guy153

The way trail works is a little bit like a gear ratio. Longer trail is like putting a higher gear on the steering-- you need to turn it less to get the same result but it takes a bit more force. The wider the handlebar however, the more you're gearing it back down, which is how MTBs end up feeling fairly normal again. They have really wide handlebars and lots of trail. But it's not exactly the same because you don't only steer with the handlebar.

You're right that the stability always increases with speed. If a bike tips a bit to one side, the wheel flop causes it to steer to that side, which rights it-- if you fall left, you steer left, which throws your weight right (and therefore back up) because of centrifugal force. The centrifugal force depends on speed. If you try to ride a bike in a straight line at low speed it always takes bigger steering corrections to keep upright than it does at higher speeds.

But sometimes you can have too much stability. If you put a really long trail on a bike then at high speeds it can feel like it's hard to make a turn if you want to like you're a passenger on a runaway train. So the amount of trail you want is sort of tuned to the kinds of speeds you expect to be doing. MTBs do actually go pretty fast downhill but I guess it's all so gnarly and bumpy and anyway you don't get that train feeling. I think shorter trail actually gives you more agility at lower speeds, but on MTBs it's confused by the wide handlebar.

Long chainstays and big tyres also give you more stability but at the expense of agility. A really long-low slack 29er is basically a monster truck that will just sail over everything. But is it as much fun? The other hugely confounding factor in all this is the human brain. If you ride a particular bike for a bit it soon starts to feel really intuitive and "just right". Get on another bike and it's weird and you don't like it. But ride it for a bit and you start to like that one more and you don't want to go back to the first one. I think this is why MTBs got gradually longer and slacker over the years. If you'd come out with a 2020 MTB in 1986 everyone would have hated it. The jump from what they were used to would have been too big. At the actual cutting edge of racing you want to make the bike that an elite rider is actually fastest on, and she will get used to and get the most out of any design you put her on very rapidly. So it is objective because you can measure it with a stopwatch. For the rest of us, well we end up with some bike or other and for some reason we either love it or we don't, but it's based on all kinds of subjective factors. People who ride a lot of bikes are maybe more tuned into being objective about the differences. For me it's just some feel weird at first, others don't, and they all feel great after a bit.

I do think the triple triangle "tech" is a gimmick. The top of the ST is reinforced because it has to hold the seat so it makes sense to attach the SS there. On a butted steel frame the TT becomes thin, like 0.5mm, about 80mm away from the ST, but the top of the ST is 1.2mm. A much better place to be attaching the stays. A lot of GT frames are aluminium and perhaps they're not butted so it doesn't make as much difference. But also why do you want the rear triangle so stiff? The seatpost is going to flex anyway, and perhaps a more "compliant" rear triangle gives you a smoother ride. But the look of those GT frames is certainly distinctive and I like diversity on an aesthetic level. There's no reason why we all need to ride the most technically optimized frames possible.