Several bikes to comment about. 380 pound team.
1998 Cannondale MT 3000. Moto fork, 100mm with 9mm QR axle. Non disc brake version. Fork has oem valving, but has been modified as follows. Fork boots from lefties, Ohlins suspension fluid, coil spring is oem but the elastomer has had a section of Judy elastomer machined to fit the upper spring end.
Preload run full firm, fork work very well, minimal if any flex is noticed, turning to steering stop allows almost 90 degrees of motion per side.
Fork requires special tools to work on it easily. Normal stuff gets accomplished, rollers lubed with Phil Wood oil, cartridge fluid replenished as needed. Had one lower triple clamp crack at the bolt hole, no failure, blended away the crack and check it frequently. The clamp is a documented problem from long ago. The on the fly adjustable rebound is sometimes utilized for different terrain.
Our 2001 MT800, I installed a Manitou Sherman single crown with 20mm axle, travel is adjustable (110mm / 150mm), but we always ran it in 110mm setting, 150 was to slack on the headtube angle. Changes made, swapped spring to extra firm, replaced damper fluid to 10 wt Motorex from oem 5 wt Motorex. Fork worked well but was used only for less than fifty miles of use, some jeep road some technical single track. Not a bad fork, but I would personally like more ramp up or progression for better mid stroke / bottoming control.
ATC, with 20mm and Avid BB7, 100mm of travel. We have ridden one while installed on our MT3000, double red springs, as delivered it had no amount of spring preload, added 10mm (?) of preload, this held the front up much better. The fork was a bit under damped for us, replaced damper fluid to 20wt. Good turning radius. The lack of external adjustments was acceptable, but made on the trail tweaks tough. Once dialed in, this was an obvious replacement for the Moto when needed. This fork was tested on technical singletrack, hardpack, sand, and other varied terrain.
We are now re outfitting our "squadron", one of our latest rides is an ECDM, it too has the ATC 100mm fork with a 20mm axle, Louise (210mm?) brake. Like the other ATC we tested, steering precision is very good, turning radius also. I need open this thing up, but speculate it has a pair of red springs (had at least one when I checked the tire to triple clamp clearance with the fork cap removed). Preload currently installed is about 8mm. This fork could use a bit more damping, it will likely also see 20wt fluid as the other fork did.
Our Fandango 29'r is here and I'm working to get it built, it too will have the ATC fork with 20mm axle and an Avid BB7 203mm disc. We have not ridden at great length any other forks on a tandem.
I have installed a Fox40 onto the MT3000, planning to shorten the stroke to 100mm, unfortunately the triple clamp / lower leg offsets did not allow ample turning radius for where we ride.
I also have a Marzocchi drop off triple, modified from 170mm to 110mm of travel. I did install this onto the MT800 but other parts of the bike were not finished, the Manitou was fitted since the forks steerer tube was a more proper length. This fork has decent turning radius, is easy to work on, is spring with air over oil, open style damper with internally adjustable rebound. I believe this fork should work decent, provided the rigidity is ample for good steering precision, but again I have not run it.
The present focus will be on further dialing in the ATC's, since this is our current setups for the most part.
In my opinion, one of the greater challenges is finding proper springs for these forks at the loads we run them. Also, some of the forks, based on single bike test, do not have much if any progression to the spring via an air column. Obviously flex is a concern, also, but is always a trade off against weight, unless you run and air spring fork setup.For some teams this may be very important.
PK