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The Project Stampede Tri-Pede

7/27/06 - Pretty Proud of Myself - This is the first time I have done a lot with fabricating aluminum so this has been pretty fun.  The plus side of this hole thing is that I thing it may actually work and would has still been fun even if it was more of a proof of concept.

What are all the parts on the swing arm?

• Drive shafts and axles (custom cut and otherwise) are all Rustler/Stampede

• Carriers are all Rear carriers with bushings (it's what I had laying around)

• The front and rear pulleys and belt are all from a 4-Tec

• The slipper is a standard Rustler/Stampede slipper

• The motor mount is and Associated/Losi (I think)

• The one camber link is a cut down front Rustler/Stampede

• Maxx hex hubs and pins for the rear wheel

Special fitting and fabrication thus far.

• The swing arm itself lots of JB Weld

• Cut down the driveshaft to about 1 1/2" to fit inside the rear axle carrier.

• Trim and fit the carriers into each end of the upper and lower swing arm axle.

• JB weld, again, again, again.

• Fit and trip the motor mount to fit and JB weld in place.

• The drive shafts are fitted to the carriers just like stock except there are axels at each end.

• A brass tube was used to increase the diameter of the rear axle to Maxx diameter and was re-drilled to accommodate the larger Maxx axel pins.

• The rear pulley (a differential from a 4-Tec) was notched to fit just as a hex wheel adapter would and bolted in place.

• The slipper assembly was drilled out to fit the axel.

• One side of the metal slipper plate also received a notch to fit like a hex adapter on the stock Stampede axels.

• The spur gear is held by two shock pistons sanded down to fit like the spur gear carrier and then drilled to fit the axel. (I bolted the pistons to my Dremel cut off bit adapter and then held them against sandpaper to assure they were round.)

• The outside slipper plate was simply bolted on to the axel.

• The motor mount was shaved on the inside to fit over the carrier and then cold welded in place.

• The top of the "top" axel was shaved where the motor makes contact.  This allow the motor to sit low enough to run a 12 pinion and 87 spur which I pray will be provide low enough gearing.

• A shock brace was cold welded on the swing arm which will mount either dual Traxxas XXL shocks or one 1/6 or 1/8 scale shock un-decided.

• A pivot joint was welded on the swing arm for attachment to the chassis

• The swing arm is long and thus the chassis will need to be shortened about 6 inches to maintain some semblance of a turning radius. 

• I will need to build a custom battery to fit onto the shortened chassis.

• A pivot joint will be bolted to the chassis.

• A shock mount will be bolted to the chassis.

• fashioned a belt tensioner out of a machine screw and 4-5 shock pistons.

I was able to get the motor mounted up geared 12/87 with the 17T Pro-Amber. My first test was to make sure the wheel turned with motor power and I didn't have parts start to fly off. The rear spun up so fast (under no load) that the unglued tire ballooned and pulled free of the rim - yeah it works. I kind of rigged the swing arm to push the Ultra-Rusty around my basement and everything seemed to work so I am extremely hopeful at this point.

I have realized how much I vehemently loathe MSCs (mechanical speed controls) what a complex mass of crap they are. These things truly make me appreciate the elegant simplicity of an ESC even an old XL-1. I may have to borrow the Rooster from the Ultra-Rusty.  I have a perfectly function MSC that I will probably never use - still part of me wants to upgrade all the wires to 12 gauge to see how much better of a piece of crap it could be.

Instead of doing everything right and taking my time with this project, part of me wants to just duct-tape everything to the chassis and at least run it around the driveway.  I practicing restraint.

7/28/06  Continuing the Build
Yesterday was fairly productive with two more cold welds put in place one for the shock(s) and one for the pivot point on the swing arm.  I decided on a pivot point on the interior of the swing arm instead of the exterior, based on some things I have read and seen used.  Due to the fact that I have some weight with motor/gearbox assembly the idea was to counterbalance a little of weight of the swing arm assembly by going with an internal pivot point and thereby hopefully increasing the overall responsiveness of the rear wheel and shorten up the wheelbase in the process.

My next big and critical decision is the connecting point pivot. Non-billet aluminum (i.e. standard extruded aluminum) is a high wear metal and deforms pretty easily, so I am a little concerned that after a very short time an all aluminum pivot with a bolt through it would loosen very quickly.  I am thinking that some replaceable plastic or hardened brass bushings/sleeves may be a better alternative for long term use of the Tri-Pede.

You will notice that the shock mount on the swing arm is a little long.  I did this because I knew I would be cutting it down later and it always easier to subtract material than add.

I was able to get the receiver mounted via some heavy duty hard plastic Velcro and bolted up a found car cell antenna that "looks" like it should work as an antenna and measures "about" right for the receiver frequency.

I also received the ultimate compliment from my wife as she stared in complete amazement of what had been put together so far.  As she inspected my creation, in a fun discovery sort of way she pointed out where all the parts came from.  The scary part was as if from memory, she recited what I still had left in inventory as to tell me she "knew" exactly what I had, and gave me that "so don't go buying more stuff or I will kill you" look.  The compliment from her was - "If only you would use your powers for good instead of evil, image what you could do".  What can I say... being bad sometimes is a good thing.

7/29/06 Testing

I decided that I had better at least test this whole monster before I spent more time and energy on it.  I bolted the swing arm to the chassis (hard tail style). I hijacked the Rooster from the Ultra-Rusty and Velcro'ed a battery in place on top of the chassis along with the ESC.  One of my biggest concerns was that there is a lot of weight pretty high on the chassis, much higher than a Stampede.  With the motor, swing arm, and battery that high, that really shifts the center of gravity pretty high off the ground and tipping might be an issue.

I was correct in my assumption.  After shooting these pics and a few tip overs, I Velcro'ed the battery under the chassis.  After that change, stability was great (stock Stampede like) and was able to so some pretty speedy swerve and slide maneuvers in the driveway without tipping. I definitely think this beast would benefit from a wide-pede mod to further enhance cornering ability.

The drive train held up and ran great, I will be improving my belt tensioner so that is provides more adjustment, as I heard the belt slip a couple times under very hard acceleration. Acceleration was good however I am sorry to say that it was a wheelie free testing.  I was able to give it a couple runs in the grass which indicated the gearing was low enough to work well.  I would still feel more comfortable with a larger spur in the 90+ range if I can find one locally for a little extra torque.  I was impressed with the overall speed, very good also about like the stock Stampede (the older version not the newer).

One thing I didn't think about that was utterly obvious on the first turn was that the rear wheel's camber link is not connected to the chassis. The pitch and rolls of the chassis were matched with a equal pitch and roll of the rear wheel.  A hard left, as an example, adds a huge amount of positive camber (this is bad) as the chassis rolls right and the rear wheel rolls on the outside right edge of the tire. This action makes the rear want to steer right as the front is steering left.  This is like a motorcyclist steering left and leaning right - not good.  Camber on one wheel is tough to explain as it is typically defined as the positive camber has the top of the wheel pointing away from the vehicle at rest and the negative being the reverse.  Assuming the one centered wheel is tuned properly at rest for a zero camber you really can only refer to the camber of the wheel as it pertains to the direction of the turn. Typically what we do in RC is give the wheel a little negative camber so the wheel will be at least somewhat flat during cornering, or mess around with the camber link placement to "tune out" some of the pitch and roll effect of the chassis. The goal would be to keep the rear wheel at least flat or preferably with some negative camber (so the wheel is turning into the corner) as it turns each way. The cornering ability of the Tri-Pede would be greatly improved if this issue could be resolved.  I think I have already figured out a solution for this, although not a simple one.

What I hope to do is construct a double pivot.  This will entail mounting a left to right pivot assembly on the rear center of the chassis with a camber link attached from the chassis top to the bottom of that pivot.  The rear swing arm will then attach to that pivot so it can pivot up and down. In theory as the chassis pitches right (in a left turn) the link will push the camber of the left to right pivot negatively (i.e. flat or a little into the turn) during the turn. The next step I think is to do some experimenting with some pivot points and then get the shock and rear end permanently mounted up for some longer term testing.

This gave me some ideas on how to get everything connected up.

Anyway it works!!!!!!!  On with the project Tri-Pede.

8/4/06 More Testing

The double pivot idea although sound, is just to freaking complex to complete without a machine shop in my back room.  Hopefully a simple and elegant solution utilizing two gears attached to the rear wheel's camber link will do the trick.  This should in theory reverse the right roll of the chassis input to the rear wheel camber during a left turn and pull the rear camber link left thereby stabilizing the chassis and keeping that rear wheel flat.  It seemed like it would work in my head anyway.  At the very least I hope this will stabilize the chassis roll so that some really high speed cornering is possible and so I can scare the living hell out of everyone at the track - [insert evil laugh].

Last night I really ran the Tri-Pede around for about 15 minutes until the battery died.  This thing is a blast and will be even more fun once it has rear suspension and a body. This setup really moves, although with the rear tires un-suspended and mounted hard tail style, there is some pogo'ing going on as I run through grass with the rear wheel bouncing about.  It is actually pretty fast, faster than I expected and has a fair amount of torque.  Still think a 90+ tooth spur would work much better though and a wide conversion would greatly improve overall handling.

I haven't mentioned this yet but the salvaged car cell phone antenna works awesome.  I figured I would have to do a little tweaking, bit it works perfect and couldn't have planned a better antenna.  As heavy as it is, I would imagine it should hold up well also.

After the rear wheel camber thing, I would say my second biggest problem will be to maintain proper belt tension. I occasionally hear the belt jump a tooth or two as the wheel is pogo'ing; grabbing and releasing traction.  Not a big problem, but I can see that I will be going through some belts over time with this setup if I don't keep that in check.

I swapped the Traxxas pivot balls at the servo saver for some ball cap pivot balls to get a little extra clearance since my chassis is not stock.  I created an under chassis battery mount/front bulkhead brace from some scrape ABS that will hold the custom 3X3 brick I am putting together.  Hopefully if all goes well tonight, I can get the battery built at least half way through the camber link gadget I am now lovingly referring to as the "Double Whammy Chassis Stabilizer".

8/7/06 Battery Pack Done and Mounted

As you can see from the pictures (swing arm still mounted hard tail style) I was able to get the battery pack built and industrial Velcro'ed in place and sandwiched between the battery mount and chassis.  At the moment I have only the one battery pack for this rig, but I would think that should be enough to get me through testing and decide if I want to sacrifice another battery pack to the project.

Next up is tackling the pivot and creating the "Double Whammy Chassis Stabilizer".  This morning I cut the pivot retainer out of some scrape 1" aluminum square tubing I have carried with me through four household moves, good to see it is finally coming in handy.  I am still wondering if I made the right decision with the pivot point of the swing arm instead of a front mount to the top axle this may limit swing arm movement, then again it may not. If it works it will work better than a front axel mount.

This project is shaping up pretty nicely as a pretty tight setup, should be a real hoot to drive once finished. Shoot it's a hoot to drive as is.

8/14/06 Stalled at the Pivot Point

I have been round and round on this wanting to do a brass pivot joint something a little fancier than picking up a hinge and the hardware store, but it looks like that will be the route I go.  Easy, simple, straight forward, strong, and easily replaceable.  To hell with elegance I want to bash.  Now I need some shocks.

8/25/06 Now Pivoting, Shocking, and Motoring

This was THE hardest part of the whole project, I engineered and built by hand and entire single sided swing arm assembly, but stalled on a freaking pivot point.  OK so an industry standard hardware store hinge is now affixed to the rear swing arm and to the chassis.  Not my preferred setup but have to say that it works much better than I had ever hoped and it should be quite durable over the long-term. 

I happened to have a couple spare XL Traxxas shocks floating around however I am still debating whether I should use just one shock or go for two.  I think I will start with one and see how that works and if it looks like it is getting overworked, I will convert to a dual rear shock setup. Still working on this, hopefully completed this weekend.

My concern in the Tri-Pede was that my gear reduction was a little off and I still needed more torque in the tall grassy areas and I was already geared 12/87 so there wasn't really any further to go.  I ran across something that will prove quite useful - a Trinity 550 Monster Max Wild 17 Turn motor sitting in the clearance bin for $25 (not bad considering it's a $70 motor).  Traxxas is now using a 550 12 Turn on a ESC that has about the same specs as my Novak Rooster, so why not.  It turns out that this is a freaking torque monster and has the RPM of a tractor motor.  In testing it worked awesome in my Ultra-Rusty geared 30/84 with alls sorts of speed and torque and the ESC wasn't even warm. I should finally be able to gear down the Tri-Pede for a little more torque and my runtimes "should" be a little better also due to the low current demand of this 14.4V motor.  For a little more fun I could easily drop another cell into the battery system without stressing the ESC or motor. Fun fun.

I am also planning to do a wide conversion to enhance stability and rig up some type of torsion/stabilizer bar since my "Double Whammy" system didn't work as I had hopped.  It worked just in a very clumsy way that I was not particularly fond of.  I may do some more tweaking on that later, but I want to get the shock(s) mounted up and do the wide conversion to see how it handles with those modifications in place first.  I may find that all the various changes have made have greatly improved or worsened the chassis role and I need to take a look at that before I go fixing a problem that may or may not exist.

8/28/06 It's Alive

This weekend I was able to get the shocks attached, the electrics situated, motor mounted, tighten up the slack in the belt drive, fabricate a suspension stabilizer bar, and did a little testing.  Although it may not look a lot different from the pictures from the last photo update, I can assure you that the Tri-Pede is world different form the last "hard-tail" picture back on the 8/14/06.  For all practical purposes the Tri-Pede is now a full working and driving RC the likes of which have every or rarely seen in the RC community.

The shocks took a little more effort than I had expected.  Fabricating the mounts out of scrape pieces of aluminum tubing was pretty simple as was mounting those to the chassis and swing arm. The problems arose when the shocks were mounted.  My intent was to use a dual XL Traxxas shocks, which after mounting were noted as being severely under dampened and also under sprung.  So I mounted them up side-by-side style, but that provided too much spring and was way over dampened.  The solution involved a multi-faceted movement of springs and shocks between various in house R/Cs and started with moving the front shocks (HPI clearance box specials somewhere between a Traxxas L and XL shock in length) to control the rear swing arm and moving the XLs up front.  The XL were way too tall of course, but a solution came as I was staring at my Ultra-Rusty and realized I had over a 1/2" of pre-load spacers on the front shocks (noting that you need this many on the stock fronts after a "Wide" conversion).  I swapped the XL springs on the front Tri-Pede for the shorter springs on the Rustler and vise versa.  The result was the Ultra-Rusty being almost perfectly tuned in the front with more spring tension and the Tri-Pede sitting at normal  ride height after internal shock extension limiters were installed.  Granted I am not using the full capacity of the shocks, but once I move to a Wide conversion, more of the shock will be utilized and the shock limiters reduced to expand the height a bit.  As you can see in the pictures I have about 2"-3" of rear suspension travel and about stock 1"-2" in the front.

The electrics were a also a bit of a pain as I was really running out of space for a receiver and the ESC.  The receiver was moved to the very front of the chassis and the ESC mounted standing on it's side.  The antenna was moved forward about an inch to make just enough room for the ESC.  There is not much room at all left on this shortened chassis.

The Monster Max Wild 17T 550 Motor was also mounted and just barely slipped into the space I allowed between the motor mount and shock mount.  I geared this at 18/87 which seemed to be a good compromise between speed and torque, but I think I could go much higher if needed and also much lower should I be faced with heavy grass situations.  This should be a good motor for this application.

I drilled a couple holes to allow for various placement of the belt tensioner and in turn adjustment of the belt drive slack.  I am a little amazed at how much the belt stretches, seems that you would go through a ton of belts.  The belt was very tight when first mounted, and now you can see there is quite a bit of slack that had to be managed.

Although my efforts to lower CG (center of gravity) on the Tri-Pede worked to some degree, chassis pitch and roll were still very apparent.  To combat this, I fabricated a suspension stabilizer bar out of some 1/8" scrape brass rod and attached it to the front arms with captured ball ends and secured it to the bulkhead.  A real quick engineering job that seemed to help quite a bit, but there was still some chassis roll during hard turns and when pushed hard enough the Tri-Pede still flips but in a very odd manner that's just so unique.  My hope is to work on this and improve my stabilizer bar at the same time that I complete the Wide conversion.

For now I need to put some hours on the Tri-Pede to really get a good idea of what I need to do to improve it's performance.  I am also going to work out a body shape which will be uniquely the Tri-Pede and echo it's three wheeled stance.