Big Rig weight distribution

[sprntrcr]

Ok, so I watched the Big Rig video from this topic.
http://derbytalk.com/viewtopic.php?t=29 ... torder=asc

BTW, it is a great little video, just wish there was a way to save it so I could show it at our pack meeting to try an spur some interest in our Big Rig event.

I noticed some of the trucks developed a shimmy on the flat part that I assume is due to how the weight is distributed. There was one truck, the black flatbed that seemed to be the smoothest, in fact I think it was the only truck that won in the red lane, at least for the runs show in the video.

Can anyone share some weight distribution suggestions for a smooth ride?

Even though I store the pack track at my house, I never test any of our cars on it out of fairness to the other competitors. We will do the normal testing on the table and slanted board, but I'm not sure I could create this condition since it seems to happen on the 2nd half portion of the run.

BTW, anybody have a set of Big Rig rules they would be willing to share?

Thanks,
Rob...

[pwdarchitect]

Rob,

Ask GoBubba......that black one is his.

Rules for the Big Rigs that ran in that race can be found here:
http://pwdracing.proboards91.com/index. ... 709&page=3

[sprntrcr]

Rob,
Ask GoBubba......that black one is his.

ah Geesh!!! And here I felt kinda bad for him when the black rig beat his military transport

OK, Bubba ..... let's here the details

[Go Bubba Go]

Rob,
Ask GoBubba......that black one is his.

ah Geesh!!! And here I felt kinda bad for him when the black rig beat his military transport

So did I . The thrill of victory and the agony of defeat, all at once. I actually thought the Army "Dragon Wagon" might be the fastest of the 3 (The Black flatbed and Dump Truck were also ours), but while it did have the fastest run of the day, the other runs..., well, I think excess "shimmy" cost us. 2nd place overall, though

OK, Bubba ..... let's here the details

The trailer for the Black Truck is actually a pretty simple design - a pine block (came with the Beta Crafts kit) that holds the 2 sets of axles in place and a couple of Spruce (Balsa too light, Pine too heavy) strips I got from the local RC Hobby shop. There are a number of different loads (Cargo Boxes, Flatbed trailer) that ride on the flatbed, but none are shown in the race video. You can see them at the end of the "October Lineup" video (you'll see the "Warthog Express" Cargo Box of Warthog's also), see October Lineup (sorry, not sure how to skip past the intro and get right to the trucks, but there is a lot of good truck footage). Mr. Slick and Warthog ran them all without the loads, not sure if they ran test runs and had issues with them coming off at the stop section or if they were just being a little extra cautious since it was a Proxy run for my trucks.

The lead (we kind of went over our PWD budget this Summer so Tungsten was out of the question) weight is mostly in a cavity that I cut out of the block between the rear axles, a little more behind the rear axle, and the final 2-1 oz. sinkers beaten into squares and disguised as "mud flaps" on the back of the trailer

When I get them back from Warthog I'll see if I can't get a few more detailed pics up (sorry, didn't take any In Process pics - my bad ).

I think part of the reason the trailer for the Dragon Wagon shook so much was a combination of 2 things:

1) It was only actually riding on one set of regular wheels on single axles with a normal (i.e. fairly narrow) track.

2) I had some weight up fairly high (a couple 1 oz. sinkers beaten into squares and attached to the bottom of the ramp that flips up on top when in "race position"). I originally put the weights on the ramp to help hold it down while it was running down the track, but in retrospect maybe I should have put the weight low and used a locking pin of some sort to keep the ramp locked down.

FYI the Dump Truck trailer seemed to track fairly smooth. It was closer in design to the black flatbed trailer (weight low in carved out pockets between and behind the rear axles).

p.s. PM me in a couple weeks about the video. Might be able to get a DVD of it for you.

[Go Bubba Go]

...Even though I store the pack track at my house, I never test any of our cars on it out of fairness to the other competitors. We will do the normal testing on the table and slanted board, but I'm not sure I could create this condition since it seems to happen on the 2nd half portion of the run...

Thanks,
Rob...

Well just set it up and invite your "competitors" over for the evening. No runs against one another, just testing.

Seriously, I would consider setting up a "test night" for the Big Rigs to work out any bugs. Ours weighed in at around 20 oz and made the track in under 3 seconds, a lot of momentum there.

One issue you will need to contend with (besides confirming the ability to clear the start and finish gates) is ensuring the trucks make it through the transition OK. If your trailer is too thick (and the hole is tight like it should be) where it attaches to the hinge pin (pseudo-5th wheel) and you don't make allowances, there will be a tendency to try to lift the rear wheels of the cab off the track during the transition. We dealt with this by starting with a drill straight up and down on the hinge pin trailer hole, then leaning it to the rear to open up space for the rear of the trailer to lift without the trailer binding at the hinge pin.

Keep us posted

Bubba

[sprntrcr]

Wow!! Thanks for all the details. I have plenty to digest there for a while.

Did you do anything special to the dually wheels. I read in the other thread you raised some of the axles so the load was primarily on a limited set of wheels.

I was thinking of perhaps gluing the two wheels together to eliminate the friction between the two, or sanding down the circumference of the inner wheel so that the outer wheel takes the weight and the inner just floats.

Thanks again,

Rob...

[Go Bubba Go]

Wow!! Thanks for all the details. I have plenty to digest there for a while.

Did you do anything special to the dually wheels. I read in the other thread you raised some of the axles so the load was primarily on a limited set of wheels.

I was thinking of perhaps gluing the two wheels together to eliminate the friction between the two, or sanding down the circumference of the inner wheel so that the outer wheel takes the weight and the inner just floats.

Thanks again,

Rob...

I did raise the forward dually axles on both the cab and trailer using the "3rd hole" on the Pro Body tool (8 wheels off the ground right away, down to 10 left...). BTW I also raised one of the very front wheels on the cab off the ground IAW basic rail riding theory (9 wheels off the ground, down to 9 left).

I also had a slight cant on the rear axles. The initial intent was to push the wheels outward as they ran for added stability, but when I was going through the alignment I found that the rigs were often riding just on the inside wheels with the outer wheels being slightly (thickness of a piece of a paper) off the track. (13 wheels off the ground, down to 5 left).

Interestingly (and to your point about gluing the wheels together), with the outside of the inner dually wheel and the inside of the outer dually wheel nicely lubed, there was very little friction between the two and (at least at tabletop speeds and with our very slight cant) the inner wheels would slightly touch the outer wheels but not generate enough friction to cause the outer wheels to spin. Bottom line: I would NOT glue the wheels together. Even if there is enough friction that at higher speeds and under real track conditions the outer wheels are eventually spun up completely, the further you can delay that loss of energy the better. In other words, if you can lessen the drag between the inner and outer wheels so that the outer wheels are spun up gradually, that would be better than gluing them together and requiring the rig to fully spin them up right away, thus slowing the start off the line. (I would back off this line of reasoning if I found that there was some sort of interference between the dually wheels that led to a corresponding decrease in performance, but so far it isn't apparent).

We were not allowed in our rules to alter the basic dimensions of the wheels, but if you are I think your idea about shrinking the height on one set of the wheels (inside or outside) might be a good idea. If you have your stability issues well in hand, though, I might consider raising the outside wheels slightly rather than the inside wheels. The reason is that if you end up with the wheels contacting the rail it would probably be better those wheels (the inner wheels) be already in motion. At some point we'll achieve "good" rail riding for the Big Rigs (i.e. the only wheel touching the rail for the whole rig is the dominant front wheel on the cab), but we're not there yet so we should plan on some inside dually wheel contact with the rail. (Of course, if your rules allow you to remove so much height that your inner wheels completely clear over the rail, that's a different story. Our rules were built largely to maintain the "realistic" 18 wheeler look and wouldn't allow for such a drastic modification).

Many similar applications learnings from the "little cars", just on a much bigger scale.

Regards,

Bubba

(Shhhhh... I'm giving away all my "Good Stuff" here).

(I did have an idea about "locking" the trailer and cab together for a race in such a way to get the rear wheels of the cab off the ground, but I just didn't have time to build and test it. See also my previous warning about the transition area. Otherwise, we would have been down to 3 wheels on the ground, 15 in the air Might have to back off on the weight a little if we did that, though )

[sprntrcr]

Bubba,

Thanks again for sharing your knowledge and experiences. You are very good at articulating your points in a way that makes them very clear and easily understood.

So I will abandon the gluing of the the duallies and focus more on the aligning and minimizing the number of rolling wheels.

I'll be the one defining our rules, but I really like "realistic look" approach of the rules that you guys used, so will probably use the same ones with the exception of unlimited height. I'll need to limit this so that we don't have finish line issues.

BTW, I ordered the big rig kits from Beta Crafts yesterday, so I guess there is no turning back now. I'm going to pitch the idea at our pack meeting on Monday to hopefully spur some interest. I'm hoping to get at least a couple of dens to build one as a den project and perhaps get sponsored by and advertise for a local business. Worst case, I'll be building a couple of trucks myself and running an exhibition race

I'm sure I'll have some more questions onces I dive in.

Rob...

[OneHour]

We have been running Big Rig for the past 6 years. If you'd PM me your email address, I'll email to you our Pack's Big Rig rules. We have never gone all out on speed. In fact, I don't think that any of the participating parents care about speed. Looks are what most of us are after in our Pack! We usually try to outdo each other in the creative department. This year, hopefully mine will be Optimus Prime carrying either Bumblebee or Megatron (have not figure out which yet).

What Bubba indicated should be the way to do it for speed.

[Go Bubba Go]

We have been running Big Rig for the past 6 years. If you'd PM me your email address, I'll email to you our Pack's Big Rig rules. We have never gone all out on speed. In fact, I don't think that any of the participating parents care about speed. Looks are what most of us are after in our Pack! We usually try to outdo each other in the creative department. This year, hopefully mine will be Optimus Prime carrying either Bumblebee or Megatron (have not figure out which yet).

If you have any pics I'd love to see them.

Bubba

[OneHour]

Megatron it will be ... just bought 6" Megatron (along with Optimus Prime) at Walmart. I am envisioning ... Megatron chained to the flatbed pulling by Optimus Prine (in truck mode). I'll try to post it in January after I finish with it. Won't start until Xmas Holidays!

[sprntrcr]

OneHour,

I got your email. Thanks for the rules document. I'm looking forward to seeing your pictures as well.


Thanks,
Rob...

[Go Bubba Go]

ah Geesh!!! And here I felt kinda bad for him when the black rig beat his military transport

So did I . The thrill of victory and the agony of defeat, all at once. I actually thought the Army "Dragon Wagon" might be the fastest of the 3 (The Black flatbed and Dump Truck were also ours), but while it did have the fastest run of the day, the other runs..., well, I think excess "shimmy" cost us. 2nd place overall, though

Sprntrcr: Little update for you on the Dragon Wagon.

When I got it back from up North I took a quick look (a little feel also) at the rearmost axles on the trailer and they we pretty loose side-to-side in the holes, at least by PWD standards. I think that led to some migration of the axles outward, which would serve to increase the slop in the axles, and so on downward.

I don't remember overdrilling the rear axle holes and having to do any kind of "cheap / quick fixes" or anything, but they were just plain loose. Kind of took me by surprise when I saw them.

Didn't have the time to really study the situation, but my suspicion is that I might have taken a little too much material out of the area surrounding the axle mounts (to make room for the lead, of course ), and that the first full speed transition might have exposed this weakness. Given that they were running on a regular Best Track that can pull up to 3Gs in the tight transition (or so I've heard, I don't know the exact figures), the area might have been a little too weak given the full load we put on it (we were pretty close to 16 oz. for the trailers when loaded, about 9-11 oz. located in the immediate vicinity of the rear axles ).

Anyway, I wouldn't let the "shimmy" runs we had with the one rig discourage you from an aggressive design, it appears there was a little poor worksmanship on my part with respect to the axle holes on the Dragon Wagon and the other 2 ran pretty darn smooth from what I could see (except for the Dump Truck vs. Starting Gate issue, but that's another story). And even with the "shimmy", the Dragon Wagon did get 2nd

Overall, it's hard to argue with a setup that brought us some pretty regular 2.9s (our first!)

Bubba