Reading this thread with interest. Count me as a hater, if you will forgive me. I have had suspension forks on several bikes, and it just never seemed worth the compromises. Still, the subject interests me....
Originally Posted by
Darth Lefty
... One of the familiar digs on disc brakes as applied to road bikes is that they require a stiffer fork due to the torque incurred. (But flexible forks are compliant and springy, don't call it suspension or the C&V will evict you!). ...?
Oh, I think plenty of C&V people are well aware of the fork as a suspension device, especially the Bates "diadrant" forks of the 30's-40's. In pre- and post-war era GB the best fork blades were Accles & Pollock "kromo" chrome-moly tubing. I have a bike with that fork, and it is the lightest steel fork I've weighed. When riding, you can see it deflecting at bumps. Almost alarming! But it works, and at 70-80 years it's not showing signs of aging.
The problem with putting a disc brake on a springy fork is that the brake is on one side of the hub. So when the brake activates, it torques the left fork blade, but not the right. This effectively twists the whole fork, making steering erratic. In normal braking that's no big deal, but in an emergency, when you hit the brake hard and things are happening fast and you can't pay attention to everything all at once, that's not a good time for the bike to veer suddenly to the left. To counteract this the builder has to make the whole fork beefy, eliminating all of its springiness.
Rim brakes vs disc brakes is a separate but not quite separate subject. On older bikes the fork is straight down to a certain point, at which it bends forward. This makes the fork springy. Nowadays forks are straight, offset at an angle from the head tube. There is no vertical flex at all. That's fine for disc brakes, but it completely negates the benefits of rim brakes.
Oh, sorry, I shoulda mentioned, I'm not just a suspension fork hater, I'm also pretty much anti-disc brake. Or I was, until I got a bike with disc brakes. I've toned down my anti-disc rhetoric lately. My disc brake bike, with 48 mm tires, is just about equal in comfort to my bike with a light springy fork and 53 mm tires. With tires that fat, the springiness of the fork is buried in the mix.
No doubt there are places where real suspension is needed, but I never ride there. I ride the roughest roads imaginable, but they are always roads. Fat tires are an absolute necessity. But suspension? Sorry, I'm not there yet.