, NC
If you look at the top ten from the recent Maribor WC (most of which are typically in the top 10) and look at bikes they were riding that should at least partially answer your question. Of those top 10, all were on bikes with a 56mm chainline. Looking at the top 20 there is only one bike with a 50mm chainline (Mondraker) and you could argue that it was built that way purely to allow for lightweight XC cranks - correct me if I'm wrong Cesar. Now a good number of those bikes are made by companies with an extensive knowledge of bike design from Road to Cross to XC to DH. You would think that, with that knowledge base, they would design the DH bikes with a narrower chainline/Q-factor if it trumped the benefits (better tire clearance, more room for linkage, etc).
 
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slowitdown

Monkey
Mar 30, 2009
553
0
You're not really suggesting chainline sameness among certain bikes measured dictates that all DH bikes' handling should always follow that chainline, are you?

If you are, you're arguing in circles. Which probably entertains smallminded people, but not many others.
 
Kanye West

Kanye West

220# bag of hacktastic
Aug 31, 2006
3,741
473
slowitdown said:
I ride all my bikes the same way. I don't know why you'd ride a "streetbike" different from any other bike. They're all bikes.
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Watch a Dakar/Baja race, then watch MotoGP. Or a WC DH race and a road race through twisty roads. That is why you'd ride the bikes differently.
 
Dogboy

Dogboy

Turbo Monkey
Apr 12, 2004
3,209
584
Durham, NC
slowitdown said:
I think it should. I'd wonder if Dogboy has spent any time playing with Q-factor.
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Missed this one. Yes, I have extensive experience with Q-factor. I ride road, XC, CX, DH, and DJ. I have bikes with all kinds of different Q-factors. I've also put in some long days riding XC on a bike with an 83mm BB and a wide Q-factor. I haven't experienced any issues, but I do acknowledge that some people are more sensitive to it and can cause issues - like your IT Band.
 
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slowmtb

Monkey
Aug 17, 2008
216
0
ChurChur, NZ
Pslide said:
ebarker - True, I may need to adjust my thinking. I suppose on flat corners in DH the technique is to lean the bike more than the rider and get your knees down, hips to the inside of the turn, and weight over the top of the bike, which is exactly opposite to motorcycle road riding. Now I just have to figure out why from a physics point of view. I'm sure it has to do with the amount of grip available...

Dogboy - Oops, my mistake, thanks for correcting me.

slowitdown - I agree with everything your saying. And yes, I doubt Q-factor enters into a DH frame designers mind. The question is, should it? Maybe Dogboy's last line is correct - maybe not.
Click to expand...
I think where your going wrong is by comparing DH to road racing. I personally would compare it more to MX, there are definite similarities there BUT you
have to factor in the "M" factor - motor.
A ( proper ) road racer has a a$$ up head down and very low flat riding
position and as a result the rider stays low and stretched out. This encourages
the rider to stay low ( low CG ) and certainly helps cornering. BUT, I raced
with a guy who raced and cornered by leaning the bike down while he sat
almost upright- very strange style and TBH he did crash a lot. It does come
down to individual style but there is deffinitely a technique that works at the
top level. Roadracing is a fairly sterile environment relatively speaking.
Another thing worth mentioning - Super Motad ( Roadracing on modified MX bikes ), these guys essentially ride like MX guys, the bikes geo dictates it. A very exciting form of motor sport.

MX - now this is the one to compare more to DH, BUT in MX you have an
engine and that fact alone is the difference. DH has a large element of balance
involved in order to successfully negotiate a course at speed. MX - cornering,
fast in and through on trailing throttle OR slow in fast out by going in fast slow through and getting on the gas hard on the way out ( these techniques also apply to Roadracing ). MX- you sit in a more upright position with wide bars and MX geo ( raked out front etc, much like DH ) the idea being better control on challenging terrain. You lay that bad boy down and get on the gas, how much your leaned over, how fast your travelling in relation to the corner dictates how much you get on the gas NB: counter steering allows you to apply more gas- it's all a complicated balancing act but having a motor really changes the game compared to DH.

A simplified reply but HTH :)

Jumps- hit the lip wrong and the front dives, get on the gas hard to get the back down; hit a lip wrong and the front sits up, use the rear brake to bring the front down - DH forget it, you don't have a motor so if you have really stuffed up then I hope your armours good !
 
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slowmtb

Monkey
Aug 17, 2008
216
0
ChurChur, NZ
Ah yes - "Q" factor. I suspect the effects on our bikes is minimal. Balance is the most important factor and a wider stance assists this- try standing straight up and down with your feet together, now lean to your left by 2 feet. Next stand straight up and down with your feet apart by 1 foot, now lean to your left by 2 feet. Which one felt better balanced?
But we are only talking 15mm here so not likely to make a significant difference.

What's most important is your chain line, if your chain falls off half way through a race run then your going to pretty damn slow LOL

My 2c
 
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slowitdown

Monkey
Mar 30, 2009
553
0
Dogboy said:
I am not suggesting that at all. We are discussing how Q-factor affects handling. Chainline dictates (to a large degree) Q-factor however.
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Only if the frame has to be made wider to accommodate the rear half's suspension system.

This is what I meant by your circular argument. Of course chainline will go wider if the BB shell must be wider. Of course. How else can it go? Shove the chain inboard? Dish the chainring to move the chainline inboard?

As far as I can tell, Pslide is suggesting --and I am certainly suggesting-- that it may be worth trying to narrow Q-factor, if possible.

Right?

Hacktastic said:
Watch a Dakar/Baja race, then watch MotoGP. Or a WC DH race and a road race through twisty roads. That is why you'd ride the bikes differently.
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Differently at what level? Good riding consists of driving the bike's footprint through your feet. Right?

The differences I can detect are that my body's position on my cross bike is different from its position on my singlespeed, and different from its position on my FS bike, and different from its position on my roadie (which I never ride any more and haven't ridden in at least 5 years).

But sure handling and predictability of the bike's path on the road/trail/track depends on the same thing: maintaining consistent traction, and absorbing irregularities in the tire/surface contact patch and its path.

Dogboy said:
Missed this one. Yes, I have extensive experience with Q-factor. I ride road, XC, CX, DH, and DJ. I have bikes with all kinds of different Q-factors. I've also put in some long days riding XC on a bike with an 83mm BB and a wide Q-factor. I haven't experienced any issues, but I do acknowledge that some people are more sensitive to it and can cause issues - like your IT Band.
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That's why I think it's a good idea to consider this possible re-think of how Q-factor and chainline and rear suspension/frame interface play out on the bikes we ride.

Every design question involves compromise.

A few people have suggested that widening the space between one's feet creates more stability. That's true within certain confines. The confines I see are that if you make the feet too widely spaced, you limit one's lower-body bump absorption and you severely restrict how far over the bike can be leaned.

Think of the BMW opposing-cylinder moto engines, and how those cylinder heads protrude. The bike can be laid over only so far before pavement grinding the cylinder heads starts to happen, right?

Or try to ride your bike in a rut that's so narrow you have to mind where your pedals & feet are, so as to avoid striking them on the rut's sidewalls. Widen the platform --the Q-factor-- and you're gonna reach a point where the rut is too narrow for that wide cylindrical plane created by your pedaling motions. Of course in this instance someone could simply say "widen the rut," but I hope you get the message I'm trying to send here.
 
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Kanye West

Kanye West

220# bag of hacktastic
Aug 31, 2006
3,741
473
Jesus christ monkeyballs....you have this incredible ability to just keep talking and not actually say anything...

Road tires rely on tire friction - flat surface to flat surface. Dirt tires rely on lateral biting edges - square edges digging into soft compliant surfaces. Lean form has to change accordingly between types of terrain, motorized or not.
 
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slowitdown

Monkey
Mar 30, 2009
553
0
Hacktastic said:
Jesus christ monkeyballs....you have this incredible ability to just keep talking and not actually say anything...
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Your ability to create an insult instead of trying to understand... priceless.

Hacktastic said:
Road tires rely on tire friction - flat surface to flat surface. Dirt tires rely on lateral biting edges - square edges digging into soft compliant surfaces. Lean form has to change accordingly between types of terrain, motorized or not.
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You're making artificial distinctions. In both cases, friction between the tire and the surface being ridden is the controlling or limiting factor. You pretend they're radically different. Your inability to see difference in types of 3-dimensional contact is pretty surprising.

The tire/surface interaction on a road bike may look much more 2-dimensional to you, but to the tire and the road, it's still 3-dimensional no matter whether you refuse to see that.
 
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P.T.W

P.T.W

Monkey
May 6, 2007
599
0
christchurch nz
Curse you people with similar name posting on the same thread...Sorry Kerry i actually agree with you. I really wanted to quote Slowitdown not Slowmtb.... my bad...here i fixxored it.

Ill try again on the next page
 
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Dogboy

Dogboy

Turbo Monkey
Apr 12, 2004
3,209
584
Durham, NC
Hacktastic said:
Jesus christ monkeyballs....you have this incredible ability to just keep talking and not actually say anything.
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Seriously, WTF man.

And my argument (?) is not circular. I was just using chainline as a reference point in regard to Q-factor - they are not mutually exclusive you know.

But P.T.W and Hacktastic pretty much sum up my thoughts.
 
zdubyadubya

zdubyadubya

Turbo Monkey
Apr 13, 2008
1,273
96
Ellicott City, MD
Anyone else read this thread, was about to respond and the realized that the argument is over 10mm!...

To all the points about leaning into the dirt and traction, if I'm going to break traction and crash or if I say that my bike behaves funny because my feet are a full centimeter further away from each other than they should be, then the joke is on me...

Long story short, like said above, Q-factor is important, but for downhill mountain biking, it dont matter. suspension performance and bike geo/setup/ability to meet other industry standards faaaar outweighs a stinkin centimeter in stance width.
 
manhattanprjkt83

manhattanprjkt83

Rusty Trombone
Jul 10, 2003
9,646
1,216
Nilbog
does it really matter? bike designers do the 83mm so they can design the bikes they want, not for pedaling efficiency...personally I like the 68/73 so i can run some XTR's on there im not that heavy (165 lbs)...

If you put the same bike regardless of bb shell size under a WC rider i doubt the times are going to differ at all...fast is fast.
 
manhattanprjkt83

manhattanprjkt83

Rusty Trombone
Jul 10, 2003
9,646
1,216
Nilbog
Dogboy said:
I am not suggesting that at all. We are discussing how Q-factor affects handling. Chainline dictates (to a large degree) Q-factor however.
Click to expand...
there are so many variables on a DH track terrain that in the end i doubt it matters...the 83 get's you're stance out a bit on the rough, the 68/73 get's you pedaling better...in the end it probably evens out...
 
P

Pslide

Turbo Monkey
Jan 14, 2002
1,373
0
Feet on the ground, head in the clouds.
It's getting a little hot in this thread for me.

I think a lot of people are making valid points. But no matter how I think about the physics and forces, and in leafing through a Dirt mag and looking at the photos of people railing corners, it seems to me a narrower Q would work better. It places the outside foot closer to where you want it for both body position and force transmission. Pedalling as well. Stability when not cornering probably not.
 
Dogboy

Dogboy

Turbo Monkey
Apr 12, 2004
3,209
584
Durham, NC
manhattanprjkt83 said:
there are so many variables on a DH track terrain that in the end i doubt it matters...the 83 get's you're stance out a bit on the rough, the 68/73 get's you pedaling better...in the end it probably evens out...
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Uh, I said in my first post that I thought Q-factor meant fvck all on a DH bike...
 
JCL

JCL

Monkey
Aug 31, 2008
696
0
Well I can definitely feel the difference between 73 and 83mm BB's. Especially on steep and rough stuff. Helps to control defections on the rear wheel similar, but not to the same degree, to the way wider bars do for front wheel deflections. I'd actually prefer another 10mm on the BB and 150mm cranks. I don't think you could go much further without negatively impacting pedaling efficiency on flatter tracks.

As far as Specialized is concerned I think we'll see the Demo with 83/150mm next year. They seem to be taking advantage of the extra width too, look at the main pivot spacing ! - http://dirt.mpora.com/news/video-specialized-demo-spy-shots.html

And if Cesar had put an 83mm on the Summum I'd have bought one :)
 
P.T.W

P.T.W

Monkey
May 6, 2007
599
0
christchurch nz
Ok think ive got it right this time :banghead::doh:

slowitdown said:
Only if the frame has to be made wider to accommodate the rear half's suspension system.

Blah Blah Blah etc etc

Or try to ride your bike in a rut that's so narrow you have to mind where your pedals & feet are, so as to avoid striking them on the rut's sidewalls. Widen the platform --the Q-factor-- and you're gonna reach a point where the rut is too narrow for that wide cylindrical plane created by your pedaling motions. Of course in this instance someone could simply say "widen the rut," but I hope you get the message I'm trying to send here.
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slowitdown said:
Your ability to create an insult instead of trying to understand... priceless.



You're making artificial distinctions. In both cases, friction between the tire and the surface being ridden is the controlling or limiting factor. You pretend they're radically different. Your inability to see difference in types of 3-dimensional contact is pretty surprising.

The tire/surface interaction on a road bike may look much more 2-dimensional to you, but to the tire and the road, it's still 3-dimensional no matter whether you refuse to see that.
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Your posting all this dribble for your own gratification...right?:rolleyes:
 
Chaingangster

Chaingangster

Chimp
Nov 11, 2007
64
0
norcal
Anyone remember the 100mm BB shells? The old Giant DH's had em so they could fit all that linkage in there. At least they had like 16" BB heights so you could still pedal out of corners. Low COG be damned!
 
no skid marks

no skid marks

Monkey
Jan 15, 2006
2,511
29
ACT Australia
73mm shell, 135mm 6speed dishless rear wheel is where it's at. But everything is a compromise with bikes, if a stiffer rear can be made with wider pivots(in turn that can be for another compromise)then 83 is probably the better compromise.
Can someone write down what chainline is made with 73mm and 83mm on a 150mm rear, this'll help some peoples decisions/understanding.
Lighter cranks is a plus for 73mm, I've heard of a couple of people facing their 83 down to 73 for this reason.
What a painful thread to read.
 
Scrub

Scrub

Turbo Monkey
Feb 4, 2003
1,453
119
NOR CAL, Sac/CoCo County
Chaingangster said:
Anyone remember the 100mm BB shells? The old Giant DH's had em so they could fit all that linkage in there. At least they had like 16" BB heights so you could still pedal out of corners. Low COG be damned!
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HA! Thats a good one, I was gonna post something along those lines. I loved that bike and still have that frame hanging on a wall. It was so stable on HIGH speed straight aways but a little tricky in TIGHT corners. Totally different than the Glory which is a great in corners and not as stable on the high speed sections. Both frames are on opposite sides of the Q-factor which I'm not sure means a whole lot. my .02
 
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