Help with understanding Soma San Marcos geometry

[handsomerob]

ATMO... The "54cm" frame is the smallest 700c size, but by the geometry chart below it appears to fit like 57cm (57.5 ctc TT). Is there something in the angles that would make it fit smaller? If so, by how much?

http://www.somafab.com/wp-content/uploads/2011/07/soma_geochart_sanmarcos.jpg

On a side note, anyone here have one?

[oldpotatoe]

ATMO... The "54cm" frame is the smallest 700c size, but by the geometry chart below it appears to fit like 57cm (57.5 ctc TT). Is there something in the angles that would make it fit smaller? If so, by how much?

http://www.somafab.com/wp-content/uploads/2011/07/soma_geochart_sanmarcos.jpg

On a side note, anyone here have one?

Sloping top tube and it fits like a frame with a 57.5 top tube.

Pretty shallow seat tube angle so if you are not particularly long femur-ed, you will need a no set back seatpost.

[handsomerob]

Sloping top tube and it fits like a frame with a 57.5 top tube.

Pretty shallow seat tube angle so if you are not particularly long femur-ed, you will need a no set back seatpost.

Thanks OP... I figured so, and with that slack seat tube... It seemed to "stretch" the effective reach even more. I just needed a 2nd opinion because it's odd that the frame would be designed for only the 6ft+ crowd to get 700c's.

[MerckxMad]

I just built up a San Marcos. I typically ride a non-Riv 55cm and a Riv 57cm. I bought a 54cm San Marcos using Riv's guidelines based on my PBH (85cm). The slack angles of the "expanded" frame make for a very long reach for this short reach rider. I'm using a 8cm stem and zero offset post and my bars are level with the saddle. I really like the large HT and the high stack steerer to get the bars up high, but I will only go so high before I start to gag on the look and feel as if I'm riding a beach cruiser.

I'm okay with the feel and the look of my set up, but it was a bit of a challenge. The high bars are most important for me as I'm recovering from a back injury. Odd ball sizing and double top tubes on larger models aside, it is a great value frameset. The Riv lugs are beautiful, the fork blades are okay, and the fit and finish, while not Riv quality, are still top notch. With 32mm tires, fenders and rack-ability, it's a great winter ride.

[Mark McM]

Thanks OP... I figured so, and with that slack seat tube... It seemed to "stretch" the effective reach even more. I just needed a 2nd opinion because it's odd that the frame would be designed for only the 6ft+ crowd to get 700c's.

I think you have that backwards - the slack seat angle will require the saddle to be mounted more forward on its rails, resulting in a shorter effective reach.

This frame has a 71.8 degree seat angle, whereas a frame of this size more typically has 73 degree seat angle. The effective reach shortens by about 1cm for each degree smaller seat tube angle, so this frame with a 71.8 deg. SA and 57.5 cm TT would fit like a typical frame with a 73 deg. SA and 56.3cm TT.

It's frames like this that make specifying frame size by Reach & Stack a much clearer method.

[handsomerob]

I think you have that backwards - the slack seat angle will require the saddle to be mounted more forward on its rails, resulting in a shorter effective reach.

This frame has a 71.8 degree seat angle, whereas a frame of this size more typically has 73 degree seat angle. The effective reach shortens by about 1cm for each degree smaller seat tube angle, so this frame with a 71.8 deg. SA and 57.5 cm TT would fit like a typical frame with a 73 deg. SA and 56.3cm TT.

It's frames like this that make specifying frame size by Reach & Stack a much clearer method.

In my head, if a seat tube is "slack" (71-72ish) it is slanted more rearward which would mean the saddle would be effectively further away from the bars and your arse further behind the BB.

You said that you would just move the saddle forward for a "shorter" reach but that more proves my understanding than conflicts it. (That unintentionally sounds argumentative). I just don't understand your math. It seems like that math would apply to the head tube not the seat tube.

[handsomerob]

I just built up a San Marcos. I typically ride a non-Riv 55cm and a Riv 57cm. I bought a 54cm San Marcos using Riv's guidelines based on my PBH (85cm). The slack angles of the "expanded" frame make for a very long reach for this short reach rider. I'm using a 8cm stem and zero offset post and my bars are level with the saddle. I really like the large HT and the high stack steerer to get the bars up high, but I will only go so high before I start to gag on the look and feel as if I'm riding a beach cruiser.

I'm okay with the feel and the look of my set up, but it was a bit of a challenge. The high bars are most important for me as I'm recovering from a back injury. Odd ball sizing and double top tubes on larger models aside, it is a great value frameset. The Riv lugs are beautiful, the fork blades are okay, and the fit and finish, while not Riv quality, are still top notch. With 32mm tires, fenders and rack-ability, it's a great winter ride.

This sounds exactly like what I was thinking I would have to do to force the 54cm to fit. I don't want to use a 0 setback post and super short stem. It certainly is pretty and well priced, but I would have to go with a 650b and I am not ready to make that leap (no matter what rcnute has to say :) )

[Puget Pounder]

In my head, if a seat tube is "slack" (71-72ish) it is slanted more rearward which would mean the saddle would be effectively further away from the bars and your arse further behind the BB.

You said that you would just move the saddle forward for a "shorter" reach but that more proves my understanding than conflicts it. (That unintentionally sounds argumentative). I just don't understand your math. It seems like that math would apply to the head tube not the seat tube.

You are right if you don't account for BB setback.

If you have 2 bikes that have the same TT length, but one has a steeper STA, you have to move your saddle back to compensate for the change in position over the BB. This effectively brings you away from your bars. I always have to pay attention to TT and STA together if I want to achieve my perfect fit. If you normally ride a bike with a 73 STA and you hop on one with 71 STA, you will be pushing that saddle forward to get your same positioning.

[Mark McM]

In my head, if a seat tube is "slack" (71-72ish) it is slanted more rearward which would mean the saddle would be effectively further away from the bars and your arse further behind the BB.

You said that you would just move the saddle forward for a "shorter" reach but that more proves my understanding than conflicts it. (That unintentionally sounds argumentative). I just don't understand your math. It seems like that math would apply to the head tube not the seat tube.

The first axiom of bike fitting is to fit the bike to the rider, not fit the rider to the bike.

The rider's position relative to the BB (bottom bracket, the center of the crank and pedal rotation) should be same, regardless of the STA (seat tube angle). So if the STA is slacker than normal, the rider must adjust their saddle more forward on the rails to compensate, and restore their position with respect to the BB. This is not actually moving the saddle forward however - it is just keeping the saddle in the same place with respect to the BB.

The traditional method to measure the TT (top tube) is from the ST (seat tube) to the HT (head tube). If the STA is slackened, the ST moves backward, pulling the HT with it. But since the saddle is adjusted relative to the fixed point of the BB, slackening the STA and adjusting the saddle to compenstate doesn't push the seat forward toward the HT, it pulls the HT backward toward the saddle - the saddle is still at a fixed point with respect to the BB. Keep in mind that pulling the HT back by slackening the STA doesn't just shorten the reach, it also shortens the front center and the wheelbase.

So, in the Soma geometry, they slackened the STA, but then lengthened to the TT to compensate, thus keeping the reach the same.

[Grant McLean]

The traditional method to measure the TT (top tube) is from the ST (seat tube) to the HT (head tube). If the STA is slackened, the ST moves backward, pulling the HT with it. But since the saddle is adjusted relative to the fixed point of the BB, slackening the STA and adjusting the saddle to compenstate doesn't push the seat forward toward the HT, it pulls the HT backward toward the saddle - the saddle is still at a fixed point with respect to the BB.


Exactly right.

It really would be more helpful if companies stopped printing the top tube
length in their geometry charts, and started listing it as two separate numbers,
setback and reach. The relevant numbers are created by a plumb
line through the center of the bb, so you can measure one without impacting
the other. 14+40=54 and 12+42=54 Top tube alone doesn't tell you much
on it's own, and people seem fixated that one number.

Add to this that many people think seat tube angle changes the handing of the
bike on it's own, and doesn't just postion the saddle relative to the bb,
and many geo charts are just a recipe for confusion.

-g

[MerckxMad]

This sounds exactly like what I was thinking I would have to do to force the 54cm to fit. I don't want to use a 0 setback post and super short stem. It certainly is pretty and well priced, but I would have to go with a 650b and I am not ready to make that leap (no matter what rcnute has to say :) )

Leap away, my friend. The 650B San Marcos is the schizzle. I have a Bleriot and a Kogswell both sporting 650B Pari Motos, and I love the ride, expecially over rough pavement. I think the San Marcos would build up into a terrific rando, or all rounder.

[rcnute]

Leap away, my friend. The 650B San Marcos is the schizzle. I have a Bleriot and a Kogswell both sporting 650B Pari Motos, and I love the ride, expecially over rough pavement. I think the San Marcos would build up into a terrific rando, or all rounder.

rcnute commands that the leap be made. :fight:

But if you're gonna stock with 700c...I tried out a 54 San Marcos and it seemed to fit me well (5'9" or so, 84 PBH). Rode real nice.

Ryan