Utility Cycling - Xtracycle: How flexible are they?

I was wondering how stiff an Xtracycled 1990's MTB is compared to a purpose built frame like a Big Dummy. Currently I have an Alpine Monitor pass fitted with a front rack for doing heavy cargo work, but I'm less than thrilled with it's handling with a full front rack. My sense of balance keeps subconsciously telling me that the weight on the fork needs to be behind me, so I'm thinking there may be either a BD (if I can come up with $1,000 plus for "One more bike") or xtracycle the Alpine. The problem is I heard a rumor that Xtras tend to be rather "Flexy" I would like to hear from those who actually own one.;

It all depends where the load is carried. If you are loaded on one side (such as towing a bike-front wheel in the pocket) , you can definately feel the flex. The flex feel gets worse when you are out of the saddle. I took my 6 and 8 year olds to school today. Could barely feel anything as they know to sit still and in the middle. When they start goofing around or try to look around my body, it gets wobbly.

I have never ridden a Dummy with a load. Somebody tried cables from the seat stays to the h-racks with marginal improvement. I have been thinking about a trying a tie rod with p-clamps under the same idea.

the dummy is stiff
in fact any flex comes from the V-racks

if you are using a suspension fork
then you add a FreeRadical

the suspension will need to have the pre-load increased.

nominally speaking: typical bicycles have about 30% of the weight on the front wheel, while 70% is on the back.

on a long tail cargo bike, its more like 50/50

so when you add an XtraCycle to a MTB with a suspension fork, the pre-load will need to be increased.

see Surly Spew #19
http://www.surlybikes.com/spew19.html

specifically:
"One thing you need to consider before putting a suspension fork on your long bike is that your body weight is more centered between the front and rear wheels than on a standard-length bike. This effectively takes some weight off the rear wheel and puts it onto the front wheel. So if you weigh 180 lbs (82kg), a suspension fork on a standard bike registers approximately 56 lbs (25kg) of your total weight. Meanwhile, a suspension fork on a Big Dummy would register approximately 90 lbs (41kg), or half your total body weight. So you’ll need a suspension fork on which you can significantly increase the spring compression to compensate for the effective weight addition. Otherwise you’ll probably be bottoming out the fork and/or it’ll feel like you’re riding something very saggy."

i've been using my Dummy for over a year now.
there is no comparison to a convert job.

while a convert job will give you an idea of what can be done
it will also highlight little things that you may want remedied
or
maybe you won't even notice.

there are plenty of people out there on XtraCycles
I used a Cannondale for my convert job, and used it for about 2yrs.
immediately, i knew what i did not like.

however, it is a viable option.

the FreeRadical holds resale value
and the rest of the kit goes onto a Dummy, if that times comes around.
which is exactly what occurred with me.

http://forums.mtbr.com/showthread.php?t=391518

I have never noticed the so called 'flex' but I've never ridden a BD as comparison. I have an old Specialized Street Stomper for the X conversion so I don't know about suspension but I carry construction stuff on mine and it is fine.

I feel the flex - it is an inevitable consequence of an "add on" design. If you need more rigidity, go for a more integrated solution.

It really depends a lot on the weight and placement, which only makes sense.

I have had/used my X for almost a year now. I only noticed flex on one occasion, turns out that my freeloaders were not tightened like they should have been. Other than that I haven't noticed any issues with flex.
I love my X,my other two bikes haven't been ridden in almost a year.

I have only had my X a few weeks but have carried adult passengers and other heavy loads.. I have not noticed any flex at all but then I am not jumping curbs and stuff with it either :-)
The more I ride it the more I like it.. Now when I go to the grocery store I push a cart around instead of the small hand basket knowing I can carry a lot more