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Bike frame geometry


Old 04-16-18, 04:24 AM
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Bike frame geometry

Been looking at some decent well known tandem manufacturers geometry diagrams and about tandems in general after not finding any decent bikes here, whether singles or tandems. I'm too big and my girlfriend is too small for 'em. LOL.

But the idea of building a frame fascinates me. One thing i've noticed with tandems is that both bottom brackets seem to be the same height off the ground : 11 to 11.5 inches ? Also the headset tube and seat angles are all pretty close on the touring tandem manufacturers, along with the bottom bracket drop measurements. The angles on these seem to be near what single touring bike frames have.

Also alot of the top touring frames have the two top tubes running in a straight line down the frame. Is that the strongest frame design?

Does all frame design and build start with the headset and forks, eg, angle and combined headset and fork length measurement and then work your way back, or on a tandem - could you start from the bottom brackets and seat tubes, and THEN adjust your front bottom bracket drop by different headset lengths??

I read that seat tube angles aren't really that important, but how bad would it be if you got a headset angle wrong by a few degrees?

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Old 04-16-18, 07:22 AM
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tandem steering geometry has the same issues as a single. If you get too much flop, then the bike likes to turn. I'm dealing with that right now on one of my bikes, an All City Macho Man Disc. It has 71.5 hta and 45mm rake which means 20mm flop. When I stand up, it really doesn't want to go straight. I'm building a fork with a lot more rake to counteract this and to work better with a handlebar bag. So, yes, start with the steering geometry and get that right. Then the top tube lengths, then everything else falls into place

You don't have to line up the top tubes, but you also don't want the bike to fold in half. The front seat tube needs to be beefed up at the junction. And this is why most tandems have diagonal structural elements.
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Old 04-16-18, 10:48 AM
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About tandem designing; On the bottom bracket height; It depends on what type of tandeming that you want to do. A road tandem should have a bottom bracket height- ground to to crank center (I see that you are tall, so I am assuming that you may use long- 175 cranks) of 10.75" higher means that with your preferred seat height it makes it higher off from the ground and a higher center of gravity (not desirable). A mountain bike tandem should have a higher bb height. The head angle is pretty critical being off A few degrees is not good. On the top tube; It is critical to have the two top tubes intersect on other sides of the captains seat tube. If you put them at separate heights you loose significant stiffness via the seat tube twisting. (definitely not recommended) I have seen other people do it. Insert frown.
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Old 04-16-18, 10:58 PM
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What singles don't have much of, and tandems do, is rear or stoker steer. With a living and significant (can be greater the captain) stoker weight any shifting about of uneven pull on the bars by the stoker will induce a steering torque. I've had tandems veer off to one side with no warning before when the stoker reached for their water as example.

Some brands want to reduce this best possible and so use a long trail geometry. Others want a more lively feel and use less trail. When I worked for Allstar Bike Shop (Raleigh, NC) we sold Santana, CoMotion, Burley, Calfee and had the odd da Vinci, Cannondale, Lightspeed and others go through our hands. We joked that the first tandem a team got was often a Santana (or Burley, budget dependent). A pre sold name and easy handling for a new team. But that team's second tandem a few seasons later was more often a CoMotion or Calfee with their quicker reaction to steering inputs. That more experienced (and better balanced/stable the riders were) the team the less they likes the dead/stable feel of the long trail geometry.

Think of the rear weight bias of a loaded touring bike and their commonly used long trail as a reference VS the short trail of a more front weighted Rando bike.

I just looked up the design blueprints for our CoMotion. It has a 73* HA, 680 diameter tires and about 51mm of trail. When I was working with CoMotion I never asked for any specific geometry but did request a quick handling feel. Our custom Speedster felt more like my singles (which all have a bit more then 60mm of trail).

The other aspect I learned was that it's hard to make a tandem frame too torsionally stiff and too long in the stoker's compartment or chain stays. Sure many will diss long rear ends, basing their opinion usually on a single bike's standard. But being able to cross chain and having that rear tire contact with the ground well behind the stoker's butt goes a long ways to make real riding more enjoyable.

I set up the captain's BB at it's lowest (assuming it's an eccentric for timing chain adjustment) so if any pedal strikes ground it's the captain's. With such a long wheel base any small rise (speed bump or driveway ramping) will get lost between the tires and pedal strike happens when not expecting it easily. If the team is good and their stopping method keeps the stoker clipped in then the rear BB can be fairly high and the taller stand over is far less an issue then the stoker's single has. My partner and I don't use this method (sometimes called "the proper method" by those who don't have a broad view of how relationships work...) so her stand over is important. This is best figured out before spending a lot of time or money.

And it is this last point that I will restate. You are a team, not a captain with a passenger. Whatever it takes to make the stoker happier will make the shared experience better. My first tandem was a Motobecane way back when. I bought it used from a pair of club members who never saw the same things when riding on it. he would wonder why they were so slow going down hills and she would complain at how fast they were going as she was pulling on the exposed brake cable to slow down. The old saying is that where ever the relationship is going it will get there faster on a tandem, for better or not... They were divorced a few years later. Andy
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