why use a Set Back Seatposts ?

[ldamelio]

I had the same question as the OP a couple of years ago. Here's a thread from ATH that I started. Includes responses from some of the deans of American frame building.

https://www.velocipedesalon.com/foru...ducate+setback


Mods please feel free to delete if not OK to cross post.

[SoCalSteve]

Originally Posted by ldamelio

I had the same question as the OP a couple of years ago. Here's a thread from ATH that I started. Includes responses from some of the deans of American frame building.

https://www.velocipedesalon.com/foru...ducate+setback


Mods please feel free to delete if not OK to cross post.

I’m not a mod ( nor play one on tv ) but if there is great information to be had, why not share that information? I’m sure the mods feel the same way. And, I also believe that there isn’t any animosity between the two forums.

[roguedog]

that was an interesting trip down the rabbit hole.

Originally Posted by ldamelio

I had the same question as the OP a couple of years ago. Here's a thread from ATH that I started. Includes responses from some of the deans of American frame building.

https://www.velocipedesalon.com/foru...ducate+setback


Mods please feel free to delete if not OK to cross post.

[booglebug]

Just a question. If you’re not fighting sliding forward or sliding back on your saddle does it mean you’re close in your setback? Thinking that pedaling style could come into play, flat vs toes down.

[ldamelio]

Originally Posted by SoCalSteve

I’m not a mod ( nor play one on tv ) but if there is great information to be had, why not share that information? I’m sure the mods feel the same way. And, I also believe that there isn’t any animosity between the two forums.

I understand, I'm active 85% over there, 15% here and enjoy both, just wanted to be respectful in case there were any considerations I was unaware of.

[Mark McM]

I totally, totally disagree with this.

You set your saddle position based on your legs, nothing else.[/QUOTE]

That clearly can't be the case at all. Your legs are not the only part of your body involved in pedalling. Here's why:

Every reaction has an equal and opposite reaction; so when you are pushing on the pedals with your legs, what is the reaction force in the opposite direction? On a recumbent, where your legs are horizontal, and the seat has a vertical seatback, you clearly push against the seatback. But on an upright bike, the reaction force the legs push against is gravity; specifically, the weight of the cyclist's body. The location of the rider's center of gravity plays a large role in determining how the rider can apply leg force during different parts of the pedal stroke.

Setback is used to place the rider's body weight over the cranks in the position for most effective pedalling. That is the basis of the previously mentioned rule of thumb about long torso/short legs using a bigger setback than short torso/long legs. A long torso projects the body weight forward from the saddle (and cranks), so the saddle should be moved back accordingly to compensate. Torso angle also plays a role in this.

Originally Posted by Kontact

You set your reach AFTER you have found saddle position.

While that has often been the simplified approach to fitting (based on strict adherence to KOPS), the reality is that setback can't be set until the upper body position is also set. Ti Designs has often discussed the relationship between saddle setback and rider CG - where is he when you need him?

Originally Posted by Kontact

Long and short legs have the same proportions and require a similar degree of set back.

This is also not true. The ratio of upper and lower leg segments also varies between people, and this also affect setback.

[Kontact]

Originally Posted by alancw3

setback seat posts are only another dimension of fitting a bike to the rider. imho if a bike is fitted properly a zero setback should be used. but most people buy a used bike and have to fit by seat post setback and stem length.

i have used both over the past 50 years and i can tell you that there is nothing like a zero set back post for a solid feel while riding. if i were having a bike custom build i would specify a zero set back seat post as part of the design.

Considering that nearly every manufacturer specs setback seat posts on road bikes, why would a properly fitted bike have the saddle moved forward an inch from spec?

I get that some people don't like set back seat posts conceptually, but you can't throw out an inch/2.5° of fit dimension without compensating for it.

[macaroon]

Originally Posted by Kontact

Considering that nearly every manufacturer specs setback seat posts on road bikes, why would a properly fitted bike have the saddle moved forward an inch from spec?

I get that some people don't like set back seat posts conceptually, but you can't throw out an inch/2.5° of fit dimension without compensating for it.

Manufacturers spec their bikes for the averages. The average person in an average position will probably require a setback seatpost.

The picture I posted on a previous page shows a position that isn't typical/isn't average, and a flipped seatpost has has been used to achieve it.

[Mark McM]

Originally Posted by Kontact

Which is why you would have the same set back whether riding on flats or hills. Because you don't want to descend with all your weight on the front wheel.

Duh.

Well you don't necessarily want the same set back for climbing and descending - you're just stuck with it.

Mountain bikes often riding on more varied terrain than road bikes - steeper climbs and steeper descents. That's why many MTB saddles are designed to allow the rider to slide both forward and backward on the saddle - forward is better for climbing, backward is better for descending. There have even been special seatposts made which allow the rider to slide the saddle forward and back as necessary. (This, by the way, is why mountain bike fitting is often considered not as critical as road bike fitting - on a mountain bike, the terrain is so varied, that there is not a single ideal rider position.)

But, even on a road bike, a single saddle setback may not be perfect. My current road bike setback works very well for most of the riding I do (which includes road and criterium racing). But I find that when road slopes up to steeply (greater than 10%), the change in the effective seat tube angle, plus sitting up more to allow better use of my glutes, my center of gravity falls too far back to allow me apply my body weight well to the pedals. When the road is really steep, I find that I have to slide forward on the saddle to be able to push down on the pedals more effectively. When I do hillclimb races on steep mountains (such as Mt Washington or Mt Ascutney, both of which have average grades of 12%), I modify my bike by pushing the saddle forward by about an inch for a better climbing position.

[Mark McM]

Originally Posted by Kontact

But there is a reason vertical dropouts came into vogue. I'm not sure who makes good use of vertical dropouts - it usually seems like people have them in them adjusted full forward.

There are lots of reasons vertical dropouts became popular - allowing shorter chainstays is just a minor one. The main reasons include:

- Allows tighter control of the position of the wheel/cassette with respect to the derailleur (this became more important with the advent of indexed shifting).

- Prevents pulling the wheel out of the dropout under chain force. (This is more important for MTBs, whose small chainrings generate far higher chain forces than road bikes, and which is why MTBs adopted vertical dropouts first.)

- Better manufacturing tolerances no longer required that axle position be adjustable for proper wheel alignment.

Chainstay lengths haven't changed much before and after vertical dropouts were adopted. If vertical dropouts allow easy wheel swaps on short chainstay bikes (without the necessity of deflating the tire), it is just a fringe benefit.

[macaroon]

Originally Posted by Mark McM

But, even on a road bike, a single saddle setback may not be perfect. My current road bike setback works very well for most of the riding I do (which includes road and criterium racing). But I find that when road slopes up to steeply (greater than 10%), the change in the effective seat tube angle, plus sitting up more to allow better use of my glutes, my center of gravity falls too far back to allow me apply my body weight well to the pedals. When the road is really steep, I find that I have to slide forward on the saddle to be able to push down on the pedals more effectively. When I do hillclimb races on steep mountains (such as Mt Washington or Mt Ascutney, both of which have average grades of 12%), I modify my bike by pushing the saddle forward by about an inch for a better climbing position.

Dropping the nose of your saddle might even help somewhat.....or fitting a 26" front wheel

Alberto Contador is someone who often looks awkward climbing in the saddle; slid forward, right on the rivet, not relaxed at all. I've always thought he had his bars too long/low...... which I myself have found makes climbing out of the saddle more comfortable, and that's something he does alot of.

[Kontact]

Originally Posted by Mark McM

Well you don't necessarily want the same set back for climbing and descending - you're just stuck with it.

Mountain bikes often riding on more varied terrain than road bikes - steeper climbs and steeper descents. That's why many MTB saddles are designed to allow the rider to slide both forward and backward on the saddle - forward is better for climbing, backward is better for descending. There have even been special seatposts made which allow the rider to slide the saddle forward and back as necessary. (This, by the way, is why mountain bike fitting is often considered not as critical as road bike fitting - on a mountain bike, the terrain is so varied, that there is not a single ideal rider position.)

But, even on a road bike, a single saddle setback may not be perfect. My current road bike setback works very well for most of the riding I do (which includes road and criterium racing). But I find that when road slopes up to steeply (greater than 10%), the change in the effective seat tube angle, plus sitting up more to allow better use of my glutes, my center of gravity falls too far back to allow me apply my body weight well to the pedals. When the road is really steep, I find that I have to slide forward on the saddle to be able to push down on the pedals more effectively. When I do hillclimb races on steep mountains (such as Mt Washington or Mt Ascutney, both of which have average grades of 12%), I modify my bike by pushing the saddle forward by about an inch for a better climbing position.

Yup, and maybe someday we'll have variable geometry bikes with angles and lengths and rakes that suit the speeds and conditions.

[macaroon]

Originally Posted by Kontact

Yup, and maybe someday we'll have variable geometry bikes with angles and lengths and rakes that suit the speeds and conditions.

Been done; see Canyon Shapeshifter........not sure how successful it's been though.

[Mark McM]

Originally Posted by Kontact

The BB is in front of the seat, because the seat tube is angled back. So from the saddle your leg is angled forward toward the crank. The number of degrees it is angled forward is pretty much the same for short and tall people, so it makes no sense to change the effective seat tube angle with different set back posts for different leg lengths.

Using the same seat tube angle for both short and tall rides would work, except for one thing:

You might think that as the legs get longer, both the height of the saddle above the BB and the setback of the saddle from the BB would change by the same proportion, and thus the angle from BB to saddle would be preserved. But we don't set the saddle setback by it's position relative to the BB, we set the saddle setback by its position relative to the pedals.

Now, if crank length also changed in proportion to leg length, then yes, seat angle could be kept constant between tall and short riders. But that would mean if a 60" tall rider with 30" legs used 165mm cranks, than a 72" tall rider with 36" legs would use 198mm cranks. Do you know any 72" tall riders using 198mm cranks? I didn't think so.

A 72" tall rider is more apt to use 175mm cranks, or 180mm cranks at the outside. Therefore, to compensate for these proportionately shorter cranks, the taller rider will need to push their saddle back (by 18mm - 23mm in this case) to maintain their same setback in relation to their pedals. To be able to use the same seatpost setbacks, the taller rider will need a shallower seat tube angle.

[Kontact]

Originally Posted by Mark McM

Using the same seat tube angle for both short and tall rides would work, except for one thing:

You might think that as the legs get longer, both the height of the saddle above the BB and the setback of the saddle from the BB would change by the same proportion, and thus the angle from BB to saddle would be preserved. But we don't set the saddle setback by it's position relative to the BB, we set the saddle setback by its position relative to the pedals.

Now, if crank length also changed in proportion to leg length, then yes, seat angle could be kept constant between tall and short riders. But that would mean if a 60" tall rider with 30" legs used 165mm cranks, than a 72" tall rider with 36" legs would use 198mm cranks. Do you know any 72" tall riders using 198mm cranks? I didn't think so.

A 72" tall rider is more apt to use 175mm cranks, or 180mm cranks at the outside. Therefore, to compensate for these proportionately shorter cranks, the taller rider will need to push their saddle back (by 18mm - 23mm in this case) to maintain their same setback in relation to their pedals. To be able to use the same seatpost setbacks, the taller rider will need a shallower seat tube angle.

I don't follow. "The cranks" have a center point, and that centerpoint doesn't change with crank length.