I've got a 58cm Madone 6.9SSL. Fits like a glove. Next year our team is switching to Tarmacs. I've crunched all the reach/stack and other numbers and think the 58cm should fit (it's my only option - the 56 and 60 are way off), but would love to hear from someone who has had both bikes.

The Tarmac's reach is a bit longer (1cm greater) and the stack a bit shorter (.7cm) than my Madone. The wheelbase is also 4cm longer, so the whole thing is basically longer and lower than the Madone. Doesn't sound like a lot but it might require a shorter stem (100mm instead of 110mm), also either a zero rise stem or more spacers. I'm concerned I'll be a bit too stretched out.

Would love to know if someone has had both bikes in 58cm and how much they had to compensate with spacers, stem length/rise.

Thanks, all!

You don't need to own either frame to answer that question. All you have to do is correct the reach value so that both frames are measured at the same stack height. The correction is 3mm for each 10mm of stack height difference. That means that the reach on the Tarmac would be about 8mm longer than Madone, once you subtract 2mm. The closest fit you could get would require a 10mm shorter stem.

How much spacer might be needed depends on the stack height of the two frame's headsets. You have to know that, since the listed stack heights do not include the headset top section.

Assuming the 3mm correction per 10mm of stack height (I didn't know that and it seems reliant on head tube angles, but I'll use it here), it would be about a 2mm adjustment. However, wouldn't it add 2mm to the approx. 10mm longer reach of the Tarmac to equal a 12mm longer reach than the Madone? How could a smaller stack height effectively reduce reach? I'm missing something. Thanks.

You're missing the fact that the Tarmac's reach was measured by the manufacturer, at a point 7mm lower than the Madone and the idea is to adjust the reach, as if it was measured at the SAME stack height. If you move up 7mm vertically, then the reach reach becomes smaller, by the cosine of the head tube angle.

The cosine of 73 degree is .292; 74 degrees .276 and 72 degrees .309. Using 3mm for each 10mm is usually plenty close enough.