Descending, bike fit, weight distribution

Discuss light weight issues concerning road bikes & parts.
GrumpyOldPizza
Posts: 48
Joined: Fri Dec 07, 2018 4:24 pm

by GrumpyOldPizza

The engineer in me needed to ask this question.

First off, I am sucky descender. Mainly due to my lack of depth perception, and hence lack of confidence.

Now I got 2 setups on my bike working well. One with a 90mm stem, and hence saddle further back. The other one with a 100mm stem and the saddle forward by pretty much 10mm. The bike in question is a SuperSix EVO2, my height is 5'8", weight is 150lbs, just to frame that properly.

Which one is likely to be better for descending ? The 100mm stem will make steering a bit slower, and perhaps give me more confidence ? The 90mm stem will put more weight on the rear (say 1% more ?), and make the whole system more stable.

What are the tradeoffs, would it matter with that small delta in the weight distribution ?

How would you go and test which setup is the objectively better one ? Long highspeed descents ? Switchbacks ? Heavy braking ?

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pdlpsher1
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Location: CO

by pdlpsher1

The saddle setback should drive the stem length, not the other way around. Given what you already have, put some wide tires on and you’ll immediately feel a positive difference in your descending.

And not all descents are equal. Some are steep and twisty. Others are long, sweeping, and fast. Each type of descent needs a specific bike geometry for optimal performance. Most production bikes are designed to suit all types, thus it’s a compromise at best.

If you have been fitted properly on your bike, the stem length won’t change the handling of your bike in a drastic way. Tire width and grip will make a far bigger difference.


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by Weenie


tarmackev
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Joined: Sat Jan 15, 2011 9:59 pm

by tarmackev

As a naturaly fast descender (I have too, I'm a terrible climber) I feel its about confidence, trust in the bike, the brakes and youself.

GrumpyOldPizza
Posts: 48
Joined: Fri Dec 07, 2018 4:24 pm

by GrumpyOldPizza

pdlpsher1 wrote:
Wed Jul 24, 2019 3:47 pm
The saddle setback should drive the stem length, not the other way around. Given what you already have, put some wide tires on and you’ll immediately feel a positive difference in your descending.
Why ... the setback and not the stem ? Looking for an explanation there. Is it the weight distribution that matters more ? If so, what is the optimum, and what does a +/- 1% change do ?

Don't think 28mm will fit, so 25mm is it.

GrumpyOldPizza
Posts: 48
Joined: Fri Dec 07, 2018 4:24 pm

by GrumpyOldPizza

tarmackev wrote:
Wed Jul 24, 2019 4:41 pm
As a naturaly fast descender (I have too, I'm a terrible climber) I feel its about confidence, trust in the bike, the brakes and youself.
Opposite here. Decent climber (so I have to wait for everybody else at the top), but terrible descender (everybody else waits for me). So whatever I can do with the bike fit is fair game ...

So what builds more configdence, a longer stem (and slightly slower steering, which could translate to more control), or moving more weight to the back (which I'd think minimized the required steering input).

TheRich
Posts: 734
Joined: Tue Jan 01, 2019 1:36 am

by TheRich

GrumpyOldPizza wrote:
Wed Jul 24, 2019 4:59 pm
tarmackev wrote:
Wed Jul 24, 2019 4:41 pm
As a naturaly fast descender (I have too, I'm a terrible climber) I feel its about confidence, trust in the bike, the brakes and youself.
Opposite here. Decent climber (so I have to wait for everybody else at the top), but terrible descender (everybody else waits for me). So whatever I can do with the bike fit is fair game ...

So what builds more configdence, a longer stem (and slightly slower steering, which could translate to more control), or moving more weight to the back (which I'd think minimized the required steering input).
Probably the latter. The stem of a road bike is effectively the stem length PLUS the reach of the bars, so a seemingly large change in stem length is effectively much smaller.

The limits of descending for the vast majority of people is inside their head, so that is what people should concentrate on.

GrumpyOldPizza
Posts: 48
Joined: Fri Dec 07, 2018 4:24 pm

by GrumpyOldPizza

TheRich wrote:
Wed Jul 24, 2019 5:22 pm
Probably the latter. The stem of a road bike is effectively the stem length PLUS the reach of the bars, so a seemingly large change in stem length is effectively much smaller.
Ah, yes. Just did the math. So a 10mm change in stem length is a 9mm change in effective reach (tip of saddle to center of the hoods). So not really "much smaller".

Conversely, if you have a wheelbase of about 1000mm, then a change in saddle setback of 10mm should only max 2% (if all your weight is in one spot), perhaps more realistically 1% ...

My weight distribution is 61/39 with the 100mm stem setup as is ... So a 90mm stem would move that perhaps to 62/38 ?

TheRich
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Joined: Tue Jan 01, 2019 1:36 am

by TheRich

GrumpyOldPizza wrote:
Wed Jul 24, 2019 5:47 pm
TheRich wrote:
Wed Jul 24, 2019 5:22 pm
Probably the latter. The stem of a road bike is effectively the stem length PLUS the reach of the bars, so a seemingly large change in stem length is effectively much smaller.
Ah, yes. Just did the math. So a 10mm change in stem length is a 9mm change in effective reach (tip of saddle to center of the hoods). So not really "much smaller".

Conversely, if you have a wheelbase of about 1000mm, then a change in saddle setback of 10mm should only max 2% (if all your weight is in one spot), perhaps more realistically 1% ...

My weight distribution is 61/39 with the 100mm stem setup as is ... So a 90mm stem would move that perhaps to 62/38 ?
The reach of my bars is 93mm, so when in the drops, stem is effectively (stem length)+(bar reach) long. The handling of a road bike is more defined by HT angle and wheelbase anyway, and is so twitchy by design that stem length has little impact on the overall feel.

You shouldn't be rigidly attached in the bike in any dynamic situation either. Your weight distribution is based where you put your weight, so it may be influenced by contact points, they don't determine weight distribution. The placement of the BB within the wheelbase has a much bigger influence.

Plus, as I said, the limit of cornering in the vast (and I do mean VAST) majority of riders is in their head. Take a slightly different bike, take a little while to get used to it, and we're going to ride it in the same way.

I'm a not bad descender, and hit 50mph regularly on one particular ride, and the 80mm stem on my 56cm Boone (to aid dirt performance) is virtually identical to the way a 110mm stem felt on the road. If your position feels good, and objectively works well, it literally IS good...people need to stop worrying about aesthetics based on faulty reasoning.

zefs
Posts: 446
Joined: Sat Aug 05, 2017 8:40 pm

by zefs

You could move your body weight further back without making any adjustments (it will be more than just 10mm of setback increase). If you are not pedalling you can even place your pelvis all the way back on the saddle which will give a lot of stability.

GrumpyOldPizza
Posts: 48
Joined: Fri Dec 07, 2018 4:24 pm

by GrumpyOldPizza

TheRich wrote:
Wed Jul 24, 2019 6:09 pm
The reach of my bars is 93mm, so when in the drops, stem is effectively (stem length)+(bar reach) long. The handling of a road bike is more defined by HT angle and wheelbase anyway, and is so twitchy by design that stem length has little impact on the overall feel.

You shouldn't be rigidly attached in the bike in any dynamic situation either. Your weight distribution is based where you put your weight, so it may be influenced by contact points, they don't determine weight distribution. The placement of the BB within the wheelbase has a much bigger influence.

Plus, as I said, the limit of cornering in the vast (and I do mean VAST) majority of riders is in their head. Take a slightly different bike, take a little while to get used to it, and we're going to ride it in the same way.

I'm a not bad descender, and hit 50mph regularly on one particular ride, and the 80mm stem on my 56cm Boone (to aid dirt performance) is virtually identical to the way a 110mm stem felt on the road. If your position feels good, and objectively works well, it literally IS good...people need to stop worrying about aesthetics based on faulty reasoning.
Couple of comments. I cited and measured the effects on the stem length and the weight distribution in the hoods. Mainly to compare it to research lterature, like this here https://scholar.colorado.edu/iphy_gradetds/43/. So yes, all of this changes if you are in the drops or the bar, but it's nice to have a point of reference.

I don't care too much about aestetics, and yes most of my personal limiter is in my head. I have ridden my other bike, a CAAD10, with 80mm stems and 90mm stems, and IMHO there is a huge difference.

The question I am posing here is really what the tradeoff between weight balance and stem length (i.e. steering) is. Yes, there are multiple components to the puzzle, but there are only a few I can control. Stem length is one, the position of the steat is another (yes, the handlebar itself, too ,.. but swapping in/out handlebars is more time consuming ...). That answer might well be that within this 10mm delta, it just does not matter ,,, I simply don't know, which is why I am aksing brighter minds than myself here on this forum.

GrumpyOldPizza
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Joined: Fri Dec 07, 2018 4:24 pm

by GrumpyOldPizza

Deleted, duplicate post ...

mattr
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Joined: Fri May 25, 2007 6:43 pm
Location: The Grim North.

by mattr

Get your saddle in the right place for your biomechanics. Not for descending.
As per other posts, you can make more than enough difference to weight distribution by moving your body round.

GrumpyOldPizza
Posts: 48
Joined: Fri Dec 07, 2018 4:24 pm

by GrumpyOldPizza

mattr wrote:
Wed Jul 24, 2019 6:38 pm
Get your saddle in the right place for your biomechanics. Not for descending.
As per other posts, you can make more than enough difference to weight distribution by moving your body round.
For this questions's shake, lets assume both fits are equally good, no noticable differentce.

Let's also assume that with both setups I could move my body weight around that same way, just with one 10mm further to the back than the other, but not being limited in any other way.

So what is better for descending, more weight on the back wheel, or a longer stem to have more fine grained control ...

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JKolmo
Posts: 581
Joined: Sat Feb 10, 2007 2:00 pm

by JKolmo

Stem length and saddle setback has nothing to do with it. Relaxed body, shoulders and arms, hands in the drops, look way ahead where you are going not right in front of you, learn how to distribute your body weight and lean your bike, weight on outer pedal six o’clock, find apex in the bends, learn correct braking technique, practice and practice and practice. There is no tech gear short cut out of it.

GrumpyOldPizza
Posts: 48
Joined: Fri Dec 07, 2018 4:24 pm

by GrumpyOldPizza

JKolmo wrote:
Wed Jul 24, 2019 7:58 pm
Stem length and saddle setback has nothing to do with it. Relaxed body, shoulders and arms, hands in the drops, look way ahead where you are going not right in front of you, learn how to distribute your body weight and lean your bike, weight on outer pedal six o’clock, find apex in the bends, learn correct braking technique, practice and practice and practice. There is no tech gear short cut out of it.
Again, all of that is irrelevant. I have 2 bike setups that are equally good in terms of speed on the flats, climbing and all other aspects I can think of. What you mention are questions of technique (other than the distribution of body weight). All I am asking whether there is an advantage in having the saddle 10mm further back with a shorter stem, or saddle further forward with a longer stem (simply assuming that the reach is the constant here (tip of saddle to center of bar for simplicity).

by Weenie


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