front steering or rear steering

[fora]

hi

I was wondering how do teams decide whether they want front steering (i.e tie rod is in front of ubj and top ) or rear steering (i.e tie rod behind ubj)...has this got any relation with Ackerman adjustment

i know this sounds stupid but i got confused with the explanation given by milliken that tie rod ball joint must be placed either on the top,rear or bottom front


can somebody help me out

thanks

[Big Bird]

Hi fora,

This may be a bit difficult to explain without drawing my own diagrams, but I'll give it a go.

Firstly, your steering axis will have a caster angle of somewhere in the vicinity of 6 degrees, with the lower upright bj forward of the upper one. The steering arm creates a moment about this steering axis, so it is advantageous to place the tie rod at a large moment arm around that axis. This means, due to space constraints inside the wheel, the steering tie rod either has to be at the top front, or the bottom rear. (You can't place your tie rod forward of the bottom because caster means that the bottom upright bj is already in the forward part of the inner rim space - and vice versa for the top).

As for Ackermann, do you want positive Ackermann (inside wheel steers a greater angle than outside), or negative ackermann (outside wheel steers more than inside)? Most go for positive Ackermann for FSAE use.

If you want positive, it means that your tie rod bj's either must be closer to the centreline of the car for a rear steer, or further from the vehicle centreline for a front steer (i.e deeper inside the wheel). The below is the first case, i.e. rear steer, postive Ackermann. Given the space constraints we have inside our wheels, it can be more difficult to bury the tie rod bj further inside the wheel to achieve positive ackermann with a top steer, (i.e. forward mount)


http://upload.wikimedia.org/wikipedia/commons/thumb/7/70/Ackermann_New.jpg/439px-
http://upload.wikimedia.org/wikipedia/commons/thumb/7/7...ermann_New.jpg/439px

The way I think about these steering arm angles is as follows. If you look at the above diagrams, for a left hand turn:
On the left front (inside) wheel, the steering tie rod is pulls the steering lever arm AWAY from the perpendicular
On the right front (outside) wheel the steering tie rod is pushing the steering lever arm TOWARDS the perpendicular
We will assume a given steering input gives the same displacement of the steering arm on both inside and outside wheels (which would only be true if the tie rods were infinitely long, but a fair assumption to get us going).

For a given steering tie rod lateral displacement, a steering lever arm swinging AWAY from perpendicular will always swing through a GREATER angle that a steering lever swinging towards the perpendicular. That is a useful rule of thumb to work by, and is a useful way to work out whether a lever arm arrangement will give positive or negative Ackermann (better than just remembering by rote anyway).

The answer is in knowing trends in your sines and cosines. If it isn't obvious why, try this exercise. Imagine the steering lever arms on the above diagram are 100mm long, and both are at 45 degrees to the axle line. (Therefore, if we projected the steering arm bj location onto the axle, it would fall 70mm inside the steering axis bj on both sides of the car). We have an infinitely long steering tie rod, and we shift it 30mm horizontally to the right to turn into a left hand corner.

On the inside wheel, we have now moved the projection on the axle line has moved 30mm to the right - so it is now 70 + 30 = 100mm inside the steering axis. Since the lever arm itself is only 100mm long, that means it is now parallel to the axle line and therefore it must have swung itself (and therefore the wheel) through 45 degrees.

On the outside wheel, the steering arm pushes the steering lever arm towards perpendicular - and the projection of the steering arm bj on the axle is now 70 - 30 = 40mm inside the steering axis. Obviously it has turned less than the inside wheel - the steering arm would have to finish perpendicular to the tie rod (i.e. move 70mm) for it to have swung through 45 degrees. In the end, it has probably only swung through around 30 degrees or so.

I hope this helps - I could explain it a lot better on a whiteboard, but we don't have that luxury.

Cheers

[fora]

thanks a ton BIG BIRD ...that was really helpful....

now if i want to keep front steer with +ve ackerman ...what method should i apply to find the geometry of the steering ( position of rack,angle between rack and tie rod ,distance between tie rod bj on upright and upper control arm bj)...

please give me direction ...how should i get these parameters

[Composites Guy]

We (like Geoff) build a super light carbon car. We choose our tie rod location so that when the wheel is heavily loaded (the outside wheel in a turn) the tie rod will be in tension. That way you can make a lighter tie-rod because you are less concerned with buckling.

[Infinity]

Originally posted by fora:
thanks a ton BIG BIRD ...that was really helpful....

now if i want to keep front steer with +ve ackerman ...what method should i apply to find the geometry of the steering ( position of rack,angle between rack and tie rod ,distance between tie rod bj on upright and upper control arm bj)...

please give me direction ...how should i get these parameters
Position of the rack can be either Top or Bottom and either Front or Rear. I guess you are clear on Front vs Rear issue but i shall elaborate on Top vs Bottom. I feel that Bottom is better because it is totally away from the driver's feet so there is no problem in entry and more importantly exit from the car. The angle of the tie rod is decided by the Bump Steer Concept i.e. the Tie rod must pass through the instantaneous centre so as to avoid any steering due to Bump/Droop.
If you wish for Frnt steer with +ve Ackermann then the Steering Ball joint would be outboard of the Steering Axis. Now Position of the Steering Ball joint(Distance b/w tie rod and Steering Axis) would depend on the Ackermann you wish to have.
I hope that answers your query about all that.

[Rishi Bigghe]

can u please upload the photo again.....i really need this help and i really appreciate ur effort.....

[Rishi Bigghe]

people .i read ur thread.....but m still confuse on how to put the rack and pinion ahead of wheel center....i have tried a lot on steering arm design but still m nowhere.....can anyone of u help me out that how to put rack and pinion ahead of wheel center.....and satisfy ackerman

[RacingManiac]

See my crude MS Paint sketch
http://www.pbase.com/racingmaniac/image/114782184/original.jpg

In practice:
http://www.fsae.utoronto.ca/20...uction/Upright01.JPG (http://www.fsae.utoronto.ca/2007/pictures/2007%20Construction/Upright01.JPG)
http://www.fsae.utoronto.ca/20...uction/Rolling05.JPG (http://www.fsae.utoronto.ca/2007/pictures/2007%20Construction/Rolling05.JPG)

As Aseem's post(from 2 years ago!) said, to achieve +ve ackermann with lead steer you need to put your toe pickup outboard of the actual steering axis, further into the wheel. Packaging wise it might limit how much ackermann you can have especially if you have a wide toebase, because you might run into clearance issue with braking components and perhaps wheel itself. We chose to package the rack up top to eliminate the u-joint on the steering column, at the expense of the ease of getting in and out of the car(no problem with egress though). They've since gone away from this in 2009 due to the templete rule. The older design also facilitate easy packaging for camber adjustment on the upright side.

[DrGadget]

Hi guys. This has made very interesting reading however I have not been able to work out why my steering will not work as it should do.

I originally had my steering attached at the rear of the wheels by the bottom BJ as installed by the Manufacturer.

I have had to change this due to an engine change and the sump being in the waty of the original steering components.

I have made some new arms that although connect at the bottom of the front strut they come up to about centre of the wheel. I have obviously had to fit a front mounted steering rack.

The problems I have is that with the front wheels in the air when turning left both wheels seem to be doing what tyhey should. When turning right however the o/s wheel does not turn as much as it should and the N/S wheel turns excessivly.

I checked to find the center point of the steering rack before starting. Then using a tape measure I adjusted the TRE's so the distance at the front of the wheel was 132cm and at the rear 131cm giving very slight toe out.

However when the vehicle was put back on the ground BOTH wheels were pointing inwards. When I tried to drive the car the O/S front wheel was straight but the N/SA front wheel was trying to turn right. Obvciously the vehicle is undriveable like this.

Pictures of my current set-up can be found here:
http://www.mk4zzoc.co.uk/wayne6.asp

Would connecting a bar with ball joints to the rear of the wheels at the original steering points help to keep the wheels operating equally?

Confused.Com!

[Dash]

Sounds like you have a slight problem with ackerman ( read the second post in this thread again ). Also, by the pictures its not clear if the end joints on the steering rack are evenly placed on either side of the centerline of the vehicle. And finally, there is something called bump steer. That would explain why the wheels change angles when its placed on the ground than when its in the air. If you read up on these things, you might be able to fix your problems.

[Adambomb]

Originally posted by DrGadget:
Would connecting a bar with ball joints to the rear of the wheels at the original steering points help to keep the wheels operating equally?


So you're talking a front mounted rack with rear mounted steering arm pickups? Aside from doing bad, bad things with Ackermann and bump steer, you will have the hilarious function of having the wheels steer left when you turn the wheel right, and vice versa. Don't do that http://fsae.com/groupee_common/emoticons/icon_wink.gif

I hate to be the bearer of bad news, but it is all gone wrong! Dash is right, bump steer is the reason your toe is changing when the suspension compresses. And by the way, 1 cm of toe out is by no means at all "slight;" never mind the amount of dynamic toe you're getting from what appears to be an enormous amount of bump steer. I would consider 0.15 cm to be a reasonable amount of toe, and can't think of a situation where I would ever want more than 0.3 cm.

But I think this is for the best, as structurally the current setup looks incredibly dangerous. Even if you got the alignment to a drivable state, the first time you hit a pothole the steering arm is going to bend. I wouldn't run that on our 375 lb car, let alone a multi-passenger car that has to survive on public roads at sustained high speeds. The effective steering arm radius is WAY too small. The smaller the distance the tie rod connection point is from the steer axis, the larger the force is on the tie rod (and thus the steering arm on the upright).

Ditto on the rack mounts. I can't see the top mounting on those pieces of flat stock the rack is bolted to, but assuming a best case scenario of them being solidly mounted at the top as well as the bottom, you have a piece of flat bar in bending along its weak axis. Remember, this will have several hundred pounds of loading on it, and what happens when the rack moves around while cornering? That's right, steering compliance, the OEM's favorite way to add understeer.

So you're going to have to totally re-do the rack mounts and the steering arms. You will have to consider bump steer for starters. When looking at it from the front, as a very rough estimate (this won't get you perfect, but a good deal closer at least), try to get the tie rods more parallel to the lower control arms. Basically you want them to be following similar arcs as the suspension moves. That will get you pretty close.

Then move the tie rod pickup points on the uprights as close to the inside of the rim as you can to get as much steering arm as you can.

As for Ackermann, that's something beyond what I feel like describing here, so you'll have to search that one out on your own. http://fsae.com/groupee_common/emoticons/icon_razz.gif If nothing else, if you don't feel like whipping up some diagrams, you can always make your steering arm mounts with, say, 3 holes in different locations in a line that follows the tie rods and play around with it until it "feels right."

As for structural design, boxes are your friend! Make everything boxed, including rack mounts, and you should be safe using 3mm sheet throughout.

[sTaRk]

I read all the replies above but didn't find the answer to the question posted about the angle between steering rack and tie-rod?? Can some one tell what will happen if I put the steering rack more towards the drive keeping the steering arm length same i.e. decreasing the angle between rack and tie rod to something less than 180 degrees??? http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Owen Thomas]

Your U-joints will bind.

But seriously, if you re-read what Big Geoff wrote in the second post and draw a quick diagram of your proposed steering system and uprights (real simple from above), the answer will become apparent to you. I just did it myself! Look at the arc each of the outboard steering pickups will take when going from 0 steer to some angle, like 45*. Now consider the arc the tie rod will have to follow, and that the rack (inboard pickup) is also translating. If you have ackermann at all, you will notice that the Y-distance between vehicle centreline and outboard steering pickups is no longer equal side to side, and one wheel has been steered more than the other.