# Dropped Boomtube/skidplate?



## Dr. Paul Proteus (Sep 26, 2004)

I've seen a few nice examples of boomtube skidplates on here, but has anyone tried making one that is dropped or lowered such that the skidplate is more or less aligned with the timing chain? Any custom frames out there that purposely lowered the boomtube so that it's a full-length skidplate?

I'm thinking of something along the lines of an old-school trials bashguard, but running all the way back to the stoker, sorta like this, but much longer


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## PMK (Oct 12, 2004)

I have spent a bunch of time considering various methods to accomplish this. Ultimately, to do it correctly the tandems design needs to be modified for best results.

In the simplest forms, such as what you have shown on that Ibis Trials bike, it could be fabricated and made to work as you describe, but truly needs the attachment points as part of the frame itself, not clamped on.

We have bent, actually folded a couple of front chainrings over the years. We have abused our timing chain across logs and rocks, drug it through dirt as we crested stuff.

Later I will post a photo I received yesterday from my good friends Tommy and Martha. The destroyed not only a chainring yesterday but snapped the chainring mount off of the crankarm.

When I penciled the concept idea for doing this, for toughness and cool white sparks I kept wanting to make it from titanium. I had other concepts, each could accomplish the task, but in the end, I decided to save the weight, and if needed avoid or walk the feature since most times careful riding will get you over the feature.

Also, a lot of this type of damage can happen from a poorly setup fork that compresses too much during the impact or as the bike clears the feature.

In the end, I fabricated a carbon fibre glide plate to save the bottom tube from dents and wear.


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## PMK (Oct 12, 2004)

This shows the folded chainring and snapped crank lug to mount the chainring.

The other photo is where it happened, this is the entrance qualifier to Gun Range section of Markham Park.

The video gives an idea of how it looks when you ride it.

https://video-mia1-1.xx.fbcdn.net/v...=fae7ce7837545bdb833658a3a426c3fb&oe=57321712


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## Fleas (Jan 19, 2006)

I've been noodling on it for awhile. It's basically one saddle piece for each bottom bracket, with maybe 2 saddles on the boom tube (or even a foam saddle running the full length) to support a spine which would have a slider attached to it. It's all in my head, though.








I would say both bottom bracket saddles would need more "meat" on them than what is shown, and maybe a larger leading surface more like a ski.
The spine could be wood or aluminum channel for the purpose of trying it out.
I am pretty sure there is a common conveyor side rail that has aluminum brackets and polyethylene or Delrin/acetal sliders.
This is for an old Cannondale MT1000. I was going to make a template out of plywood or even cardboard based on this drawing to see how it matched the chainrings, then actually build something, but it has been on the back burner for a long time. The idea was just to protect the boom tube and the chainrings.
It's not a big stretch to think it could be made from PVC pipe or something.

-F


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## Procter (Feb 3, 2012)

Not advisable with butted tubing. The tube isn't intended to take stress there.


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## Fleas (Jan 19, 2006)

Procter said:


> Not advisable with butted tubing. The tube isn't intended to take stress there.


Agree. I was leaning toward a full-length support, but then you start getting into more weight. Someone on here made a slider out of carbon fiber sheet. If it had rigid foam support it may be fairly light and at least be able to take a few hits before replacing the foam support. We just haven't taken so many hits that the need justified the effort.

-F


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## Dr. Paul Proteus (Sep 26, 2004)

Boom tube is going to take a beating either way. Something that aims to distribute the load across the entire tube and/or shift the impact to the BB shell could only be an improvement. I'm still in disbelief that our boom tube looks the way it does, but our timing chain somehow (mostly) survives.

That's some impressive work on the chainring. One really needs to be committed to getting over the obstacle to manage to do that... I'm guessing they made it over?


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## PMK (Oct 12, 2004)

They did not crash.

I spoke with my friend today, I posted the incorrect feature that stopped them. The feature I posted they did ride and get over clean without a dab. The second feature which is much smaller, is what did the damage. Basically an log pile that is shaped just wrong for a tandem.

I am sure next time he rides those features he will stoke the stoker and use the feature as a jump...


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## Dr. Paul Proteus (Sep 26, 2004)

They crashed... they just didn't fall over when the did so.


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## Procter (Feb 3, 2012)

Dr. Paul Proteus said:


> Boom tube is going to take a beating either way. Something that aims to distribute the load across the entire tube and/or shift the impact to the BB shell could only be an improvement.


Agreed, you want to dissipate across a large portion of the tube, unlike the pic with the trials bike where the stress will be concentrated at the collar.


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## Okayfine (Sep 7, 2010)

IF there's a weak point in the ECdM frame, or at least a point under stress already, it's the stoker's BB area. We've cracked a frame at the stoker's BB shell; luckily for us I caught it before the frame failed. We were behind a couple who didn't catch it before it failed; luckily for them they were riding straight and slow when it did ('cause we were headed toward some less-smooth sections...).

Adding stress to this part might well accelerate any potential for failure here.


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## DHMASTER (Oct 12, 2010)

I could see making one out of wood. Shaping it to cradle the bottom tube and brackets. Some double sided tape and some clamps. It could work. I would definitely add a front bash ring too.


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## laksboy (Sep 4, 2007)

Just the thread I was looking for... 

Joe what are you using for that skid plate? Anyone else have any good ideas for a round boom tube on a Ventana El Testigo? I could see maybe ABS or PVC pipe? ABS would certainly look better. We have a few features that we definitely hi-center on. My thought is mainly to protect the frame as we bottom out and see-saw/slide/scrape across.

I'm going to be sending in the frame to Sherwood for a new tapered headtube so now would be the time to add some underside water bottle bosses if I wanted to make a nice bolted connection as opposed to hose clamps.

I'm also going to have stealth dropper ports added. And the frame will be re-heat treated and painted. Might as well go all-in.


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## unikid28 (Oct 11, 2017)

laksboy said:


> now would be the time to add some underside water bottle bosses if I wanted to make a nice bolted connection as opposed to hose clamps.
> .


Cool idea!

Maybe space them out so you could attach standard skateboard deck rails like this?

https://www.tactics.com/powell-peralta/rib-bones-deck-rails/black


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## 1x1_Speed_Craig (Jan 14, 2004)

I rode mountain tandems for 15 years, but never ran into any chainring or boom tube issues (and yes, I drug the boom tube over logs from time-to-time). In the Jeep world, UMHW is commonly used as durable, lightweight skid material. B.O.B. trailers even use small pieces for the same purpose. I'd investigate that material, as it slides nicely over obstacles.


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## switchbacktrog (May 10, 2013)

We used plastic guttering covered with carbon fibre effect sheet. Fixed to frame with double sided tape.


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## laksboy (Sep 4, 2007)

1x1_Speed_Craig said:


> I rode mountain tandems for 15 years, but never ran into any chainring or boom tube issues (and yes, I drug the boom tube over logs from time-to-time). In the Jeep world, UMHW is commonly used as durable, lightweight skid material. B.O.B. trailers even use small pieces for the same purpose. I'd investigate that material, as it slides nicely over obstacles.


Just pulled the trigger on some 1.5" W x 0.75" UHMW bar stock in black. I'll have it machined to match the 2.5" diameter of the tube so it will end up 0.5" lower to hopefully keep us from breaking a chain or bending timing chainrings. I had Sherwood add mounting bolts under the Stoker and Captain BBs.


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## 1x1_Speed_Craig (Jan 14, 2004)

laksboy said:


> Just pulled the trigger on some 1.5" W x 0.75" UHMW bar stock in black. I'll have it machined to match the 2.5" diameter of the tube so it will end up 0.5" lower to hopefully keep us from breaking a chain or bending timing chainrings. I had Sherwood add mounting bolts under the Stoker and Captain BBs.


Awesome. I'm looking forward to see the final solution, and hear how it works.


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## PMK (Oct 12, 2004)

Planning to do the prototype cabon fibre protector in a few days. Certainly will not be 3/4” thick. Tubes I will cut them from are 3.5mm wall thickness. This will be substantially thicker than the aluminum tube which is supposedly 1/16” (1.5mm) thick.


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## PMK (Oct 12, 2004)

Ours currently and with years and miles of use still going strong.

http://forums.mtbr.com/tandem-mount...-1000-words-only-few-words-needed-759595.html


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