STIFFY chassis assembly... final cut!

[zeropointbug]

Hey guys, just wanted to post a screenshot of my chassis i designed on 3D CAD. It's not a great pic... is there a free image hosting website i can use?

Anyways the chassis weights 420 grams LEST the motor, the motor weights another 410 grams too.

I have tested it using simulation software ALGOR DesignCheck, and the chassis braces have a breaking strength of roughly 3000 lbs. However, i made the front connecting plate relatively weaker (intentionally) to buckle before that, and not to snap the screws. I wanted the weak link to safe the rest, because the plate would be much less expensive to replace.

The rear connect plate is for the Quark 125B controller, I will be applying Arctic Silver 5 to it, and using zip ties, strapping it on there; the entire chassis will act as a heatsink integration ;)

The pic is a back view, the motor on it is the 7XL 540C, i modeled it up quickly just to get a relative measure.

zeropointbug

[Gustav]

looks good!

couple of things,you'll want provision for a steering servo in the front upper plate probably.And i'd want to extend the lower plate forward so you've got cell slots all the way to just behind the steering posts,even if you only planned to use 6 cells per side it'd be worth having the ability to shift weight forward/backward,apart from that you might get power hungry.

The only problem with using a chassis as a heatsink for escs is that you've then got no shock protection between the chassis and the esc(ie not good).On one of my trucks i have the esc mounted to a metal plate which acts as a heatsink which is then attached to the chassis via little posts that have rubber bushes between the plate and the posts,that gives some shock protection.the posts themselves can flex a fair bit too.They're actually fuel tank posts with the rubber bushes to stop the fuel foaming.

[zeropointbug]

hey Gustav, im glad you like it!

About the battery slots, i don't ever plan on using 'cells' anymore anyways. I know it may look like it was meant for NiMH, but they're not. In fact I'll be standing my cells (A123 M1's) upright close in the center, there is still room for 'ultra capacitors' if want to invest in them. $130 for a 20V system, not bad, and extreme power.

The servo mounts, I kinda like having a barrier between the servo and the ground, that's just me though, most ppl probably prefer on the upper plate though like you said.

The ESC mount, rubber mounts are not going to stop shocks to the electronics, they will merely damper higher frequency vibrations; nothing much really. If you really want to protect it from shocks, make a mount that is suspended with springs from either side, and the springs connect with rubber grommets. :027: I've actually though about doing that...

I was originally going to make the parts from 7075-T6, but i've read around the web about horror stories that it can just blow about with a bang without warning. That is of course with excessive fatigue, and, the 6061-T6 is better at copping with fatigue. 7075 would have a clean break, were as 6061 will bend more before it breaks. If i could afford it, i would use 2024-T3 aluminum, this stuff is is like steel, it's light, and has superior fatigue resistance.

Here is another pic of top view. If anyone wants to view a 3D file of the assembly, just let me know. I can email it to you.

zeropointbug

[mothman]

Looks awesome! DO you have any particular place to get the chasis machined? I have some projects too that I need some one with CNC mill to cut for me.

[zeropointbug]

Mothman, thanks for the comments. Yes, we have a machinist in town, (I live in Canada), he has CNC, he can machine anything you want really. I would suggest finding someone locally though, just a suggestion.

zeropointbug

[rc4x4nut]

Originally Posted by zeropointbug

hey Gustav, im glad you like it!


I was originally going to make the parts from 7075-T6, but i've read around the web about horror stories that it can just blow about with a bang with not warning. That is of course with excessive fatigue, and, the 6061-T6 is better at copping with fatigue. 7075 would have a clean break, were as 6061 will bend more before it breaks. If i could afford it, i would use 2024-T3 aluminum, this stuff is is like steel, it's light, and has superior fatigue resistance.

Here is another pic of top view. If anyone wants to view a 3D file of the assembly, just let me know. I can email it to you.

zeropointbug

IMO the 7075-T6 would be the way to go if designed right. Look at the supermaxx parts. You rarely ever hear about their parts failing. Beautiful chassis though. I wouldnt skimp out of its design and material. Of course this drives the cost up.

[Gustav]

That mount i was talking about definately provides a lot of protection against big shocks too,the whole assembly is very springy,the bushes are squishy and not done up tight and you can flex the whole thing considerably with one finger.The posts are plastic and flexy and the whole thing can spring about in a tumble,it takes the edge off the blows.It seems to have proved reliable in the truck,in real life.A casio G-Shock floats in a rubber bush i believe,similar principle.

Anyways..the 6061 vs 7075 thing,i've always looked at it like this,(disclaimer,it's just a theory from my little brain);it's possible to make a 6061 part equally strong as the 7075 part at very similar weight,it simply takes more volume of material.7075 is a similar percentage stronger as it is heavier to 6061.So using 7075 is not strictly necessary to get a part that's of similar strength and weight unless you're restricted to paticular dimensions or specifically need the surface hardness,such as in a diff cup where 7075 is required.7075-T6 still seems to bend long before it snaps in RC car chassis,so i'm not sure i'd be worry too much about metal fatique,it doesn't look like it'll be flexing a whole lot.i'd just descide based on strength:weight,cost,availability,machinabilty.

[zeropointbug]

rc4x4nut, thanks!

that's true, i guess it doesn't matter if 6061 is not as brittle as 7075, if it bends, it bends, it still fails. I suppose a bent up part(6061) is just as useless as a broken part(7075)... :007:

I'll see how much difference it will cost on the entire chassis if i made just the bottom chassis plate out of 7075, and the rest 6061.

What do you think, should i go with 7075-T6 for the bottom chassis plate?

It would prob only be $15 more... really. The .25" (chassis braces) stock is too expensive though. If i had ENOUGH money, i would make it out of 6Al 6-6-2 Titanium, ~200,000 psi yield strength. :017: And more flex than a chinese acrobat!

zeropointbug

[zeropointbug]

Gustav, that's true about 6061 and 7075 typically being similar in strength for the "typical" part design. HOWEVER, IF you design it right (which takes skill, and experience, and 'feeling') the stresses (such as impact) should be transfered in such a way as to avoid modulus stress, and basically transfer it as a tensile force. And use the material to it's capability. That would allow 7075 to fully show it's stuff.

6061-T6 typical tensile yield: 40,000psi 2.7g/cm3

7075-T6 typical tensile yield: 83,000psi 2.74g/cm3

2024-T3 typical tensile yield: 70,000psi, 2.77g/cm3 superior fatigue

I have tested the bottom chassis plate mayn times with ALGOR designcheck, and managed to get VERY distributed stress, while at the same time keeping the motto "form follows function", and design dictating functionality.

triangulating, circular coring, and tapering are the main features you want to keep in mind, and the most important part, knowing how to apply, and integrate them.

So i think 7075 it is then...

zeropointbug

[coolhandcountry]

how do you plan to make the trans or shaft work?

[rc4x4nut]

I would definitely stick with 7075 with the entire chassis.

[Gustav]

What material was the 420 grams for?

It looks like it would be extremely beefy in 7075,maybe a little over engineered if you see what i mean,unless you're wanting it to last forever,it probably would in the the 7075-t6.
2.5mm 6061 might be enough for the upper plates if you wanted to keep those sacrificial,before they started to bend screws and knacker the threads in the chassis rails.

it's like a viscous cycle though in terms of weight.The stronger and heavier you make those upper plates and the skids,the stronger and heavier the rails need to be,if you kept the thickness of those down then you might be able to lighten up the rails more since the the plates would be the sacrificial part.crumple zones:005: I would want it lighter since you're going to all this effort to make an efficient design,otherwise you're probably overengineering in terms of the strength it needs.A g-maxx comes in at like 230 grams doesn't it? it's something like that.

[zeropointbug]

Yah, it is a viscous circle really! I just want 7075-T6 on the bottom plate strictly for abuse reasons, after all it sees all the action.

Here are the weights on each part:

Drive shaft (steel): 20 grams

Chassis plate (7075): 140 grams

Chassis braces: 93 grams (x2)

Front connect plate: 7 grams

Rear connect plate: 27 grams

motor/gear mount: 23 grams

rear bearing mount: 10 grams

Total weight: 413 grams

HOWEVER, i do not need a transmission. I am using a direct pinion/spur setup as it is the simplest, and strong enough. Im thinking i am going to make a new shaft setup to allow for Revo slipper, and maybe a V1 slipperential.

But, if anyone likes the general design, and would like to have one, i could perhaps make a more 'standard' design. Such that it will take a tranny, slipperential, etc. and upper level servo mounts.

cheers!

zeropointbug

[zeropointbug]

Oh, and my wheel base is 350mm

[jnev]

Very nice work with the design. It looks liek it should work well. Are you sure though, that you only want room for 6 cells on each side? Or is this enough room for lipo or other kinds of batteries?