Where to mount the steering rack in a formula car

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imdni8
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Joined: Mon Mar 21, 2016 6:18 pm

Where to mount the steering rack in a formula car

Post by imdni8 » Mon Mar 21, 2016 7:27 pm

Hi, we are participating in a student formula event, and we are currently designing the frame. I am the responsible for the steering and while deciding the place for mounting it (keeping in mind the pedals, feet of the driver and a arms), I realized I don't know enough about it. And so, here I am, writing this question, hoping to know more about the engineering behind -
1. How to decide where to mount the rack if what we need is to minimize toe during jounce and rebounds and bump steer.
2. How does the tie rods (its length, its inclination w.r.t rack) effect the steering as a whole.

Thanks :)

Greg Locock
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Joined: Fri Jun 29, 2012 11:48 pm

Re: Where to mount the steering rack in a formula car

Post by Greg Locock » Tue Mar 22, 2016 7:13 am

Time to get your books out. But I'll give you some advice. First decide on the front view geometry of your suspensions. Then see if you can package a rack to suit your targets for bump steer (and a target of zero deg/m is fine at this stage, the changes you are likely to need are a few mm here or there in height once you have a neutral location and the correct tie rod length).

Now in plan view decide how much Ackerman you want, and shift the rack backwards and forwards until you get that. If that all seems to work and you haven't installed the rack where the driver's feet are, or other more important problems, you need to refine your targets a bit and come up with some accurate dimensions. Otherwise you go back to the first step and try something else.

There are theoretical advantages in having the rack behind or in front of the wheel centre, for FSAE don't worry about that, packaging is far more important.
Last edited by Greg Locock on Tue Mar 22, 2016 11:07 am, edited 1 time in total.

Jolle
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Joined: Wed Jan 29, 2014 9:58 pm
Location: Dordrecht

Re: Where to mount the steering rack in a formula car

Post by Jolle » Tue Mar 22, 2016 9:45 am

Don't forget about safety. With a head on crash the steering column must not come toward the driver.

imdni8
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Joined: Mon Mar 21, 2016 6:18 pm

Re: Where to mount the steering rack in a formula car

Post by imdni8 » Wed Mar 23, 2016 10:33 am

@Greg Lolock
I don't really have much choice with the height, but I'll try to address the problem the way you said.

Now in the second para, you say "decide how much Ackerman you want", actually how I go about it is that we first decide the track and wheel base roughly and then using simple trigonometric relations, the Ackerman angle is found. Can you elaborate your way of deciding Ackerman.

Tim.Wright
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Joined: Fri Feb 13, 2009 5:29 am

Re: Where to mount the steering rack in a formula car

Post by Tim.Wright » Wed Mar 23, 2016 11:00 am

If you are restricted with the rack height you are pretty stuffed. Have you designed your uprights yet?
Not the engineer at Force India

Greg Locock
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Joined: Fri Jun 29, 2012 11:48 pm

Re: Where to mount the steering rack in a formula car

Post by Greg Locock » Wed Mar 23, 2016 11:06 pm

Yes I could tell you all sorts of things about Ackerman, but for a race car they don't matter. At least one successful builder/racer reckons parallel steer is best, and then uses toe out as fake ackerman, depending on the circuit. Another successful race engineer uses a swing on toe out (tries 3 different settings) to get a handle on Ackerman, when he has time.

If you haven't designed the spindles yet you may be able to position the outer tie rod and steering arm correctly relative to the rack.