Zinn on Frame Geometry

Over at Velo News I ran across an excerpt from Lennard Zinn’s Cycling Primer that looks at frame geometry and how it relates to bike stability. It’s an interesting read if you’re interested in this sort of thing.

Frame Geometry and Bike Stability [PDF] →

7 Responses to “Zinn on Frame Geometry”

  • Elliott @ Violet Crown Cycles says:

    Zinn’s comments are appropriate for high speed, skinny tire road bikes with their 60-40 rear/front weight placement and shorter wheelbase. Throw what he says out the window if you are talking about putting weight on the front of the bike with a rack or TT setup where you are in the aerobars or you are riding low speed or if you have a longer than normal wheel base. Steep head angle and short trail are more stable in these instances. Take a look at the Spring 2005 (Vol 3 No 3) issue of Bicycle Quarterly for some real world tests of bikes with a wide variety of head tube angles and fork rake. Heine tests some old French rando bikes with very small trail that by Zinn’s standards should be unrideble but are in fact incredibly stable.

  • Alan says:

    Hi Elliott,

    I’ve read the articles in Bicycle Quarterly as well as followed the discussions on low trail designs on the Kogswell forum and other places. Interesting discussions, but it seems the assumption is often that the load will be carried on the front of the bike, something that I rarely see out in the real world of bike commuters. Still, by a very wide margin, mostly I see a rear rack with a pannier or two and possibly a small saddle bag. And if I see a front rack, it’s almost always a lowrider with a touring pannier as a supplement to rear panniers. Maybe in towns such as Portland where a higher percentage of commuters are riding boutique bikes, the front cargo rack is more widely used. I’m sure my Surly LHT with its Pass & Stow rack would benefit from a fork with more rake – it’s a handful with a heavy load up front.

    Alan

  • themador says:

    Elliott makes some good points here. Because of the steering correction that long trail gives you, a lot of weight at the front end is going to make that steering correction effect swing the steering about, creating serious wobble, especially at low speeds. Can you elaborate more on how wheelbase and trail affect one another?

    Regarding what you see day to day, I do agree with Alan in that most people (even here in Portland lol) don’t ride with much weight over the front. Those fancypants Oregon Manifest bikes aren’t really representative of what you see people riding – it’s mostly old mountain bikes, old road bikes, and fixies. People who ride $4,000 boutique bikes are too afraid of having their bikes stolen.

  • ToddBS says:

    Loading the front is particular to the French philosophy of riding and more pointedly at Randonneuring. When you are riding 200km+ with a time constraint, you don’t want to have to stop and fish through your saddle bag or panniers every time you need a PBJ sandwich or to put on your rain jacket. With all that stored in a handlebar bag you can get to it without stopping. And the low trail stability afforded by such a bike means you can take your hands off the handlebars to put that rain jacket on without stopping, too. Most bikes simply aren’t designed for that purpose, which is why a great many people who are die-hard Randos get custom bikes.

    It gets confusing when you start talking about trail relating to stability. Supposedly more = more, but a good test is riding no-hands with no load. I’ve found that my lowest trail bike (88 Centurion) is infinitely more stable than my highest trail bike (09 LHT) when riding no-hands. Riding no-hands on the LHT, I find myself getting skittish and grabbing the handlebar after just a few seconds. On the Centurion, I can ride pretty much non-stop that way, even going through corners no-handed.

    Trail is one of those things that no matter how many times it’s explained to me I never quite get full comprehension of it.

  • Alan says:

    “Loading the front is particular to the French philosophy of riding and more pointedly at Randonneuring.”

    And delivering newspapers… ;-)

  • ToddBS says:

    And delivering newspapers… ;-)

    Well, yes that is where the porteur style of bicycle came from :-)

    I can just see those guys dropping off their papers then stopping at a cafe for a quick cigarette and some cheese!

  • Elliott @ Violet Crown Cycles says:

    Alan,
    That geometry is also most stable for slow riding which is more prevalent with transportation cycling. As for the lack of front cargo, I’d say most transportation cyclists could benefit from having at least a front basket or handle bar bag. Handling would be improved with such geometry.

    Zinn is right if you aren’t putting a load on the front, but his assumption is that more trail is always better which is not true. He is writing to a performance/recreational audience so that’s understandable, but he probably shouldn’t make such a blanket statement. The BQ article also found pneumatic trail to have an effect (i.e. the size of your tires effects the wheel diameter thus the trail.) Zinn seems to dismiss this as well.

    themador,
    I’ve found longer wheel bases are less effected by the undesirable characteristics of low trail without front load. This is especially true of extremely long wheel bases like Xtracycles but is still true with longer chain stays in conventional designs. This means you get the best of both worlds in that you have a fairly stable ride whether you choose to put a load on the front or not. I may have misspoken a little as this is more a characteristic of the inherit stability of long wheel base than trail.

 
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