My B4M
 

I made a mid motor conversion of my B4.2. It started out as a fully printed chassis, but I could never get it durable enough. After many revisions, I ended up making something like my own Centro... go figure.

I've been running it for about 6 months and haven't broken a single part I have made.

The printed parts are PETT (Taulman T-glase), and the aluminum bits (chassis, rear camber plate, and rear bulkhead) are 7075.

After getting sick of breaking stuff in the earlier revisions, I started FEA stress analyzing everything in Solidworks with a 2000 psi limit (and lots of optimizations based on this and all failures stopped. I was really amazed at how much difference the various tapers, etc. made on the printed parts (or any part, for that matter). Sometimes a gram of plastic here or there could reduce a stress hot spot 5:1. I guess with small parts on 1:10 cars, lots of features are stress concentrations! I was also surprised at how much material wasn't needed. There was certainly more material removed in optimization than removed.

http://i58.tinypic.com/288v7ma.jpg
http://i61.tinypic.com/ibz7r5.jpg
http://i62.tinypic.com/14918a8.jpg

Here is my original plan to make a fully printed chassis. The chassis broke in the middle (delamination of layers). Eventually, I bonded thin aluminum sheet to the top and bottom. The failures just moved to the shock tower mounts and hinge pin mounts. Oh well -- it was a fun design.

http://i59.tinypic.com/2j3jiqc.jpg

Very intrested project.
Jerzi

Nice work :thumbsup: I like the idea of the tub - and I suppose you did re-use the gear casing (=3-gear mid-motor)? How does it drive?

The tub is very nice, so... Have you thought about reinforcing the weak areas of the 3D print with some layers of fibre mats and resin? Or using the 3D printing for moulds to make a CF tub and keep using a 3D printed top chassis?

Anyway, keep us postoed! And I'd be curious to see more pictures or a video of it :)

3d printing a mould for carbon would be a good idea, oh for a 3d printer to play with..... mdf for me for now. carbon matting isnt that hard to work with really, have a play.

YOu could do a fan mount for the motor (post/pic 1) in the upper part. Kinda like rdrp did a printed one for the 210 that hasn't come out

Just a bunch of thoughts with no particular overall message:

Using carbon is interesting to me -- I have made some carbon things, years ago, but it's been a while. When I look at carbon, I see two reasons to use it:
1) It's light
2) It's easy to make crazy shapes with it

The light part wasn't a big driver for me at the beginning. I'm actually using ballast weight on this car, so it's a matter of where to put the weight instead of just lowering it.

Having said that, I'm interested in centralizing and lowering the mass, so I suppose that just means I'm motivated to lower the weight if it is not in the middle, bottom of the car. Maybe making a chassis that is focused on a lower part count chassis that has a really light front end would be interesting.

The making crazy shapes part is really addressed well by 3d printing, so I'm less motivated there. I think 3d printing a mold sounds really fun, but I don't have enough time to attach new technology, just to do it

That is a very doable project. It's a bit of an old picture; the fan was cooling a 17.5 motor that had seen better days. I have a new motor in there now and cooling isn't much of an issue. I do use a fan on my mod car's ESC. It seems that ventilation is something I need to think more about in future projects...

I do use the 3 gear transmission. I think it drives well, but my driving ability limits how well I can tell. I'm typically back of the A main level at my local club (before and after making my own car). When I first made it, it was very clear that it how lower inertia -- it starts turning in very fast, but in a stable way. I find it generally easy to drive. The biggest challenge for me is the lack of setup sheets. It really shows how good or bad I am at setup. I lately have found that extending the wheelbase and moving the weight rearward helps. I find that helps making it a bit lazier, which helps with my middle aged reflexes.

I ended up making it about 2 mm shorter wheelbase than a B4.2 (by accident), so I suppose that I'm just finding that stock might be right.

I don't think that the 3/4 gear makes such a huge difference, but don't have back to back comparisons on the same car to really tell. Having said that, I focus on 17.5 classes, and they have lower gear ratios, which lowers torque reactions to motor acceleration, compared to typical mod gear ratios. I always feel like my mod car (same design) wants to kidnap me and murder me, so maybe that's the 3 gear and higher gear ratio at play...

I haven't really seen the need, in my project. With PETT and some of the newer materials, overall strength isn't such a big deal. But I find areas with stress concentrations are not doable with 3d printing, like hinge pin pivots and shock tower mounts. Reinforcing doesn't help much in those situations, as tearing out threaded holes and crushing from hinge pins it a stress concentration issue, unless I'm missing something.

Having said that, I would try it in a second, if I could figure out a way it would give me a genuine performance increase. I think there is a compelling reason to use CF shock towers because of how high the mass is, even if I don't currently.

keeping the weight low down is one advantage of the layout my 2wd project uses, the slipper and spur gear are very close to the chassis plate, the only things really that are of any weight high up are the shocks. weight can be a problem with a carbon chassis, my latest chassis is really like a feather, whole car is under 1300g so it needs a good bit of weight added. i'm sure i read somewhere of a method of strengthening 3d printed parts by bathing in something like acetone breifly, to effectively increase the bond between layers by chemically melting them together, not sure what the print material was though

I REALLY like your project. It actually got me talking to people to see what I could scrounge up to make a project like that. I have a leftover B5R chassis and a Durango DESC410 rear end sitting on my desk to see if I could come up with a slightly different spin on the same idea :)