Friday, March 23, 2007


Just for the record - HyMDT Cruz stats

More Specs

The trail measurement is 125mm. and the seat pan has been move on the mounting bracket one set of holes and it comes close to John Tolhurst's suggestion, "The seat pan should point to a spot about a foot above the crank."

Post # 1569 to the Yahoo Group
Establishing the seat-handlebar config correctly

I have the FWD operating nicely although I have not connected the dérailleur so I can shift yet, just clamped into a single gear.

Preliminary Test Ride
The ride skills are improving rapidly spinning in low gears. I have adjusted the handlebars to meet knee clearance [It feels like the stem choice should raise the bars about 40-50mm and move them forward about 25). I will say as it is feels pretty comfortable when I relax and sit back like I'm in my recliner.

Seat position
I am a bit confused about the suggestion for seat placement. It says, "Position the lower part of the the arc to be 8" in front of the seat post tube."

Well here is what I did. I matched up one set of holes on the seat support bracket with the water bottle mounting taps in front of the Downtube Folding bracket--the folding bracket is still operable with this placement. I chose the bracket holes so that it fit as far back as possible. Then I mounted the seat pan so that the back of the pan is 18 3/4 inches forward of the rear axle vertical bisector and 27.5 inches from the ground. The wheel base is 43 1/2 inches, the crank axle is 10 in forward of the front axle vertical bisector and 28 inches from the ground.

Weight distribution
I weighed (bathroom scale) the front and back with rider in pedaling position to get a figure for weight distribution, rear = 128# front=102#. I knew the short rear chain stays for the 20" would have the effect of needing to move the seat forward to have the same moment, but I just guessed where to place it and took advantage of the water bottle tap holes.

The general information speaks to a nearly 50/50 weight distribution and what I have puts the center of weight about over the orginal BB. The long trail induced by the change of head angle from 72 to 65 from the 3" wheel radius increase has some effect on quickness of response but I haven't ridden enough to know if it is a substantive issue.

I do like the way a 26" wheel rolls through gutters and feels on roughed surfaces. I still have the original rear wheel in place but I don't expect the change to the front wheel will produce any discernable change.

Seat Back Mount

The seat back is at about 38 degrees as it sits (sic) and I intend to bend some rod into a seat mount that adapts to a horizontal precision seat post. It will have some milder suspension effect and fore/aft adjustment as well as seat back angle from the vertical seat post adjustment, but
that will not be undertaken until the seat placement is optimized. I could easily cut a piece of wood and drill holes to make fore/aft adjustment seasier (the old backwards seat is as far back as it will go :D) if you people think it is a critical issue it getting things optimized for handling and maneuverability .

So what do you think? I can make some additional measurements and pics too if that is needed.

Thanks for your help.

Post #1570 in response:

Re: Establishing the seat-handlebar config correctly

I think you've got everything in the range of values where the
design becomes pretty insensitive to small changes. The thing that
will most influence the operation of the bike at this stage is
riding experience.

What I didn't do, and should have, was to get the bike in about the
stage of completion that you have, get the ancillaries hooked up and
fettled (i.e., derailleurs and seat post), and then put a minimum of
about 100 miles on it. This will give you enough time to decide what
is most comfortable for the long-term, and what parts of the riding
dynamic you want to tweek, and why.

Tom Traylor uses fairly large trail dimensions on his bikes with
good results. The design is pretty insensitive to large trail
because the influence of the weight of your legs, combined with the
dampinig effect provided by your leg muscles, is a much larger
signal than the traditional inputs that result in "wheel flop" on a
conventional recumbent. Small trail can make the bike nervous at
high speed, just like a conventional bike, though you need to be
almost at zero or negative to encounter it.

All in all I think you're in good shape. It's an interesting and
versatile application of the kit.

Have fun,

"Doug Burton" <hardtailcruzer@...>

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