Frederik_b

Lars:Yeah you are right there is not enough space to put a bearing anywhere there. I will see if I can machine a diff housing, with my lathe and my buddy's cnc'd milling I should be good. But I think I will solve this later and just start with the alloy diff in the rear, and plastic one in the front.

Okay so today while looking at the aluminium pieces I have collected I thought about doing some kind of roll cage as suggested by C.Lyra. I would make it out ofL bars because those allow bolting from 2 angles which is simpler. L bars are also a pretty strong shape I believe. Instead of cutting real aluminium I figured cardboard would fit nicely with wood for testing


Note the 2 long upper parts running front to rear on each side (the 2 parts that form the top of the cage) are one piece. I slit a small V in the cardboard on one of the sides of the L allowing it to fold and have an angle at 2 places: once at the base of the windscreen to make the chassis narrower around the front wheels, and once on the top to have a downward angle to the rear:

Then I would bolt a small aluminium plate where I cut the L shaped bar to make it strong again and hold the angle. I hope my description makes sense lol

Instead of mounting the top strut ball joint to the diff assembly, it would be bolted to this cage. I figured I could fold an aluminium plate to make a nose for the car. that piecewould make the front stronger (avoid side to side flex of the top of the cage). I'd do the same in the rear.

I'd probably also screw a thinner X shaped plate on the left and right where the doors of the car would be to give more strenght.

I would have to add some bracing in the middle of the car as well because right now with this design, a side impact on the top of the cage (like when rollingthe car)would most likely collapse or bend the cageon one side or the other (I have only vertical and horizontal bracings so it not that strong laterally). I would probably add at least 2 diagonal pieces, one in front of the engine and another behind.

I did not calculate the total weight of the required L bars, if I find out it's too heavy I will probably drill holes at places where there is less stress.Also the addedstrengt would maybe allowto drill some holesin the bottom of the chassis.I like the fact that even with the cardboard cage, I can feel by pushing with my hand where the chassis is stronger or weaker.







Pics of some of thealuminium parts I have collected. I also have a large aluminium plate for the frame bottom which could easily do 2 maybe 3 bottom plates.



A small 21 ccengine I planned to use in the beginning. But it has no clutch and was fairly cheap made (multi-layer stamped steel connecting rod!) so I decided to find a pocket bike motor instead.



This is my small lathe. I bought a small milling attachment for it on ebay but I have to make something to put it in place of the tool holder.




Do you have any idea on how I will be able to properly break in the engine as I will probably not be able to run the car without issues the first times!

Let me know if you have any ideas of improvement


Frederik

Frederik_b

I bought some additional aluminium L bars,2 sizes, I started with the smaller one for the cage to see if it will be strong enough. I also cut the bottom plate to size, except for the wheels cutout, I will cut those later when I am exactly sure if the dimensions.

I used duct tape to test the assembly

It's pretty much the same setup as the cardboard one. The aluminium is 1/8 thick. I test fit the body on it and it fits correctly. Still have to make the front, side and rear supports.

Also I noticed therectangle aluminium tubing you see in my last post in the aluminium parts pic is exactly the width of the FG diff so I should be able to use it to make the diff holder.

Frederik_b

I did some modifications to the cage, I added a fold in the rear to narrow it down a bit to make space for the rear struts.

I also thought about using whole 1/8 plates on both sides instead of separate posts at each corner, and added bracing near at the angle near the front struts (1/8 inch plate running left to right and a L bar under it for more strenght). I will most likely remove some material from the sides as it is not needed I think that those panels be full.

Still have to find a way to make something in the front to link those bars to the bottom plate, let me know if you have ideas. Those 2 L bars in front end up with an angle downwards and an angle inwards so.. may end up doing just like the cardboard model.





I have been thinking for a long time about how to make the upper strut mount strong enough give the fact I have a ball joint instead of the bearing only. Ithought about adrilling a hole the size of the outer diameter of the ball joint plus another 1/4 inch hole in the bottom. Then I slide the ball joint in the hole and I can bolt it from the other side of the block.



a pic with the body




Fred

Frederik_b

Just wondering, I remember the Lotus Elise actual car is built with mostly aluminium which is bonded with some kind of supernatural glue. Do you think some of the parts could be held with glue if it was strong enough glue?

Also I was thinking that for some longer parts like the side plates attached to top, would it be suitable to use rivets instead of bolts,like each 1.5 inch ? For parts that never need disassembly that would be simpler than drilling and bolting everywhere. Maybe if I glue with contact cement and add rivets that would work on some of the parts ?

But then when stuff breaks is't harder to repair.. Just thinking out loud.

Fred

troppo

aluminiun is a real ******* to glue unless you use something like a sikaflex type adhesive, the problem is those products take days to cure, the curing time is a ******* but we use it at work for building truck bodies (in combination with rivets).
Bolts are always a good bet for fixings with aly just remember to put washers under the nut and bolt head to prevent chafing, rivets are good but tend to stretch and loosen over time with any stress

Frederik_b

Thanks for the tip, so is this kind of glue available to ordinary people ?Is it insanely expensive? Bonded+rivets may be a nice solution for some parts of the chassis that I will never have to disassemble (never say never though). I understand that rivets alone would loosen so I won't go that route.

Today I made the nose with a 1/8 plate and a L bar to join it to the bottom plate (it''s not really visible in the pics)

Added another brace on top of the cage on the last fold close to the rear wheels.

Also made the rear link between cage and bottom plate.

The chassis as it is in those pics is 12.8 lbs. It's kind of hard to tell how strong it will be, I think it will be pretty stiff, I just dont know how it will resist to shocks, especially with the heavier engine.

Am I going to be ok with 2 large scale servos to run the steering ? I bought them used on ebay but don't really know what theyre worth.. One is am HPI SF-1 and the other one is a MS-X7 BB. I also have the red MCD one I snapped a pic in the parts pic in the previous page.

I am also going to need a proper radio(with fail safe), I have to read about this as I know there is information in here on this topic.

Fred

Frederik_b

After googling a little bit I found out those servos I just bought on ebay are not large scale servos as I have thought... the seller only sell 1/5 stuff so I took for granted that those were large ones but didnt think it could be tiny throttle servos...

The MS-X7 BB(did not find much info)

Multiplex MS-7-BB-Servo

Specification:
Size: (L X B X H) 40.5 x 20 x 37.5mm
Weight: 47g
Torque: 70/84 (5/6VNcm) on another side I found a torque of 7.0 Kg
Speed: 0.15/0.12 (4.8/6V40sec
Double Ballraced

And the hpi SF1 iswhat is installed stock on small hpi rc. Low speed low torque.. It sucks.. will do my research before buying next time..

Fabe

Hey Frederik, looks like things are coming along well over there.

For servos, a relatively inexpensive choice is the Hitec 805BB servos. I've got a single one to steer my large scale (20+ pounds) and it's alright. For the price it's definitely not bad.

In terms of failsafe and rx/tx, there's also a lot of options. Typically most use GHz systems for the large scales. I've got a Spektrum DX3.0 that has it's "return to preset positions" failsafe. Although that's a step in the right direction, I'd suggest adding an additional component such as the picoswitch from Dimensional Engineering (can be obtained from robotshop.ca), which can be set to kill the engine if signal is lost.

For the aluminum, I'd say go with a few bolts for the moment then when you know where you want everything for sure based on trials, have it welded by someone with a dedicated welder.

Keep ot up though, it's looking good...

Frederik_b

Thanks for the tips on servos and rx-tx. I plan for mechanical disk brakes too, I would think a big servo would be required for braking, am I right ?Also the throttle on the engine is pretty hard to pull. Makes for a built in spring return fail sage Iguess

Also if the chassis now weights close to 13 lbs with nothing on it I expect it to be pretty heavy. If I ever want to put dual servos on steering, do they need to be the same model ?

edit: I see arout 170$ for the Spektrum DX3.0 on ebay, are those the normal prices for this kind of radio ? What would you pay for a used one ? IS there any thing to watch out for when looking on ebay for those ? Like when new, they come with the receiver only ? (no servos)

I also see one for 120$but it says SPEKTRUM DX3.0 DSM 3CH Radio with SR300 RECEIVER while other ones says SR3000 receiver, I don't know if its a typo or a different receiver,

Thanks

Fred

troppo

Sikaflex should be available in any decent hardware place, its just a high quality liquid nails type adhesive. I`d rather spend a little more to know i have a decent quality bond than save a few bucks and have it fall apart while its being used.
The big buck compounds are the best of course They use them to glue real cars together) but theres no need to pay that much when a little ingenuity can make a joint thats just as strong.....

da mad maori

looking good man... love the scale.. looks very tough too...

Fabe

Can't say I'm too familiar with the other brands, but I've heard of others using Hitec, Futaba and JR radio systems as well. Someone might be able to chime in with a suggestion for cost-benefit analysis...you're basically looking for something that will not be susceptible to engine interference, and will have a large range. DX 3.0 radios are a tad expensive - you don't need their servos becuase they're too small. The SR300 is their "budget friendly version" whereas the SR3000 is a standard rx included in the box with the DX3.0 (at least the one I got).

A large scale servo is required for braking depending on your system requirements. For example, an HPI Baja can get away with a standard size servo. However, a bit extra torque, and perhaps more importantly, speed, never hurt anyone. If your throttle is hard to pull and you can't fiix that then yeah, you'll need at least that much torque to pull the cable. However, don't only rely on the spring for a failsafe, since the servo may get locked in place and not return to neutral if signal is lost. If the spring IS strong enough, then chances are you're also going to eat up some power just pulling the spring during normal driving.

Ttam Says Blarg

Man keep going! I love it when I see custom builds like this.

Frederik_b

Fabe:

Thanks for the information about the servos and receiver, this might be just the radio model I am going to order, price seems ok and it appears to be proven a good radio. I will probably wait a while before buying because I have lots of mechanical work to do for now but thanks for this valuable info.

Troppo:

Yeah I think I will stick to bolts and nuts simple and effective

Ttam Says Blarg:

Thanks! I did some testing of the subaru type suspension with solidworks, seems to work well. Only problem is the camber is ok when in middle position, but when compressed or extended, the strut have a negative camber which is not good for handling (see third pic)

This is due I think to the fact that the strut is completely vertical, my guess is I should put the shock at an angle with the knuckle so it gets some camber when compressing. At least I see I dont have mechanical interference between the trailing link (the one attached to the bottom plate) and the other arms.

Last pic is an example of what is on a production wrx (modified parts but the geometry is standard). What I like with the ball joints and multilink/macpherson suspension is there is less suspension components to machine, more simple threaded rods. Ans also the fact that it's what is on the actual car I'm trying to replicate !

Also yesterday I started to make the engine mounts out of aluminium flatbar (3/8 thick I think) but not much more than what is on the previous pics.









Fred

Frederik_b

Figured out the way I am going to mount the diffs. Will use 3/16 aluminium plates with holes to fit bearing supports in it. The bearing supports will be bolted to the diff support plates allowing some play for adjustment.

Please ignore the 2 out of place blocks that holds the strut top and the chassis to the lower arms, theyre there just to hold the assembly.

I copied the bearing support piece so you can see how it will be made. It will go slightly inside the diff support plate to give enough room for the bearing.

The part you see on the right is what is going to hold the larger sprocket, because there is some distance I thougt the shaft might flex so I will install one bearing on each end of a machined holder for this shaft. I also mounted the diff the wrong way in the assembly... but you get the idea



Fred

Frederik_b

I found a picture the other day of a cutaway shock and I thought about doing my shocks the same way. There arerubber seals and also 2 brass sleeves.I have 3/4 inch aluminium tubing with 1/2 inch inside diameter that I will use as the shock body,



I thought about finding large 1/2 bolts that I could machine to make a cap at the bottom of the shock tube, and the other end of the shock.
Step1 : the bolt, complete
Step2: remove excess material
Step3: Drill a hole slightly larger than the shock rod
Step4: Machine the inside of the bolt to hold the brass bushings and rubber seals shown on step 5
Step5: Machine the brass bushings and find appropriate rubber seals
Step6: the final result that would screw in the top part of the shock tube

Pretty much the same for the bottom cap exceptthere is no hole and there is a small notch for a rubber seal (see the complete shock assembly, there is a small notch at the bottom). Thinking about it, there should probably be one seal like this on top too.

Then would have to find clips that would hold the brass bushings. What do you think about it ? I will try to find an correctly sized and good grade bolt type that I can machine. I will probably buy extra of those bolts and maybe try to tap the aluminium tube with actual bolts.. not sure it's going to work, I will probaly have to buy a tap.



And some tests of the general layout, with one front wheel and one rear wheel suspension done





Frederik

Lars from Norway

Looks like you have a very good plan on the shocks!

Frederik_b

Thanks, the easy part is the plans!Now I need to try it and see if it works. There are 2 things I am worried about in the design:

1- Is this0.2 inch diameter steel rod going to be strongenough for this strut design (a lot of stress is on this rod I think because it has to endure side forces, like hitting bumps sideways). I have not found which kind of steel I will use for this part.

2- Oil being incompressible, when the piston push in, the rod takes space inside the shock body so there is less space for the oil, that oil have to go somewhere? Should I simply leave some air on top of the shock (not filling theshock completely)so the air can take the compression caused by the rod? Then the air will probably mix with the oil and affect the dampening, but eh, it will still dampen

On my smaller shocks (1/8 kyosho mp6 and the old 1/10 nitro stampede) there was a rubber diaphragm separating the oil from the air but I dont want to go that complicated. Let me know if you have suggestions about this.

Thanks

Frederik

traxxaspede

I know it would be hard, but some shocks have the little reservoirs that help with that. I think the reservoirs are filled with air, and have a little spring with a seal plate that get pushed back and allows oil from the main shock into the reservoir.

Frederik_b

Yeah I understand how it would work, but having a strut design I am not sure how I could attach a reservoir, the bottom must stay free to allow space for the driveshaft, and I cant really drill the side because this is where the piston will travel. Maybe I'll get a burst of imagination at some point to add a reservoir

Actually I might be able to machine the bottom capso it goes out of it sideways(machine an elbow inside the cap). A small center hole inside, and a small hole on the outside that would join with the center hole, then tap the side hole and screw in a small angle tube and attache a reservoir. Why not Might start with a simpler shock with the mixing air to start with. Will see later how it turns out. Thanks for the idea!

Fred

Frederik_b

Ok so reading Lucian75's home made project thread made me realize the gear ratio I was about to use was very high, like 17:1 high...

Engine sprocket: 7 teeth
Mid-sprocket: 30 teeth
Small mid gear: 18 teeth
Diff gear: 48 teeth



EDIT: ok after verification all my calculations are wrong used the wrong formula for circumference lol

So a random google search it looks likea chinese pocket bike engine revs around 8000-9000 rmp max, but Im really not sure about that. What would be a good starting point ratio considering I'd be using MCD rally wheels ? DVP you mentioned around 6 is usually good but I have much smaller wheels (5.5 inch diameter)

So I did some calculations, please tell me if I'm correct:

5.5 inch diameter = 3.1416 x (2.75 x 2.75) = 14.13 inches circumference
9000 rmp / 6 = 1500 wheels rotation per minute = 32 kmh

Is it the 9000 rmp that is not realistic ?? Looks like I would need like 4:1 to have a speed of 50 kmh...

Let me knowif my assumptions make sense

Thanks!

Fred

Dirty_Vinylpusher

I've never been one to do calculations on stuff like this.
Just try it and see.
Set it up so that you can easily change one of the sprockets to change the ratio.

I recently got hold of an old pocketbike and it was geared at 10:1 actually.
I guess they don't rev that hard but have pretty good torque.
Just think, if one of those pocketbikes can carry 100kg+ at 50km/h with 10 inch wheels, at a gear ratio of 10:1, I think you can gear your car at 4 or 5:1 and get it to move pretty quick.







But someone who's more mathematically minded can probably shed a bit more light on the situation.


Loving the project, but I wouldn't have the patience to build everyting out of wood and cardboard first.
I'd go crazy within a day.

[&:]

VassilisG

Dirty_Vinylpusher

I was searching a bit about this earlier and found this

Frederik_b

Dirty_Vinylpusher: Yeah I think the second sprocket will be the one to change easily to change the gear ratio. I'll try to find a smaller one so I get a better starting point then my current ratio and we'll see then. I'll be looking for a total ratio of 4 or 5 (3.8 would give 50kmh at 9000 rpm)

About the wood haha yes I noticed you testing material is preferablysteel I usually have a lot of preparation before doing somehting to make sure it works out ok. It can still not work when I do it lol but still Ienjoy doing the testing with all those materials. One could argue that I do too much drawings but I also enjoy the drawings hahahah so that's all good.

Thanks for the pic about the twin tube shocks, I never realized before that there was this extra space for oil (and air I guess because something has to compress). I wonder what happens if you take this kind of shock upside down, will the air come into the main cylinder ?


VassilisG : Thanks for the tip on the washer, that's a neat idea! Also I'll try to find some good steel rod material for the strut.

Fred