Has anyone done the calculations (or taken measurements) to figure out what track lengths are possible (from center to center of sprockets)?
It looks like the tracks are assembled from pieces about 1" long that are connected with rivets. Given this configuration, there will be wheelbase lengths that work, and others that will be either too long or too short.
The calculation will be different for the two different sprocket sizes, and the result will be a function of the number of track sections to be connected. Does anyone have this function for each sprocket?
I should probably just buy the tracks before designing the chassis, but I thought I’d see if someone has already worked this out.
if its to long, well you just add an idler between the two sprockets, that pull the track toward the sky. if its too small well you add links till its to big then do the same thing than when it’s to long.
I would suggest many idlers on the botton of the track (the one that is on the floor) so that it will hold and only one on the top so it will be kept well.
If you absolutly want to do your desing before hand and, lets say you want to do one triangular like Jim’s, than you will have to ask him the exact positioning, either way you should put idlers on the botton, it will help.
The sprockets shouldn’t really affect the wheel base. The two sprocket sizes both should be matched to the length between the sprocket holes in the tracks. Track size will be a multiple of the size of the section that have the sprocket holes.
This will be true for the 6 link sprocket as 3 links will wrap exactly half way round one and 3 around the other socket. this would mean an exact fit from centre line to centre of each sprocket. I’m not sure it will work for the 9 link sprocket.
From experience though, designing to absolute measurements can cause real problems. If there is no leeway in the distance from centre to centre then the tread may be too tight. The ideal would be to construc something similar to the tension adjuster on a motorcycle chain. This would allow for a degree of inaccuracy and also for wear and tear as the treads age.
Idlers serve more than one use. On a tank, they serve to control the sideways movement of the treads as well as keeping the treads pressed against the ground.
If you dont have idlers then all the weight will sit on the front and rear sprocket. The links in between will not really achieve any traction.
The idlers should in reality be spring loaded to help press them against the surface. Fixed position idlers could cause a rather bumpy ride over an uneven surface.
That’s exactly the kind of imprecision I’m trying to avoid.
A given number of track sections will definitely have a different wheelbase (or track length to say it differently) with the two different sprocket sizes - the larger sprockets will yield a shorter wheelbase because more of the track is in the vertical dimension.
I don’t follow what you’re getting at here. I thought the large and small sprockets both were compatible with the same tracks? I was talking about the LM tracks in case you missed that, or maybe I have a bad assumption.
I guess I’m looking for something a little more scientific- as I eluded to, a function of n where n is the number of track sections. I just did some math and came up with numbers that I think are pretty close, but I had to make some assumptions that may be not correct. I admit I was reaching here … I just wanted to design it all first to make sure I don’t get the wrong stuff.
I guess will definitely need the tracks in hand to make anything final. Once the tracks are assembled on the sprockets it will be easy enough to take precise measurements.
Thanks for sharing your thoughts. I’d still like to hear from anyone who may have taken more of a mathematical approach to this problem.
scientific… it would be fun to have it perfect, but if you calculate it like this, its almost sure it wont fit if you dont have the tracks in hand. but, imagine you wna tot put a lot of weith on the robot, then Idlers on the botton would help to support, instead of only 2 sprockets we have lets say 5 to support the same weight. so the sprockets wont bend and make you loose
I had considered this in my design. I was aleady planning to overbuild the in-between part.
This bot will be a lightweight carpet rover, so while precision matters to me, I don’t think spring loading the idlers is necessary. A chassis suspension is another story though (for another thread)…
This brings up another question I have… what is the ideal amount of tension? I’ve seen a lot of real tanks in action, and there is definitely some noticable slack there. I agree that too tight is bad.
I do have CAD software, but I don’t have much confidence that the track models are completely to scale. If they are off a little, I won’t be any better off than I am with pencil and paper.
Cool, that’s what I thought. I’ll just order them and work it out. My chassis is going to be costly to fab so I need a solid prototype. I’m about to order some electronics so I’ll just grab a little more track gear than I think I need…
1). Tooth spacing on the sprocket and the tooth holes in the track must be matched up. You can have sprockets with varying numbers of teeth fit the same track (look at the sprockets on a 10 speed bicycle rear wheel).
2). In large tracked machines, the the sprocket drives are not on the ground. The wheels between the sprockets support the weight of the machine and roll along on the hard surface provided by the tracks.
That’s an interesting point! Ground clearance can be a lot better if the motors don’t have to be at the bottom, but the CG is higher. I’m guessing it also reduces the load on the drivetrain.
A lot of tanks do seem to have the trapezoidal track layout with a raised drive sprocket(s). The triangular Johnny5 design is another option. I need to meditate on this some more and decide how much extra compexity it’s worth building into the design.
The main reason for raised drive sprockets is for the drive mechanism to be fitted.
If you imagine on a 4 by 4 vehicle how the axles are always the limiting factor on the ground clearance. Essentially your ground clearance becomes half your wheel diameter because the drive shafts always have to reach that point. On solid beam axles the bulge of the differential was your ground clearance.
By driving wheels that are elevated you eleiminate that problem.
If you think of the Jonny 5 design with the motor at the top of the triangle, in theory you have a vehicle that is water-safe up to the outer casing of the motor. Obviously in practice its not quite that much but…
Personal recommendation based on my own recent experience, if you want a good fit with a minimum of tweaking of idlers and hole spacing then buy a length of track and a couple pair of sprockets and build a prototype from wood or spare plastic. I went the do it in CAD route and while I am not really disappointed I certainly can see where some changes could be made for the better. I may eventually go back and have new mounts made but right now I am moving forward with what I have to find out if I have other things that need to be changed.
It’s for timing belts but the exact same theory applies. The distance between the 2 axles. Double it then ad half the circumference of both sprockets.
The problem is calculating the proper distance between axles to match the spacing in the track sections. Each section is roughly 1 inch as I believe it is a 1" pitch. I think in reality the distance between each track joint is just a hair over an inch. You would have to confirm that with someone who has the track.
I know just what you mean. When I was designing my aluminum bot head, I had several revisions and still found things I would change. Designing new products involves trial and error AKA R&D. This can get expensive real fast.
According to the advert for the track sections, a 21 link piece measures 23 inches which gives 1.095 inches per link but that doesn’t say whether the 23 inches is from centre to centre of the hinges or the overall length of the plastic mouldings.
The information you are referring to is simply the length if the tracks linked together laying on the table. however what you want is the pitch, which is 1.07" I do not know, or have the ability to accurately measure the actual radius the track follows around the sprockets.
Very well put zoomkat! Some people confuse the term idler with passive sprocket. It’s only required to have a sprocket, driven or passive, at each end of a tank style, or at three points in a triangular style set up. The sprockets should not be used to support the vehicle. Doing so will work, but it will go “whump, whump, whump” as it moves, not very smooth… Use idlers to support the vehicle, with or without suspension. Softer rubbery tires can be used as idlers for a cheap “suspension system”.
it would be fun to have it perfect, but if you calculate it like this, its almost sure it wont fit if you dont have the tracks in hand. 
but, imagine you wna tot put a lot of weith on the robot, then Idlers on the botton would help to support, instead of only 2 sprockets we have lets say 5 to support the same weight. so the sprockets wont bend and make you loose