Bicycle Mechanics - fixing the frame flex

ok,

so i've got an xtracycle hooked up to a Wheeler 5900 mtb (i got it second hand, i'd say it is mid 90s mid range level)
now most of the time it's totally fine but the other day, hauling a big ol' monitor and pc box home from the tip shop i noticed more flex then normal. fine, i had more weight then normal, plus it was pretty badly balanced.
so i had a better look at the whole setup, and althopugh im sure the xtracycle itself is giving me a bit of flex, a substantial part of it seemes to be coming from the rear triangle of the donor bike. at least, it is when i grab the bike and twist it!
the wheeler is by no means overbuilt compared to a lot of mtbs out there - i see my issue mostly related to my donor frame, not the xtracycle although obviously it puts a lot of un-designed for forces through the frame)

so my real question is this: how could i got about stiffening up this rear triangle, bearing in mind i dont haveto worry about the wheel in there.
my two main thoughts have been: a 'high' tension cable with p-clamps or brazeons in a zig-zag kind of pattern along a section of the seat stays. i wouldnt know how many zigzags id need or how far along the seatstays i should extend my addition
OR
a solid plate of some kind (eg some aluminum sheet) somehow similarly attached firmly between the stays. again, i wouldnt know how big it would eed to be, nor how thick. perhaps i could drill out lightening holes, but then again, maybe if my holes were in the wrong spot for the tension produced it could be bad.?
actually, having written that I guess the middle option would be tension straps (like in the building industry) but i think they'd be a bit hard to get proper tension on in this case.

so, any ideas? i guess im searching for a strength-weight ratio here, in a method that will be effective!
(2 hours searching hasnt helped yet!)

cheers
Lance

Without a clear picture of what your rig really looks like I can't offer any specifice but if you want to try stiffening the rear with cables, you can tension them with turnbuckles. Any hardware store has them in a variety of sizes.

its a standard frame with an xtracycle attached. i did forget to say its steel, but thats kind of implied in the flex isnt it!
do you mean, you want to see the size of the seat stays? they're defineitly on the skinny side of mtb (like i said, mid level, mid 90s), i cant take a snap now, its too wet and dark out there!


cheers
Lance

oh, also, i realise that if im going the cable route it will be marine type cable and turnbuckles, not one of my old brake cables from the parts bin :) i did initally think of using old brake cables, but if the frame flexes, i dont reckon theyll hold tension for long!

lance

A brake booster, which is a horeshoe-shaped arch attached (along with the brake) to the brake bosses, would probably help some. It's designed to greatly reduce flexing of the seatstays during hard braking and in most cases where flex is a problem it results in noticeably better braking performance. Braking performance may not be one of your concerns, but a brake booster will certainly help reduce seatstay flex-

ah yes, i had considered tahat too! i know ive got one lying around. ill give it a go asap before i go any further.

cheers
lance

Sheets of metal (need not be thick) attached to seat/chain stays with pop rivets. This will be a lot more effective than a cable which would have a lot of stretch in the attachment at the ends. Epock glue between these sheets and the stays would also help.