AWD Pushrod front suspension design

SpecBC · Nov 22, 2011

Hey Everyone,

So I have tried to do some searching but haven't come up with concrete information. My only current book is Chassis Engineering by Herb Adams which is no help in this question.

I wanted to ask people with first hand knowledge of this. I am looking to build an AWD tube chassis super car and for packaging purposes I believe a pushrod design will be necessary. Unfortunately there are not a lot of real world examples I can use to copy.

I was looking at the Aventador suspension for inspiration and I was wondering does the angle of the pushrod matter as long as the bellcrank is at the same angle as well?

It seems the ideal(common) design has the push rod originating from the center of the lower control arm at an angle to pass the axle up to the bell crank. I was thinking why not use a pushrod that is offset either to the left or right side of the control arm so the pushrod is moving 100% vertical with the suspension. I assume this is because a torque would be caused about the axle centerline if you have it offset? Or is it some other issue?

I have seen rear pushrod setups with offset rods but they also have lateral links and the upright does not turn.

Is it also not possible to mount the pushrod to the top control arm to transmit the movement? What is the issue with doing this? Clearly there must be a disadvantage for people to avoid it.

Lastly, what type of connection would you recommend for the pushrod ends? A Heim joint?

Sorry for all the questions I just don't see a lot of examples of this and figured this would be the best place to ask.

thanks for the help! Any other suggestions would be greatly appreciated.

reference images I have found:

32 hot rod

hrdp_0905_03_z+1932_ford_coupe+brakes.jpg

Aventador:

full link to lambo setup:

http://blogs.insideline.com/straigh...borghini-aventador-suspension-walkaround.html

Ben

WolfHR · Nov 23, 2011

Just a quick note before more knowledgeable members join in. Re. attaching pushrod to upper control arm- you have a terminology issue here... Most commonly, when such mechanism is attached to the upper control arm the bellcrank is usually placed below the control arm and then the link is being pulled instead of pushed in bump, and then it's called pullrod, not pushrod. I think Newey's most recent Red Bull F1 cars feature such design on rear suspension.

Toddr8541 · Nov 23, 2011

Wolf you are partially correct. You can still have a pushrod system by placing the attachment on top of the A-Arm. You could do it several ways but the a-arm would incorporate both the a-arm function and bell crank function in one part. Early F1 cars did this in the 60's or so.

WolfHR · Nov 23, 2011

Hehe, but it doesn't have a rod in it. (if it's what I'm thinking about, like Lotus 25 front suspension with inboard mounted coil-overs... it seems to me the downside might be maximum bending momentum on that control arm) On a more serious note, Pagani Zonda used proper bell-crank shaped UCA on rear suspension, as can be seen here:

http://1.bp.blogspot.com/_hHNtbIcXa...Zonda_C12_S%2BUnderhood_Engine_Suspension.jpg

.

There was a variation of pullrod mechanism in early '70ies*- a bit earlier than, in my knowledge, 'proper' pushrod/pullrod linkages were used, designed by Len Terry in order to get a progressive spring characteristic. I think he might have designed it as an improvement of rather complex progressive rate system one of McLaren F1 cars used in '71... It didn't use bellcrank as one would expect, so I'll attach a picture in case it's of any interest.

SpecBC · Nov 23, 2011

Thanks WolfHR, that is actually like a design I was thinking of(on the Pagani but obviously not exactly like that).

In terms of the terminology I still don't understand how the setup on the Zonda would be a pull rod setup. Unless in this situation the control arm is technically the "pushrod" and they don't count that...

Is there an issue in terms of motion? As in would this be less than a 1:1 ratio? I would assume it would be very slightly less than 1:1 if you had unequal A arms. Would this cause a potential issue and if it is can a 1:1 ratio be re-captured with the right bell crank geometry?

thanks again for the help this is giving me great design ideas.

Ben

GregLocock · Nov 23, 2011

There's no particular magic about a 1:1 MR, except it is the only unambiguous MR (half the world talks about wheel/shock, the other half the inverse).

Cheers

Greg Locock

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SpecBC · Nov 26, 2011

The reason there is discussion on motion ratio though is to achieve acceptable amount of travel correct?

I assume you want to avoid a situation where you say only have half the motion of the wheel translated to the suspension otherwise you need twice the spring rate/damping ability?

thanks for the help and input!

Ben

NormPeterson · Nov 27, 2011

Try four times the rate . . .

Norm

SpecBC · Nov 27, 2011

^^^Was that supposed to be a helpful suggestion or a challenge, I'm not sure what your intent was.

thanks

Ben

GregLocock · Nov 27, 2011

It is the correct answer, if you change the spring/wheel MR to 0.5:1 you will need to change rates etc by a factor of 4 compared with 1:1

Cheers

Greg Locock

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SpecBC · Nov 27, 2011

Good to know thanks I assumed that was the other possibility haha. It's been years since I did any real engineering calculations(obviously). Luckily I did find a lot of my old engineering textbooks from when I was in ME to reference but I figure a lot of this specific motorsports info is not available in those. I have a large cart full of books to order on amazon as well.

I think I like the design and simplicity of the Pagani upper control arm design as it was close to what I originally envisioned but wondered why more people did not utilize it like on the Aventador if it was a viable solution.

thanks again for the help, this will be a long project but it will be fun.

Ben

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