pls see bottom posts -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Michael Wise Sent: Tuesday, December 30, 2008 1:54 PM To: [email protected] Subject: {BL} Re: Front-loading a Rivendell
On Tue, Dec 30, 2008 at 10:11, Doug Shaker <[email protected]> wrote: > > At 05:54 AM 12/30/2008, you wrote: >>I am interested in what design >>considerations go into making a frame more rear-weight than front- >>weight friendly. > > Wheel flop/trail are the main considerations that I know of. > > As I understand it, trail (the distance between the tire-ground > contact patch and where the headset turning axis would hit the > ground) effects handling in that the larger the trail, the more the > front wheel wants to go straight. High trail is, in that sense, > stabilizing. > > However, the more trail you have, the more wheel flop you have. > Wheel flop comes from the way that the wheel lowers itself slightly > when you turn it any direction from straight. Gravity pulls the bike > into wheel flops, so wheel flop tends to de-stabilize bikes. > > At high speed, the stabilizing effects of high trail dominate. At low > speeds and high front loads (more weight, more flop) the > de-stabilizing effects of wheel flop dominate. Since high trail and > wheel flop go together, a bike that tracks beautifully at high speed > may be very difficult to control at low speed. > > I have one bike with high trail that is perfectly fine 99% of the > time, and pleasant 95% of the time. But with a front load and going > up hill, it just wheel flops all over the place - I have to get off > and walk it rather than risk driving into traffic. > > At least this is my understanding. This understanding might be > completely wrong. If so, I would love to be corrected. > > > > - Doug "Anonymous" Shaker Doug's answer matches my experience. I have one of the original Kogswell P/R with the low trail fork (30mm trail). I've configured it with a large front rack, and have carried loads in excess of 50 pounds with very little loss of handling: it doesn't require me to manhandle the handlebars to keep the bike on track, even when I am just starting from a stop. This has never been my experience with high heavy front loads on any other sort of bike. I have a Heron with more typical high trail that would probably be hard to handle with that same sort of load. Instead, it has a pair of panniers on a low-rider rack, and seems to handle that just fine. I got a low-trail replacement fork for an older Kogswell G, and find that it too handles heavy front loads well. The increased rake on that fork also eliminated my toe-clip overlap. Rivendells are spec'd with high trail, because that's the way Grant likes it. I suspect that there are a number of reasons for his preference, but I would like to say first and foremost that he probably considers trail to be insignificant in comparison to other design considerations; in fact, I've heard him say as much. That said, some of what I think might be informing that is his preference for rear loading (the proportion of rear/front load does indeed affect the way the bike handles), his feeling that toe-clip overlap is not significant, the speeds he likes on downhills, and that he prefers stability and the ability of a bike to hold a line well. ======================================= I agree with Michael's summary re trail and stability as well as his interpretation of Grant's design MO. On the other hand, I don't believe that wheel flop matters a darn while riding the bike. It is simply the outcome of front end geometry, tire choice and loading. But more on that later. For now, here's a summary of my experience loading my 58 Rambouillet with stock and modified low trail forks: Stock geometry (high trail fork): Rear load - stable with light to moderate loading, any speed with panniers or Carradice Nelson Longflap bag Front load - unstable at low speeds, fine at moderate to high speed. Mark's rack, Ostrich Bag. Unacceptable toe clip overlap with SKS fenders compromises low speed maneuvering in tight areas. Low trail fork with integrated rack and decaleur: Front load - stable at all speeds. Handling is slower than stock, quite stable and secure and varies with tire choice / pressure. Rear load alone - (Carradice) stable at slow speeds; shimmy at moderate speed; afraid to try this at higher speeds Mixed front rear load - stable provided a substantial amount of weight remains up front Minimal TCO (increased with Berthoud fenders that have a higher profile than previous SKS fenders. Conclusion: There is a nonlinear relationship between trail and stability. This relationship is further complicated by varying loads and loading styles. What is clear to me is that lower trail forks allow more stable handling at low speeds with a front load. However, there are trade offs to low trail forks in my sample of 1, namely cost (new fork w/ custom rack), substandard handling with rear load alone and load carrying capability. This last issue is because I conceptualized a front load in terms of a Berthoud or Ostrich bag which limit ability to carry bulky loads. On the other hand, there are a host of racks out there designed to facilitate carrying bulky things. Finally, I found a significant personal commitment to (working with the builder to) modify an already fine road bike. Jon Shinefeld PhillyPA --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Bicycle Lifestyle" group. 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