22-06-2016 15:11

asid61

Re: pic: Another Belt-In-Tube Drivetrain

I love this thing! Very clever to counterbore the pulley so that it can use the full width of the tube.
Looks super compact and lightweight, and above all very clean, great work. Interesting that you used OnShape for this, I have a few friends who love it or hate it.

22-06-2016 17:47

Ty Tremblay

Re: pic: Another Belt-In-Tube Drivetrain

Thanks. I'm a little bit concerned about the lack of flanges on the pulleys, but with less than .035" of space on either side I just couldn't fit them in. That said, the .035" is small enough to let the tube wall be the "flange" if anything goes wrong and the belt tries to walk.

As for OnShape, I've expressed my thoughts here.

22-06-2016 19:59

InFlight

Re: pic: Another Belt-In-Tube Drivetrain

2 inch square .125 wall 6061-T6 is readily available ( On-line Metals, et al). Seems like this would eliminate machining of cots parts, and potential rubbing of the belts.

It seems like many teams just use 1.5x2, but there's no need to constrain yourself to it.

22-06-2016 22:23

asid61

Re: pic: Another Belt-In-Tube Drivetrain

I would think the reasoning is to save space rather than just fit the belts in the tube. Losing 1" of width doesn't seem like a lot, but especially for this year that 1" would have been the deal breaker for me.

23-06-2016 07:28

Ty Tremblay

Re: pic: Another Belt-In-Tube Drivetrain

Saving space is nice, but my main reason for designing this was an exercise in whether or not I could fit a belt-in-tube drivetrain with 15mm belts into a 2x1.5. When I have the time (aka, offseason) I like to give myself tight constraints in an effort to force myself to think outside the box.

(or inside the tube... eyyyyy)

23-06-2016 07:55

notmattlythgoe

Re: pic: Another Belt-In-Tube Drivetrain

23-06-2016 09:17

Chris is me

Re: pic: Another Belt-In-Tube Drivetrain

If you're doing 15mm belts with a single pulley, you already have to machine your own pulleys from stock. You also already want to leave a nub on the pulley to touch the inner race of the bearing. So adding the counterbore for the bearing isn't THAT much harder, and it makes the entire chassis more compact to do it this way.

The extra inch of space really does help sometimes.

23-06-2016 09:28

Sh1ine

Re: pic: Another Belt-In-Tube Drivetrain

How do you plan to attach your belly pan to this setup? We use belt in tube and I have wanted to go to a smaller tube but I always worry about the rivets rubbing the belt.

An alternative to pocketing the belts is to add .125" thick washer (spacers) to the bearings before putting them into the tube, that makes them flush with the interior or the tube.

23-06-2016 09:42

Ty Tremblay

Re: pic: Another Belt-In-Tube Drivetrain

We're fortunate enough to have the ability to weld our belly pan on. You could get clever with staggering your rivets for your belly pan as the belts only take up one side of the tube between the pulleys, but it would make it almost impossible to disassemble if you needed to.

23-06-2016 09:43

notmattlythgoe

Re: pic: Another Belt-In-Tube Drivetrain

Everywhere there is a hole it is safe to mount to.

23-06-2016 10:34

Ty Tremblay

Re: pic: Another Belt-In-Tube Drivetrain

what if you have to pull the chain/belt out of the tube?

23-06-2016 10:39

notmattlythgoe

Re: pic: Another Belt-In-Tube Drivetrain

23-06-2016 14:58

asid61

Re: pic: Another Belt-In-Tube Drivetrain

Wouldn't you just pull the shafts out and pull them out that way? I always thought that how everybody did it. You can pull them out to avoid the rivets pretty easily, I would think, especially given that you only have 2 belt runs in a tube.

23-06-2016 15:05

Chris is me

Re: pic: Another Belt-In-Tube Drivetrain

Are you saying to pull the belt out through the bearing hole? That's kind of a tight fit.

Regardless, I think this is mostly an academic exercise - this particular belt in tube configuration has by now been battle tested on dozens of FRC robots without failures. It's a very reliable drivetrain.

23-06-2016 18:20

Ty Tremblay

Re: pic: Another Belt-In-Tube Drivetrain

Chris, what're your thoughts on this belt in tube design? IIRC, 2791 has had success with belt in tube.

23-06-2016 22:04

asid61

Re: pic: Another Belt-In-Tube Drivetrain

I mean pull them out from the front or back of the tube. It looks like the front and back are open, so one should be able to just draw them out from there.

23-06-2016 22:13

Ty Tremblay

Re: pic: Another Belt-In-Tube Drivetrain

The issue is the pulleys. There's only .030"-.060" of space around the pulleys, so they'd get caught on any rivets poking into the tube (like rivets do).

23-06-2016 22:39

kaliken

Re: pic: Another Belt-In-Tube Drivetrain

Could you pocket the tube right above the pulley such that you could pull it out that direction?

24-06-2016 00:46

Joey Milia

Re: pic: Another Belt-In-Tube Drivetrain

I think you'll be fine with the 1.5 tube and no flanges, good way to do it. I am more concerned about fitting the 24T pulleys in there, are you sure your belt thickness is correct? When I was choosing 22T for our drives I was pretty sure that was as large as could fit. Maybe i was using a different drop than you or something, but there isn't much room when we assemble, they're almost rubbing the tube on some of our drives.

I don't know when this will be but next time i get access to one of the robots i'll try and take lots of pictures and show how we do assembly and everything. We have it so assembly and disassembly are really really fast.

24-06-2016 03:03

asid61

Re: pic: Another Belt-In-Tube Drivetrain

OH.
I get it now... whoops.

Maybe if you attached the bellypan using 4-40 buttonheads and tapping the tube? If you use an 1/8" bellypan, 1/4" 4-40s would be long enough. Then it wouldn't stick into the tube.

24-06-2016 03:41

DaveL

Re: pic: Another Belt-In-Tube Drivetrain

Hi Ty:
Can you talk about the bumper supports?
(How are they assembled?
Have they been used in the past?)

Dave

24-06-2016 07:46

Ty Tremblay

Re: pic: Another Belt-In-Tube Drivetrain

This might let you get the pulleys out, but there's so little space around the pulleys in the tube that you wouldn't be able to get the belts off the pulleys before taking the pulleys out of the pockets.

Excellent, thanks Joey. I'll double check the thickness of the belts in the CAD, do you know of any resources online that will tell me the outside radius of a pulley with belt on it?

These bumper supports are an evolution of a concept I copied from 118 last year. The thread I linked has a more in depth explanation, but the short of it is that single-piece bumper assemblies with reinforced corners are plenty strong enough to withstand the rigors of an FRC season and you don't need much extra structure behind the .75" plywood.

In 2016, 319 ran this bumper support setup, which is all 1"x1"x.0625" tubing, along with a single-piece bumper. We didn't have any issues with bumpers all season despite playing in over 100 matches. One thing we did notice, however, is that putting chain or belt inside your chassis rails puts a lot of limitations on where you can mount things to your chassis. We ended up mounting more than we wanted to the .0625" wall bumper supports in 2016.

For this evolution, I copied 558's concept of a single piece of 1"x2"x.125" for bumper supports on the side, and the chassis rails for bumper supports on the front. This gives us a rigid place to mount things, and means we don't have weight on the robot dedicated solely to holding bumpers. The angled 2"x1" supports were also 558's idea, but I modified ours a bit since 319 has welding capability in-house. If you remove the cutout, you can bolt the bumper rail to the top of the angled piece, and then bold the angled piece to the top of your chassis. I'll get ahold of 558 and see if they can post a few pictures of their setup.

24-06-2016 08:08

jwfoss

Re: pic: Another Belt-In-Tube Drivetrain

Ty, thanks for the hat tip on design inspiration. You can see 558's 2016 chassis was a little different because of the angled front cuts so the front and rear rails were on top of the drive rails. We did a fully bolted together construction on the chassis and rivets on the belly pan. A version for use on the flat floor is shown here. Again, all bolt together construction. Feel free to ask any questions via PM or we can start a new thread if needed.

24-06-2016 09:46

Chris is me

Re: pic: Another Belt-In-Tube Drivetrain

This design should work very well for you. 24 tooth pulleys and 15mm wide belts are basically bulletproof in my experience, and with your welded design you can make the tubing and frame as compact as possible. 2791 has used 24T pulleys and 15mm belts with both 4 and 6" wheels without any ratcheting or belt failure at all. Other teams in our area even used this setup for this year's game, with 8" pneumatic wheels, without any failures. This is a very robust setup and you shouldn't have any issues with it that I can see here.

The only slight design tweak I would suggest would be to switch the bearings out for ThunderHex and 1/2" round. In exchange for a little bit more lathe work on your shafts you can use a 1/2" round bearing on the inside shaft and a ThunderHex bearing on the outside. Round bearings tend to be more robust in WCD setups, and as a small bonus if you design the shaft correctly you can retain the entire shaft using a single snap ring.

24-06-2016 09:51

Ty Tremblay

Re: pic: Another Belt-In-Tube Drivetrain

Thanks. What size tubing do you use for your rails? is it 2" tall? If so, what kind of drop are you using and do you run into the issues that Joey mentioned above?

Agreed on the ThunderHex. we ran Thunderhex on our chain-in-tube this year and loved it. I just already had the hex bearings imported into OnShape.

24-06-2016 09:58

Chris is me

Re: pic: Another Belt-In-Tube Drivetrain

Shaker's drives haven't been welded for the past several years, so we have used 3" tubes to allow ample rivet clearance. In flat field games like 2014, we can use 4" wheels with 3" tubes no problem. In other years we had to switch to 6" wheels for extra clearance. With the 3" tubing, 1/8" drop was just fine (if anything a little more than we needed in 2014). If we went to 2.5" tubing or smaller we would have to stagger the drop or even switch to a zero drop drive, sanding outer wheels down to a smaller diameter if we needed to add drop after the fact.

The designers on the team are looking into ways to use smaller tubes in future drivetrains, to save weight and to allow adequate ground clearance with 4" wheels. 2.5" tall tall tubing, with rivets staggered to avoid the belts and pulleys, are a possibility. Experimenting with smaller tooth count pulleys is another option, albeit a risky one. 21T pulleys have a number of teeth divisible by 3 for easy machining, and they allow JUST enough clearance for a 2" tall tube if you stagger the rivets, but without testing I can't be certain they would be robust enough.

24-06-2016 10:11

Ty Tremblay

Re: pic: Another Belt-In-Tube Drivetrain

Here's a 22t 5mm HTD pulley with the counterbore in it. This only leaves .0195" of wall at its thinnest point, and only .005" of clearance from the radius of the bearing.

SDP/SI does sell 23t 5mm HTD pulley stock, however.

24-06-2016 10:18

Chris is me

Re: pic: Another Belt-In-Tube Drivetrain

Ah, I didn't think about the counterboring issue. That makes things more difficult. On 2791 we just used 1/8 washers between the bearing flange and the tube to get everything to fit. This is a bit more compact than going to 2" wide tubing but lets you use smaller pulleys.

24-06-2016 11:20

Cothron Theiss

Re: pic: Another Belt-In-Tube Drivetrain

What are your concerns with using smaller pulleys? I assume that the belts would slip before the pulleys or belt would fail, but is belt slippage your concern with a pulley smaller than 24t?

24-06-2016 13:57

1493kd

Re: pic: Another Belt-In-Tube Drivetrain

We learned everything we know about belt in tube from 2791 but I will agree 100% that it is very robust and actually pretty simple once you get the hang of it... (And have machining capabilities)

2015- 1.5x3 side rails, 6" Colson wheels, 1/8" drop- 24 tooth pulley stock used to make custom pulleys that we counter bored to eliminate the need for a spacer on the bearing. Image below:
http://imgur.com/goFii7G


2016- 1.5x3 rails, 6" West Coast Pneumatic wheels, 1/8" drop, 24 tooth pulley with the counter bore.

The only negative I have with this set up is how much it weights, but the trade off of not having to touch it after we set it up is worth it to me.

24-06-2016 14:04

Chris is me

Re: pic: Another Belt-In-Tube Drivetrain

Belt skipping / ratcheting often damages the belts as the teeth are being loaded in an odd shocking manner, so from a robustness standpoint ratcheting is to be avoided as it is itself a precursor to belt failure.

Belt / pulley strength is a function of two things - the diameter (tooth count) of the pulley, and the width of the belt. A larger diameter pulley decreases the force put on the belt teeth a given output torque. A wider belt decreases the stress placed on the tooth by spreading the force out to a wider area. As belt sizes get thinner and as pulleys get smaller, eventually you start to skip teeth via ratcheting, and eventually you'll fail a belt.

In a drivetrain, with HTD belts, 24T pulleys and 9mm wide belts are riding on the edge of feasibility - these drives tend to fail belts by ratcheting and eventually tensile failure. I haven't experimentally determined how small of a pulley you can go to in order to safely use a 15mm wide belt - I'm sure I could estimate it with some math, I just haven't done it. I seem to recall 18T pulleys would cause 15mm belts to skip in a drivetrain, but I don't remember if that was speculation I heard someone else say or something actually based on real world experience.

24-06-2016 15:04

Cothron Theiss

Re: pic: Another Belt-In-Tube Drivetrain

Hm, it would be nice to know whether an 18t HTD pulley will work with 15mm wide belts. I'm currently working on a WCD that would be using that setup, so I have a bit of a vested interest. For your purposes, since you're already machining your pulleys out of stock, couldn't you switch to GT3 belts and GT2 pulleys? there's not a huge difference, but it would give you some margin of safety for the the setups that are on the cusp of failure.

24-06-2016 15:26

Mark Sheridan

Re: pic: Another Belt-In-Tube Drivetrain

yeah we have broken HTD 15mm with 18 tooth. This year we made the pulleys almost the same diameter as our wheels to have a beefy safety factor with HTD.

Most likely, if we stick with belts, is to go with GT3/GT2 like 971.

24-06-2016 15:56

Ty Tremblay

Re: pic: Another Belt-In-Tube Drivetrain

How much stronger is GT3? Looking at the profiles, I can't tell the difference.

Edit: Google says 2x as strong.

24-06-2016 16:03

asid61

Re: pic: Another Belt-In-Tube Drivetrain

Cripes, that's a ton! Who knew that such tiny changes could have such an effect...
The difference between trapezoidal and HTD is just as great somehow. It's a shame that GT2/GT3 is patented, otherwise Vex would be able to produce them (I think?). Does anybody know why Vex doesn't use GT2?
Also, can GT2/GT3 belts be used with HTD pulleys, and what effect does that have?

24-06-2016 16:50

Cothron Theiss

Re: pic: Another Belt-In-Tube Drivetrain

Do you mind providing details on what parts you were using and how it failed? The 18t VexPro timing pulleys are plenty strong enough for use in an FRC drivetrain under normal conditions, but as Chris explained earlier, there must have been some slippage and racheting going on that lead to your failure.

Yes, the GT belt profile is trademarked. (GT stands for "Gates Tooth," so they made sure everyone would know who came up with it.) This means that Vex cannot produce ACTUAL GT belts or profiles. But there is nothing against measuring the belt teeth and coming up with a very very close approximation to the GT profile. So if you look at WCP's page on the GT2 belts and pulleys they sell, they refer to it as "GT2 compatible profile." I assume that this is just a legal workaround to use the GT profile.
And yes, I believe that GT (3mm) belts can be used with HTD (3mm) pulleys, and if I recall correctly, it performs about as well as HTD with HTD.

24-06-2016 19:11

Mark Sheridan

Re: pic: Another Belt-In-Tube Drivetrain

No slippage or ratcheting. We have good control on build quality to get our Center to center right. its was overloading with too many cycles and the belts snapped on our drive. Our practice bot easily runs 100 simulated matches in a week. So we always have the practice bot on a lifecycle far ahead of the comp bot to catch these sort of failures.

971 helped us out and pointed out that these belts are being loaded beyond their recommended limits. We built a calculator just to double check belt loading. We also swapped a bunch of belts on the same drive rail and noticed despite the same CC, the tensions were different. We try to reduce belt load and build in tensioners which is what we did this year. The last conversation I had with the students is that we are in the mood of having the drive on the practice bot survive with minimal maintenance for over 1000 simulated matches. we are just sick of worry about this stuff, so keep in mind that perspective.

I think using 22 tooth pulley and 15mm wide GT profile belt will be a step in the right direction for Ty's design, which is pretty sweet.

24-06-2016 19:58

R.C.

Re: pic: Another Belt-In-Tube Drivetrain

Pretty much,the belts are not produced by Gates and the profile is not the same as the GT2 or GT3 profile by gates, it slightly different. Will be putting up test data over the next few months to compare the profile we sell vs Gates GT2/G3.

24-06-2016 20:07

Cothron Theiss

Re: pic: Another Belt-In-Tube Drivetrain

That's excellent, thank you! Any chance you can also throw in testing of HTD so that teams can compare your data against something most teams are more familiar with?

24-06-2016 20:18

R.C.

Re: pic: Another Belt-In-Tube Drivetrain

Will do. Let me know if there is anything specific you'd like in terms of testing.

As a side note the pulleys we sell and future pulleys we will be releasing are the actual GT profile. The pulleys are free game for anyone to sell/make just not the belts sadly.

Thanks!

24-06-2016 21:32

Cothron Theiss

Re: pic: Another Belt-In-Tube Drivetrain

That's great news! It'll be interesting if teams can use the more compact GT pulleys to make belt-in-tube drivetrains using the 2x1 VersaFrame.

24-06-2016 23:24

Joey Milia

Re: pic: Another Belt-In-Tube Drivetrain

I don't think having that thin wall is an issue at all. I'd go with that.

For the correct belt thickness I made my belt drawings using the tooth heights and overall thicknesses of the belts given in the gates belt design manuals.

27-06-2016 14:20

Steven Smith

Re: pic: Another Belt-In-Tube Drivetrain

Hopefully not de-railing the conversation, but if anyone could humor me and take this discussion up a level, or link the relevant threads if it has been adequately discussed before, I'd certainly appreciate it.

#1: What are the pro/con of going to belt in tube if you are currently running chain in tube?
#2: Do you believe the differences are significant? And why?

As a data point, since I've been on 3005:

In 2014 we ran a WCD style 3.25" 6WD tank with internal 9mmxHTD5x20T pulleys (Vex) on the inside of the robot. Never had any ratcheting issues (or would expect). Used VexPro/WCP bearing blocks (without retention cams) and had enough slippage issues (block vs. tube, our fault) to cause pulley misalignment and the press-on walls of the pulleys to come off.

In 2015, we ran a chain in tube for an H drive configuration in 1x3x.125" extrusion, #25 chain, ~22-25T sprockets as I recall, geared at maybe ~10ft/sec. No issues to note.

In 2016, we ran a chain in tube, 8WD with #35 chain, 8" pneumatic tires, in a 2x3x.125" extrusion. No real issues to note.

Looking forward, though we may play around with "other" drive train options, it is highly likely we will prefer to similar drivetrain styles. We have the manufacturing capability to handle most designs (lathe, cnc mill, Al welding, etc), it just requires the investment of resources to create in the offseason and prove out. We value robustness over maneuverability, and think we can make up for any loss of mobility with extra driver practice in most games.

What advantages might we be missing out on by continuing to run chain in tube?

The things I can think of:
- We have run 3" high tubing two years in a row, to accommodate extra center drop as well as a slightly larger sprocket to reduce chain loads/sprocket wear/etc. I have justified this in my head by saying the extra profile yields a stronger frame (torsional), but is it needlessly stronger? Would running belts make it easier to go to 2" profile and maintain the high safety factor I would like, even at large diameter wheels?
- Are belts (when properly tensioned) more accurate with less slop when it comes to measuring distance traveled (for auton)? Or is the difference to chain not worth mentioning?
- Is the system more efficient? More robust? Overall lighter (I know the belts are, but the hubs look heavier than an equivalent plate sprocket).
- Other?

Any rate, we will probably do another iteration of our drivetrain before build season next year, and the discussions on the belt in tube have me intrigued. Thanks for any input!

27-06-2016 14:35

asid61

Re: pic: Another Belt-In-Tube Drivetrain

Chain is a bit less efficient than belts. Look up Team 234's paper on Chain vs. Belt, it has some good info in it.
Because both chain and belt are positive interference/have engaging teeth, neither is more accurate in auton. Timing belts might give you better performance by a couple hundreths of an inch, but that's about it.

Belts are lighter than chain, but you're right that the pulleys are not. In my experience the weight difference is negligible.

If you're considering switching drivetrains, running 9mm belts on 36 tooth pulleys or something could be a good swap for you. If you're already using 3" tall tubing, using the largest pulley (around 36 tooth) would net you a good factor of safety. I'm only going off the "24 tooth 9mm" being the absolute dangerous bare minimum for belts, so doing your own testing in the offseason would be a good idea.

Personally, I prefer chain in tube for the compact factor and the strength that it offers; I've never broken a #25 chain (well I did once, but not in a drivetrain, and certainly not in a normal application). But if you're already used to designing with 3" tall tubing, maybe large pulley belts are the way to go for you. 2x2" tubing, or a setup like the one in this thread, are also options you can pursue. The low noise of belts is also a big appeal factor for me personally.

27-06-2016 14:42

Ty Tremblay

Re: pic: Another Belt-In-Tube Drivetrain

In no particular order:

• There really isn't a weight savings with this specific design. The pulleys are about .5lbs heavier than the sprockets would be, and the belts are about .5lbs lighter than the chain would be.
• While you should always strive to do it right, chain can be a little more forgiving in terms of tension, proper spacing, and alignment, in my opinion.
• You can run chain-in-tube with 1.5"x2" rails like this design. There are some great posts and videos by 2363 in the forums.
• I would say yes, 3" tall tube is needlessly strong. If you've got a welded chassis, 2"x1" is plenty strong even if the front and rear tubes are .0625" wall. A lot of the strength of your chassis comes from a properly designed and attached belly pan.
• When properly tensioned, belts are no more or less accurate than chain in terms of odometry.
• Chain wears in (which some people perceive as stretching) over time. This can often lead to less tension in the chain later in the season, and can sometimes be a cause of issues depending on chain load. Belts wear too, but I don't think they wear as fast and the effects of belt wear aren't as apparent in an FRC robot.
• My main reason for wanting to go with belt-in-tube over chain-in-tube is for pulley wear. In FRC, a chain drive will wear both the chain and the sprocket because most of our sprockets are made from AL. In a belt drive, the AL pulley is much harder than the rubber belt and won't wear nearly as fast (if at all).

27-06-2016 17:48

pchild

Re: pic: Another Belt-In-Tube Drivetrain

FYI, the OnShape model linked in the original post has 23 tooth pulleys, not 24 tooth. Maybe this is the discrepency?

I compared your belt thickness to the Gates GT3 Design Manual, specifically this image from page 9 and everything looked fine.

27-06-2016 17:54

Ty Tremblay

Re: pic: Another Belt-In-Tube Drivetrain

Sorry about that. I had realized that my belt model wasn't thick enough so I changed it and then realized that I had about .003" of space between the belt and the "roof" of the tube. I found 23T GT3 pulley stock on SDP/SI and decided to make that change since it gives me enough roof clearance (.043") and still gives me room for the bearing counterbore.

28-06-2016 13:03

nuclearnerd

Re: pic: Another Belt-In-Tube Drivetrain

I'm also concerned about not having rotating flanges on at least the center pulley. My old team used the Vex belt upgrade on the AM14U chassis in 2014. It worked for the season, but the belts showed some pretty hairy wear, as the sides of the two belts rubbed past each other on the common, center pulley.

28-06-2016 13:17

Chris is me

Re: pic: Another Belt-In-Tube Drivetrain

You have nothing to worry about with flangeless pulleys. 2791 has never had flanges on the pulleys and has never had a problem at all. This is one of the benefits of belt-in-tube.

As for GT vs HTD belts - the biggest thing to keep in mind is availability of particular belt lengths. Sourcing GT2 and GT3 belts can be difficult, but it is extremely easy to get HTD belting. I have heard that the HTD's deeper tooth profile is supposed to be better for reversing loads / ratcheting prevention as well, but I have no data to back this up.

If belts are properly aligned and tensioned, this just shouldn't happen. When the belts are on the pulley, they aren't moving relative to each other at all. I've never run into significant problems with a setup like this.

28-06-2016 19:25

Ty Tremblay

Re: pic: Another Belt-In-Tube Drivetrain

Does 2791 add any additional spacing between centers in their drivetrains?

28-06-2016 19:43

GeeTwo

Re: pic: Another Belt-In-Tube Drivetrain

This looks to be like a quite tight definition of "properly". If the holes aren't bored a lot closer to square than many teams can probably manage, the belts on the drive shaft will migrate one way or the other, and shall end up rubbing on the bearings (not TOO bad, but not optimal), or each other (as described), or both (one goes to a bearing, the other is jammed against the first belt, with the same results). The 'both' case actually seems rather likely as I work through the variations.

29-06-2016 10:31

Chris is me

Re: pic: Another Belt-In-Tube Drivetrain

A quality live axle drivetrain should be striving to get those bearing holes as concentric as possible for maximum efficiency and best performance anyway. This is significantly easier in a tube drivetrain, where both "sides" of the drive are actually the same piece, than it is in a drivetrain with multiple side plates. These efficiency gains are one of the subtle benefits of a "west coast" tube style drivetrain, and it absolutely isn't out of reach of teams who have some access to machining resources and put an emphasis on quality.

I've heard adding .005-.010 can help, but 2791 has always run them exact center with no additional length. The belts initially seem a little looser than a brand new chain, but that's just how belts like to run. One of the pitfalls of manually tensioning belts is that it is somewhat easy to over-tension a belt, weakening the whole system, because it seems like belts should be tighter than they actually need to be.