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Forks(5 posts)

Sep 13, 2001 4:48 PM
OK guys I know this has been discussed in great detail lately, but I wanted to post a real world scenario. I'm going threadless, I picked up a brand new Bianchi carbon steerer, the carbon steerer is the subject of another post. Any way my bike has a certain geometry that produces a certain ride. Now the new fork has a 45mm off set vs the stock geometry 40mm. Now first to 5mm not even a 1/4 of an inch seems a very small change. Theoretically though, this will effectively lengthen my wheel base by the 5mm thus tending to slow the steering some and make the bike more stable at speed. I that a correct assumption?
backwards again...C-40
Sep 13, 2001 7:12 PM
Increasing the offset by 5mm will lengthen the wheelbase by 7mm (less that 1%), which won't make a noticeable difference. The trail will be reduced by the same 7mm, or about 12%. Reducing trail speeds up the steering and reduces stability at speed.

IMO, you should have purchased the same rake originally supplied with the frame.

For reference, the formula for trail is:

Trail = (tire radius/tanH)- (offset/sinH) where H is the head tube angle. As you can see, all offset reduces trail. The more offset the smaller the trail.
backwards again...mackgoo
Sep 14, 2001 6:38 AM
So, if you pull a wheel closer into the frame by reducing off set all other things being equal. The bike will steer slower and be more stable at speed?
re: Forksmackgoo
Sep 14, 2001 7:32 AM
Well, my dilema has been solved. I just talked with Bianchi and it turns out Bicycleguide had the specs wrong, at least the fork rake. The rake was actually 45, wich is good, as I really didn't want to mess with the steering characteristics of the bike. I really like the way the bike handles now, seems very stable.
I must say I'm sure the math and physics are correct, but it seems counter intuitive to me how all this works together.
a picture helps...C-40
Sep 14, 2001 9:39 AM
It helps to make a drawing. Another way to think of it is that increasing rake moves the tire's contact point forward and closer to the point of intersection of the steering axis with the road, thus making the trail smaller and the steering quicker. By the way, my math was off a small amount. The difference between a 40 and 45mm rake will lengthen the wheelbase by only about 5.2mm or .5%, and reduces the trail by about 9%.