Disc brake frame design

[jpw]

I'm a little curious about how different disc brake frame designs could influence the feel of the rear end of a bicycle. This Serotta 29er on ebay has a small strut tube between the stays fore of the dropout to provide additional support to the rear triangle under braking. Would it in any way change the vertical feel of the rear end of the bicycle when riding along , over bumps, et.c.? This is a titanium frame and I wonder if the dampening/ flexing characteristics of the metal are being 'locked out' by the strut?

http://www.ebay.com/itm/2010-Serotta-TiMax-29er-/150691008980?pt=Mountain_Bikes&hash=item2315e251d4#ht_894wt_698

[Kontact]

With a few exceptions, I don't think there are any "ride" or "damping" characteristics to rigid framed mountain bikes that are very important. What little movement the rear end is capable of is completely lost in the compression of the large, low pressure tires.

I think concerns about frame and fork dynamics are going to be a rather big deal when people start pushing road disc brake bikes, but that's because 100 psi plus tires contribute so much less to ride feel than a 2" tire.

[HenryA]

No problem -- you want that strut for best brake performance. The bike will still feel right.

[MapleWelder]

I've been wondering about this too, particularly on soft tail designs. I notice moots ybb's don't have this strut...

[jpw]

I used the Serotta 29er on ebay as an example because it's the only decent photo I've come across that shows how Serotta fabricates ti disc brake frames these days. I am thinking about non suspension dirt road/ cross frames with 700 wheels and how the ride might feel with that strut. I'm sold on th eidea of road disc braking and I await with anticipation the mooted Shimano development of high quality road disc equipment that will be compatible with Dura Ace/ Di2 levers.

Moots seems to do it differently, and I was a little surprised to see such a long and continuous bead weld attaching the Serotta disc tab to the stay tube. Is that necessary?

[HenryA]

Serotta actually made some frames with a very tall, somewhat fancy machined dropout that eliminated the welding and brace. It gave some chirping in the rear brake. I rode one a good bit before and after it had the diagonal brake brace added. Adding the brace made the chirp go away and did not change the ride in any way that I could tell.

There is a lot of force to overcome when the rear brake is applied, both perpendicular to the rear axle and twisting, and this brace deals with it effectively. I own one of the more recent frames as pictured with brace and its absolutely wonderful in every way.

[Chance]

I've been wondering about this too, particularly on soft tail designs. I notice moots ybb's don't have this strut...
Seemingly minor details can sometimes make a big difference. Soft tail should be different and require different design considerations.

In the case of the E-Bay bike mentioned in OP the straight seatstays won’t compress appreciably so the angle between left chainstay and seatstay won’t vary significantly. The brace should therefore not affect the frame’s ride quality enough to notice.

On the other hand if a frame was built with curved seatstays to improve ride the angle between the seatstays and chainstays would vary when bumps were hit. That’s why some frames with very curved seatstays use bearings in the dropout area – to allow articulation between seatstay and chainstay. In these cases placing a knee brace between the stays would change the ride significantly and would undermine use of a bearing.

In a soft tail bike design the angle between the seatstays and chainstays also changes significantly as the suspension compresses. A knee brace between the left seatstay and chainstay would not only change the ride but would also help bind the suspension mechanism (shock absorber unit).

[Dustin]

I know one of the reasons Matt Chester says he won't build a bike with disks is that he would have to change the feel of his bikes by beefing up the rear end. I think he otherwise uses especially thin and flexy stays.

[markie]

I know one of the reasons Matt Chester says he won't build a bike with disks is that he would have to change the feel of his bikes by beefing up the rear end. I think he otherwise uses especially thin and flexy stays.

But with all due respect, he is the only person who seems to hold that opinion, and he has issues.