Tarmac SL8 vs SL6 geo head scratcher

[kgreene10]

I need some help figuring out something that seems inexplicable to me. I have been riding a Tarmac SL6 for years. The fit is dialed. I am now building a Tarmac SL8 and it seems impossible to replicate the saddle and bar positions.

The two frames have identical stack and reach. The stock seat posts have the same setback. I am using the same saddle. The bars differ by just 2.5mm of reach.

However, to achieve the same very moderate saddle setback from the BB on the SL8, I would have to push the saddle further back than the rails will permit. What??

If that’s not weird enough, once the saddle height and angle replicates the SL6 perfectly, the saddle-to-bar drop on the SL8 is still 25mm further. How in the world??

I have stared at this thing for hours. How could any of this be? What obvious stupidity have I committed?

Seriously, any ideas?

[EB]

Depending on which size frames you’re working with, the SL8 has a steeper seat tube angle (by 0.5 degrees) in some sizes than the SL6.

Additionally, based on the imagery I am looking at, the SL6 uses a headset with stack - can’t tell how much - vs the fully zero stack headset on the SL8.

[Xrslug]

On the bar drop discrepancy, a couple of ideas:

Stem — Are the stems identical? Same distance from base of steering tube clamp to centerline of stem extension? Stem angle and extension exactly the same (as measured, not per what’s stamped on the stem, which can be inaccurate)?

Stack — you say stack is identical, but I’m assuming that’s per Specialized and not as measured. Assuming no headset spacers or equal number, is there any difference in the height of the headset components (e.g. top cap covering the drop-in bearing? Or different type of headset?).

One thing about stack measurements that isn’t obvious when comparing frame specs in the abstract is that there can be a fairly large difference caused by headset types (e.g. integrated versus external cups can be a couple of centimeters difference in actual stack vs frame spec stack).

If none of the above, any measured difference in fork length, from axle to bottom of head tube?

[deluz]

I am also in the process of building an SL8.
I looked at the geometry charts and the STA, stack and reach are pretty much the same.
The one thing that is different is SL6 has a longer seat tube and taller stand over. Not sure if that has anything to do with what you are experiencing.

[kppolich]

There's a tool for that
https://geometrygeeks.bike/compare/s...c-sl8-2024-54/

[christian]

Originally Posted by kgreene10

I need some help figuring out something that seems inexplicable to me.

I'm not sure what kind of skin rash that is... pictures would help!

No but seriously, that does seem curious. My expectation is that you'd need a couple more headset spacers on an SL8 to match SL6 position, but that the geometry is otherwise identical. Is something going on with the seat posts or clamps? Which sizes are the bikes?

[John H.]

Could be a couple things- Other have mentioned this.
1.) Did you achieve the exact same saddle setback? If not, a relatively further forward saddle (at the same saddle height) is more vertical relative to the bars.
2.) Top cap spacers and stem. What top cap and spacer setup does each bike have? I have seen SL6 with both 8mm top cap and 15mm top cap.
And if you moved to the new one piece bar/stem the stem itself has pretty low stack. Also might be more negative angled than -6 it is advertised to be.

I have an SL8 on the way. I am prepared to go through all of the same calculations. Though I was lucky enough to check out a friends bike in the same size. I also took measurements of his to make sure it would adjust as I need it to.