Rack and pinion right location VS Bump Steer

[Arturo Mendieta J]

Hey guys, just wondering...
I`m confused about the right location of the Rack, well we decided to put it upwards to the drivers legs (not in the floor anymore) and backwards from the front wheels axis I`m concerned about its effect in the following:
1. Bump steer
2. Toe in / Toe out
3. Right Ackerman geometry
Is there any math model to find the right height of the rack?, I read that the track rod joint must to travel parallel to the upper ball joint describing parallel arcs to prevent BS, in what cases is this valid?

http://fsae.com/groupee_common/emoticons/icon_confused.gif

[Kevin Hayward]

With all of these concerns doing studies in a kinematics package is a really good idea. Either use something off the shelf or program one yourself. The maths for a double a-arm is not too difficult (think 4-bar linkages in 3-d).

To locate the rack for minimum bump steer the general rule is have the steering arm lie on a line from the steering rack attachment to the instantaneous centre of the suspension. Also draw a line front view of the suspension between the inboard mounting points of the a-arms. The rack point of the steering rack should fall on this line. Do the same for the upright points and the outboard steering rod points. This is easy to see in a diagram and there are a fe diagrams in some of the useful texts.

However Ackermann will screw around with this simplification a little. My suggestion is to use this ass a starting point then fiddle around with it in a kinematics package until you are happy. No matter what you do in some point (especially some applied steering) there will be some change in toe due to vertical movement of the wheel relative to the chassis.

As for Ackermann I believe it is becoming almost standard to have 100% or more. 100% being where the intersection of the steering arms meets at the centre of the rear axle line. It can be handy to build an upright that has adjustable ackermann if your team has never played around with it.

As for toe-in/out ... test ... it's changeable afterall.

A couple of extra points on the location of the steering rack. Look at what a side load will do in terms of compliance. Your rack and joints etc will have some slop ... unavoidable, but can be minimized. It is useful to look at what will happen toe and camber wise when a side load is applied to your tyres with the rack in the location it will be. Last point I want to bring up is toe change due to camber change. A good way of placing your rack is to have it parrallel with one of the a-arms, with the camber adjustment system on the other arm (or chassis/upright mounts for the other arm). This means that when you change camber there will be minimal or no change in toe. This can be very handy in testing when changes need to be made quite quickly to get the most out of testing. Simple rule when testing cars if a change is difficult and/or takes a lot of time it will not be made.

Hope some of this helps.

Kev

[Arturo Mendieta J]

Thanks a lot Kev your orientation was very useful I'll try as you told me, now I'm going back to work, next iteration is awaiting for us

Greetings from the other side of the ocean

[J. Vinella]

You also might want to look at how your driver's legs will hit the rack.

We have our rack mounted in a similar position. Our tallest drivers have problems with their legs hitting the rack. I'm pretty short but in bumps my legs sometimes hit the rack. Nothing some good padding couldn't fix. http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Arturo Mendieta J]

Thank U, we are making our best for keeping safe the driver's integrity, and you are right I most think in Adan: the tallest driver we count with (1,92 m) I'm short too (1,63 m) and I take in account the padding stuff, I believe that a hit in that zone of the leg shuold be very painful http://fsae.com/groupee_common/emoticons/icon_eek.gif and have a negative effect in drivers concentration http://fsae.com/groupee_common/emoticons/icon_rolleyes.gif, thnks my friend http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Infinity]

Regarding same matter, I have a problem. I adjusted my rack parallel to the upper suspension so that i had the simplest of the minimal bump steer geometry but later at the manufacturing stage i realised that the rack cannot be placed at the same level but at a level slightly up aur slightly below the desried position. The previous positon was just above the driver's legs so there may be problem in lowering and moving it up will make the rack out of the chassis.Suggest something?

[pablo180]

To locate the rack for minimum bump steer the general rule is have the steering arm lie on a line from the steering rack attachment to the instantaneous centre of the suspension. Also draw a line front view of the suspension between the inboard mounting points

Infinity,
I would suggest that you first take into consideration all the drivers legs positions and then begin to design your system after a consensus among the drivers with respect to the rack location has been met. After that you can then setup you links in the way that was descibed by Kevin a couple of posts earlier to address the bumpsteer issue.

An image to help out the description above.
http://www.thedirtforum.com/images/bsdrawing.jpg

Determining Ackerman setup is a little more difficult and is best determined by testing. As an initial guide though I would recommend at least 100% or greater.

[Infinity]

Thanks Pablo,
But I have kept my steering rack at the top not at the bottom. I have considered all these but since it is our first time so we came across a problem while installing it. So i still need to keep it at the top position but i have to move it a little upwards or downwards. So i need to know that if i keep my rack little above the chasis does it make any effect?

[pablo180]

Originally posted by Infinity:
So i need to know that if i keep my rack little above the chasis does it make any effect?

As long as your tie rod projection meets up with the instantaneous center of the A-Arms then you will have minimal bumpsteer.

If you put your rack above the chassis, like mentioned above, you are going to have a tricky time figuring out how to get the tie rod projection to line up with your instant center depending on the orientation of your a-arms. Specifically at the joint at the upright.

[rjwoods77]

I coined a term for it. Instant center convergance. Sounds good to say in the design report.

[C.Zinke]

But instant center convergence does not mean that you will have no bump steer. In our car we have no convergence http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Infinity]

In any case my bump steer cant be zero as the mounting cannot be done at the specified place but still does my shifting of rack produce large effect or a minimal effect? Also How is the track( We will participate in the June-2007 in California) so is the track bumpy enough to provide for this??

[Ax]

hey guys,
i know im bringing up a topic thats bin pretty much dragged by now, but anybody thought bout the forces acting on the rack when u keep it where u've kept it? As in considering yer bump steer and ackermann is set as desired, do u have to worry bout the forces now?
for ex, when u have a floor mounted rack in which the steerin tie rod outboard joint is in the plane of the lower ball joint which is highly loaded, shouldnt u worry bout kickback at the steering wheel?