Hi All,
I rode my TT-X21 for about 8 months and put around 2000 miles on it. It's a 56cm frameset and the final build was all Shimano Ultegra r8070 with a Quarg PM and DUB Crankset. I run a set of Scribe Aero Wide + 60D wheels tubeless with Conti 5000s. I ran a 52/36 with an 11/28 cassette initially, then switched the cassette out for an 11/30 to do more climbing over the summer.
The TT-X21 is fast. If Strava results hold any weight for you, the bike got me enough KOMs, top 5s, and top 10s, over mostly short sprints, sprint climbs, shorter time trial segments, and a couple of descents, to confirm for me that the set up I had around a 56cm TT-X21 frame was fast. Compared to a mainstream Aero specific frame that retails for 3 times as much? Yeah, the TanTan TT-X21 is still a fast bike. That's the good news, and if you're perfectly happy with Chinese made carbon frames, you can stop reading here. Happy riding! =)
The Propel with same set-up, though, is quantitatively faster...so far. I'm way off my summer fitness peak and I haven't dialed the bike in yet, but for short sprints at top speed, yeah, markedly faster, anywhere from 1 to 3 mph faster. All in all, speed is up and wattage to achieve that same speed is down.
The direct qualitative comparison? The TT-X21 is fast, but it's a stiff and harsh ride. It's stiff, but not strategically so. That translates to good power transfer, but you really take a beating in exchange for speed. The bike doesn't exactly lumber, it gets to top end but it takes a lot of effort to hold speed. At all out effort, on the other hand, the fork flexes and the rotors will grind. It's not a lot of flex, but the grinding is consistent when you're out of the saddle and putting the power down. It handles well enough though. Descending is rewarding. I'm not a very confident descender, but it's not because the TT-X-21 doesn't inspire. I have no complaints about the handling at speed.
The Propel accelerates really quickly in comparison. The speed comes easy, 0 flex anywhere that it matters, but it's not nearly as harsh a ride. Once it's going, it just keeps going and the handling is noticeably better. No drama, you can feel it lurch forward with every pedal stroke and you don't pay for it in ride quality. I'm not sure about the relationship between frame stiffness and power measurements at the spider, but my power readings are more consistent overall, not necessarily higher, just more consistent over the course of a ride. I wouldn't call these marginal gains. They smacked me in the ass the first time I really rode the Propel. That being said, I wouldn't call it a surprise for a frame that retails for 3 times as much.
In retrospect, would I still be happy riding the TT-X21? Absolutely! The bang for your buck is phenomenal compared to the Propel. The main reason I switched was because I felt like a 56 cm frame was a bit big for me, after all. I would have been fine trying out a 54cm TT-X21. On the other hand, I have no regrets after plopping down my hard earned cash for a mainstream frame either. As an older rider, I can appreciate the difference in ride quality without having to sacrifice the aero geometry.