I will make the rockers for my front suspension this week. I orginally planned to use 4130, but I'm now considering 6061.

I'm using bike dampers that use 550 lbs springs but I leveraged it down to the normal wheelrate.

The rockers will be two plates on either side of the damper, thus not requiring any welding. My question is this: Does 6061 seem up to the job? It would be much easier to work with than the 4130...

--Chris

I have no clue if the aluminum would be strong enuff, but the thicker the better.

On another note, Ive been playing with a bunch of designs, but I just cant settle on the ratio, and the distances. You wouldnt happen to be able to post a drawing of your design, could you?

It would be hard to say whether 6061 is good for your application. I assume you are talking about buying 6061-T6 plate and just cutting it to shape? Are you intending to bush the joints??

The T6 material is much better than the T4 material.T6 is going to be less than 1/2 strength of 4130 steel and probabl a third as good in bearing, i.e. at the bolted joints.

What im saying is that if you had a 4130 part 3mm thick which was just strong enough in terms of bearing then you may well need 9mm of 6061, given the material density you wont save much weight or energy in terms of manufacture. You can make steel bushes for the alloy and reduce the thickness but youve then added extra work in manufacture and it needs to be accurate.

As i say, really you need to do some quick sums to see what loads and thickness you needin order to build some confidence. These are some of the highest loaded parts you will find on your car!

It might be worth you looking at getting 7075-T6, its probably about 85% tensile strength of 4130 and 80% of the bearing strength, i.e. more than double the 6061.

I hope ths is of some help chris, feel free to contact me.

Cheers

Chris

Thanks for the pointers, Stressy. I would like to use the ali because it is easier to work with and it is much cheaper. In my mind, I'm having a difficult time figuring out how I would use a bushing.

I'm using R6 bike dampers, and some that I bought came with the steel rockers from the bike. The ratios won't work but the idea will (I hope!). It does not look like Yamaha used any kind of bushing. Do you have any suggestion as what can be used? Oilite?

To figure out my ratios, I used this thread. I worked on the front for quite a while. Have a look at Matthew_1's photo archive. I basically swiped his design idea and applied my specs. In CAD--at least--it works a treat. If you have a broadband connection, I can send you my SW files in zipped format.

7075 it it (if I can find it)!

--Chris

Chris F says he wants to us Ali because it is easy to work with. Isn't 7075-T6 Ali hard to cut because of the alloy and heat treatment?

Another thing to look at might be softer springs with your existing rockers, or at least something a bit lighter than the bridge girders 550lb springs may require .

I know the FSAE kids here in town got custom coils wound for less than US $25 apiece.

Cheers, Ted

I may be exploiting my inexperience here, but surely 7075 is easier to saw through than 4130.

I suppose I will try it. If it looks questionable, I can switch to 7075 at a later time.

Any ideas on how to use bearings?

--Chris

Alan B would be a good guy to ask about oilites as I think he uses them a lot in his day job (machine designer), and he's used them for his shock rockers, as has Steve Graber.

Pete

Hi there

My suspension is rocking top arm as opposed to pushrod but the figures are similar. If you're using 550 lb springs you must have quite a leverage there (3:1?) unless you're going for super high wheel rates. That's gonna put huge loads on the rocker, especially at the rocker/chassis pivot, even when the car is just sitting there!

I personally wouldn't want to see a piece of aluminium doing that job in any car of mine. Remember, unlike steel, ali has no endurance limit - whatever its loading, its failure is just a matter of time. You've got to know exactly what you're doing and have your numbers spot on if you don't want that failure to be in the lifetime of the car. Also, ali isn't likely to bend to give you a warning - it'll just snap. If you're using solidworks - have you run your rocker through the Cosmos express analysis demo? Personally i'd use steel here, and it needn't be 4130 neccesarilly.

Having said that, your design can't be that dissimilar to the downhill bike in me garage in terms of the loads and leverages. Pushrod, 550 lb spring and it has two 11mm thick 6000 series ali rockers. Nowt wrong with ali but make sure your design is spot on.

liam
Rescued attachment 2004_0809_190742AA.JPG

The comment you made as to the bike rocker being made of steel and having no apparent bushes agrees with what i was trying to suggest. i will try and explain a little better.

when i was refering to bearing i was meaning the "bearing stress" imposed on the side of a hole due to a bolt being pulled sideways against it, in shear, i.e. as you have on your rocker arms.

The capability of a joint to take these bearing stresses is dependant upon the type of material (its bearing strength) and the contact area of the bolt in the hole. Often in aluminium parts this bearing issue will be the weak link rather than say the bolt pulling off the lug or breaking itself.

In the simplest sense you can calculate the maximum load you can apply to a joint by multipling the contact area (bolt diameter x material thickness) by the material bearing strength. This number must then be divided by appropriate factors of safety!!!!!!

Imagine now you calculate you need a 2mm steel plate for your choosen bolt diameter and applied load. If you were to use ally you would need either a 6mm plate or a bolt 3 times the diameter. Clearly you would not use the huge bolt.....

......but you could use a bush.......

if you had a steel disc 3 times the diamter of the bolt diameter, which you pressed into the alumium plate, and which the bolt passed through, you would only require the steel disc to be the original 2mm thick, then when you consider the bearing stress at the joint between the disc edge and the alumium the the area would be 3x bigger than before so you would only need the 2mm of ally thickness.

Hopefully that makes sense.

If i were looking to achieve what you are after i would probably use steel for the rockers and eliminate any busing issues that may occur. Working with a thick peice of 7075 would be pretty hard anyway. I would consider running the either oilites or needle rollers at the fulcrum point on the chassis.

I was unable to get to the files you refered me to so im still not too sure of you situation but this might be some food for thought.

If you want a chat chris just drop me a u2u with a contact number and let me know good times to call, i'll see if i can be more usefull!

Cheers mate,
Chris

Thanks for the picture. My idea was much simpler, though less elegant. I don’t think I’m stressing the rocker too much and my ratio seems reasonable. This is how I came to my rocker ratio.

Leverage 1: Pushrod angle= (1/cos 51.44)^2
Leverage 2: SAL/IC=(98.3/95.65)^2
Leverage 3: Chassis to ball joint/chassis to pushrod pickup= (16.25/12.73)^2
Leverage 4: Rocker ratio= (1.25)^2

Wheel rate (Leverage 1* Leverage2* Leverage3* Leverage4) = 554 coil rate

So I get a wheel rate around 80. All my bits come in this week, so I hope I did this correctly! Just to get it rolling, I may use the 6061 or 7075 (though it seems a bit more difficult to come by.

--Chris

Chris just as a word of caution.
Chances are the rocker will be the most highly loaded part on you car, if the geometry of a rocker deviates too far from "good practice" the loads will go through the roof and you can get problems with bearing stiction.

As i say before iwould stick with steel unless i was confident in the loads applied and had done the calcs.

I dont mean to put you off in anyway i just wanted to make sure you dont get any nasty suprises.

Happy sawing,
Chris

a simple thing to consider

car mfr's spend many hours simulating and testing bits before a car is launched.

you get one shot, and your life could be on the line if its not right.

err on the side of safety with a large margin for error.

atb

steve

[Edited on 10/8/04 by stephen_gusterson]

nicely put steve

Thanks for the advice. I think you folk are right in that steel is a better choice. You commented that the rocker may be the most highly-stressed area of the car. Thinking about it, you are probably right. So I think a prototype in 3mm mild steel will be just the ticket. If it feels well, then I will make the final bit in Chromemoly.

The advice on brearing stress was very good ideed. For suspension pickups and the like, I took Uncle Ron's advice and just used what the book suggested. But I see what you mean about bearing stress. I was afraid the rocker would eventually gouge the bolt if no bush was used. If I understand correctly, this is not really the case.

I finally created a photo archive. You can see my rear suspension design that I hope to complete this week. The front will be done next week. Many thanks to all.

--Chris

[Edited on 10/8/04 by chrisf]

Nice to see the pictures you have put up. I would really like to use inboard suspension on the front when I get that far but am not sure if I know enough about it to be prepared to go that way. Would be great if someone made a kit for it - how about it GTS!