# Suspension fork for mtn tandem



## morganfletcher (Jul 22, 2005)

I picked up a 1995 Santana Rio tandem mountain bike a couple days ago, real cheap and kinda beat. I ride off-road with my 9-year-old, and he rides with me on our road tandem. (kid stoker adapter and short cranks in the back) I'm thinking I'll put a Thudbuster seatpost on the back of the Santana and have my 7-year-old ride it, with a kid stoker adapter, so that we can go for rides with the 9-year-old.

I'm already looking at getting disc mounts added to the rear of the frame, and a Chris King Devolution headset up front, so I can convert it from 1-1/4" to 1-1/8". I need (want) to figure out a suspension fork. I am big, 198lbs, and it's possible I could have an adult on the back of the tandem. I want something with a fork with a fair amount of travel, beefy, like the dual-crown look, needs to handle up to 410lbs or so total bike + riders.

Riding with the little one would be pretty tame, some easy local singletrack, fire roads. If I put the 9-year-old on the back we might get a little more hairball; rockier stuff, faster stuff. (I have in mind the Turkey Day Appetite Seminar loop on Pine Mountain near Fairfax, CA, with the Tamarancho trail system for dessert.)

I can get good prices on a couple fork makes: Marzocchi, Maverick, Rock Shox, White Brothers. And I don't mind buying used either. 

Got any recommendations?

Morgan


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## Octane (Mar 16, 2004)

Sweet!!! Nothing like flinging around a mtn tandem! It'll be a blast!

Here's my opinion on what fork to use:

You ARE going to stress the fork more than usual because it is a tandem, but you are NOT going to be doing crazy technical stuff. I would suggest using a fork with a 20mm thru-axle. This will give you some strength and piece of mind when bombing over the rough stuff. Also you'll be able to run an 8" rotor to slow you down. 

I would also suggest lookin into an air fork (not coil spring). This will allow you to tune the springrate based on terrain and weight of the riders. 

You can find some Marzocchi's on eBay that have an air preload (but coil main springs) which should work in your application. Look into the "Dirt Jumper" line of forks for something inexpensive. You could also try something from White Bros too.

Good luck and have fun!

-B


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## morganfletcher (Jul 22, 2005)

Thanks, Octane! I think I recognize you from the Norcal board. I looked at the Marzocchi Dirt Jumper, and Ebay has some new ones cheap. (2004) They're also listed here:

http://mtbtandems.com/Forks.html

I thought I might need a dual-crown fork, for strength I guess. Do you think single-crown is beefy enough for a mountain tandem?

Morgan


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## Octane (Mar 16, 2004)

Morgan

The additional length/travel of a Dual Crown fork will stress the headtube more than it is worth it. A single-crown fork with a thru-axle will work great. 

Wait till you ride your tandem. You'll realize that you won't be leaving the ground, and the suspension fork works really well for control and comfort.

Have fun!

-B


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## TandemNut (Mar 12, 2004)

Octane said:


> Morgan
> 
> The additional length/travel of a Dual Crown fork will stress the headtube more than it is worth it. A single-crown fork with a thru-axle will work great.
> 
> -B


That's not necessarily true. Modern mtb tandem frames are built with a double-crown fork in mind. Also, the additional weight and shear forces created by a tandem will stress most single crown forks more than they're designed for. The DJ and 66 are exceptions, as well as Manitou's Gold Lable series, but I'd stay away from the rest of them. If you look, you can find the Manitou's pretty cheap.


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## Hardtails Are Better (May 4, 2005)

Uhhh... it's a '95 frame. Not exactly modern. Though I have no experience with tandems, I have to imagine that a 20mm axle and air sprung are both good ideas. This leads to a Rock Shox Pike. Adjustable travel, if you want to pay for it, stiff, light, nice dampening, reliable.


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## Jayem (Jul 16, 2005)

morganfletcher said:


> I thought I might need a dual-crown fork, for strength I guess. Do you think single-crown is beefy enough for a mountain tandem?
> 
> Morgan


If marzocchi thinks a 66 single crown fork is strong enough for downhill and big dropoffs, it will probably handle tandem ok. 66SL might even be a great fork due to the fact that you can reduce the travel with negative air pressure.


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## grawbass (Aug 23, 2004)

In my opinion, the thing that stresses the front end of an off road tandem the most is not so much the extra weight, but the fact that you can NOT lift the front wheel at all, so the wheel just slams into everything.

Personally, I'd go with an older dual crown DH fork. Some of them have less travel than modern single crown long travel forks, but they're much more stout. As long as the travel isn't huge, it won't stress the headtube too much.


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## TandemNut (Mar 12, 2004)

Jayem said:


> 66SL might even be a great fork due to the fact that you can reduce the travel with negative air pressure.


Depends on the team weight. Most tandem teams are heavier than single riders (go figure), and using the negative spring to reduce travel to a manageable level also makes the spring rate so high the fork doesn't perform very well. I tried a 66SL on our Ellsworth Witness demo tandem. It worked okay on longer travel setting and we cleared some stuff we never could before, but the 16" front bb height really screwed with the handling, and made for some painful dabs. 
airing up the negative spring helped some, but made the fork too stiff to react to anything but big hits. 
Now a lighter tandem team might get better results (I weigh 260, as much as some lighter teams).
There is no 66SL for '07, though, so if that's the route you're going, get one quick. We're down to one left and no more available. We're talking to Marzocchi about making some with 150mm travel, but even with that setup, the fork's too long for most tandem frames. 
For the original post, I'd go with a DJ with 20mm thru axle and air assist, not air spring. 
A great fork for tandems would be 66 with 120mm travel.


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## Jayem (Jul 16, 2005)

BigNut said:


> There is no 66SL for '07, though, so if that's the route you're going, get one quick. .


Uh no, it's called the 66SL ATA for 2007, and you don't need a pump this time to adjust the travel.

BTW, you can run the 66SL at about 130mm of travel, 150 is no problem, and 170 is only the "max".


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## Hardtails Are Better (May 4, 2005)

It would seem to me that you would need to lower the positive air pressure if you tried to reduce the travel to soften the fork up to the point that it would move again. I'm just guessing, since I don't own one, but that makes sense to me.


And yes, Jayem's right, there is an '07 66 SL.


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## TandemNut (Mar 12, 2004)

Jayem said:


> Uh no, it's called the 66SL ATA for 2007, and you don't need a pump this time to adjust the travel.


Correct; I was thinking about a conversation with Marzocchi about '07 stuff, but that was referring to OEM only. I stand corrected.



Jayem said:


> BTW, you can run the 66SL at about 130mm of travel, 150 is no problem, and 170 is only the "max".


 Uh yeah, you can run the fork at 10mm of travel if you want. What I said was that with the weight of a tandem team, the pressure needed in the negative spring and positive spring setup for that weight and that travel makes the fork very stiff. I was relating actual experience with setting one up that way.


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## TandemNut (Mar 12, 2004)

Hardtails Are Better said:


> It would seem to me that you would need to lower the positive air pressure if you tried to reduce the travel to soften the fork up to the point that it would move again. I'm just guessing, since I don't own one, but that makes sense to me.
> 
> And yes, Jayem's right, there is an '07 66 SL.


When you increase the air pressure in the negative spring, that has the effect of increasing the air pressure in the positive spring. So you lower the air pressure in the positive spring, and that lowers the travel. So far, so good. 
However, as you increase the negative pressure, in effect you're increasing the load on the positive spring, same as adding a heavier rider. So you have to increase the air pressure on the positive side to compensate. If you set it up for the weight of a light tandem team, it will work, but a 300+ lb team won't be able to get much performance out of it. 
There are limits to the negative air spring pressure, and in attempting to set up the fork we were at or exceeding that limit (the unofficial higher limit that we discussed with Marzocchi tech during this process). We found that the fork, when set up with sufficient pressure in the positive side to support our team weight, overloaded the negative spring and thus raised the travel/ride height. increasing the negative pressure pushed it far past it's max rating. And yes, we tried approaching the setup with both sides empty. 
Problem is that once you adjust the negative or positive pressure, it has a direct effect on the opposite pressure.
If the '07 is a wind-down type travel reducer, that solves the problem. We'll have to get one in and try it out.


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