Bump Steer, Steering Rack and Tie Rod location

[Briggs09hp]

Hello,

2009-2010 is my first year designing the suspension for our car. I have read Miliken's book and a few others, but aside from that, I really do not have much experience or knowledge about the details of steering systems. I am wondering if you guys could tell me how you are avoiding or dealing with this problem:

From what I understand from the 2010 rules, there are only two possible positions that the steering rack can be mounted: either above the drivers legs (high mount), or in front of the pedals (low mount). If you mount it in front of the pedals, you would have to construct some sort of awkward linkage to bring the steering column to the rack. I am thinking most people are putting the rack above the drivers legs this year.

Because of the Figure 9 template rule, it is impossible to have a steering column between your legs, correct?

In my design, I am mounting the steering rack above the drivers legs such that the tie rods and steering rack form a straight line from the top view due to Ackermann reasons.

But, since figure 9 must pass under the steering rack, the rack is higher than the upright, so the tie rod end at the upright is lower than the tie rod end at the steering rack. Because of this, when I raise and lower the suspension, the wheels steer themselves (Bump Steer).

From my understanding, the best way to avoid bump steer is to mount the tie rod ends at the same vertical height, so the steering rack, tie rods, and tie rod ends are pretty much level.

How are you guys dealing with this? One simple solution I could think of would be to build some strange arm coming off the upright that raises the outer tie rod up higher.

Is seems like 2010 is the first year that you cannot have a steering column between your legs, so i am wondering what people are doing.

Thanks for any help,

Matt Nayda
Clarkson FSAE

[Pennyman]

Because of the Figure 9 template rule, it is impossible to have a steering column between your legs, correct?

Incorrect. Make the chassis big enough for the template to clear the rack and put the rack between your legs. Read the rules again and you'll notice that the steering column can encroach on the template.

Look at some pictures of cars from last year and you'll start to get an idea of what to do. No mysterious linkage needed. http://fsae.com/groupee_common/emoticons/icon_smile.gif

[D.J.]

I have been dealing with the same problem and it is my first year designing the suspension as well.

In figure 9 there is a slot for the steering column that only goes so far (i think 350mm or something) but from my understanding it is just a guideline and the steering column can actually go all the way down to the floor. so the rack can be mounted above or below the template.

also the template goes to 4 in before the pedals not all the way to them.

I have done a lot of analysis trying to get a top mounted steering rack to work but the best I have been able to come up with was about 5 degrees of bump steer per inch travel. (i know it sounds awful but I am not sure how much it will really hurt us)

also it is not that you want the steering rack at the same height as the tie rod on the upright
i think you want the steering arm parallel to an a-arm so that the geometry stays constant.

hope this helps
let me know what you decide to go with i would be interested to know what another first year guy comes up with

[Matthew Bell]

You might want to find a copy of Competition Car Suspension by Allain Stainforth. It details the process for "eliminating" bumpsteer. We have ours down to a few thousands toe-in for one inch of rack travel up and down. This is with the rack mounted on the bottom of the chassis.

[R. Alexander]

Originally posted by Briggs09hp:
Hello,

2009-2010 is my first year designing the suspension for our car. I have read Miliken's book and a few others, but aside from that, I really do not have much experience or knowledge about the details of steering systems. I am wondering if you guys could tell me how you are avoiding or dealing with this problem:

From what I understand from the 2010 rules, there are only two possible positions that the steering rack can be mounted: either above the drivers legs (high mount), or in front of the pedals (low mount). If you mount it in front of the pedals, you would have to construct some sort of awkward linkage to bring the steering column to the rack. I am thinking most people are putting the rack above the drivers legs this year.

Because of the Figure 9 template rule, it is impossible to have a steering column between your legs, correct?

In my design, I am mounting the steering rack above the drivers legs such that the tie rods and steering rack form a straight line from the top view due to Ackermann reasons.

But, since figure 9 must pass under the steering rack, the rack is higher than the upright, so the tie rod end at the upright is lower than the tie rod end at the steering rack. Because of this, when I raise and lower the suspension, the wheels steer themselves (Bump Steer).

From my understanding, the best way to avoid bump steer is to mount the tie rod ends at the same vertical height, so the steering rack, tie rods, and tie rod ends are pretty much level.

How are you guys dealing with this? One simple solution I could think of would be to build some strange arm coming off the upright that raises the outer tie rod up higher.

Is seems like 2010 is the first year that you cannot have a steering column between your legs, so i am wondering what people are doing.

Thanks for any help,

Matt Nayda
Clarkson FSAE

Bump steer is minimized when the line of the tie rod intersects with the wheel instant centre. It's up to you to figure out why.

The page here should answer most of your steering questions.

http://www.woodwardsteering.co...05%20pdf%2064-79.pdf (http://www.woodwardsteering.com/images/cat05%20pdf%2064-79.pdf)

PS our steering rack is roughly under our knees.

Why do you say that you can't have it between your legs?

[Tech Guy]

Matt,
A low mount rack does not have to be in front of the pedals. It just has to be below the envelope of the template as the template passes forward through the cockpit.

I think the answer to your question about the slot in the template is answered in the FAQ's on the official FSAE web site, so you should have looked there first! However, to save you the trouble, if you notice, the drawing of the Cockpit Cross Section template in Figure 9 of the Rules has "UP" on it twice, once at the top and once at the bottom. From what I saw on how the template was used last year, with a low mount rack, the tech inspectors start with the slot in the template upwards (and it is a slot not a flap on the template I saw). They pass the template forward with the steering column passing down the slot. Once they get to about half way, they flip the template over in the cockpit so the slot is on the down side, and continue towards the pedals.

[Briggs09hp]

I did not know how they did the test at competition. Thanks Tech Guy for explaining how they invert the template. That's pretty tricky, I didn't think of it.

The reason I thought you couldn't put the steering column between your legs to go to a low mounted steering rack was because I thought the template would hit the column. But with the inversion of the template, it is fine.

I think I will change to a low mount steering rack to make it easier.

Now, I have another question. There is a figure 19.1 in Milliken on page 710 that shows a "typical tie rod location" for front steer and rear steer. It shows that for a low mount you are supposed to run front steer, and a high mount runs rear steer.

Can anyone explain why this is? Or is that just a recommendation and can you run front and rear both ways?

Also, how would you achieve ackermann with a front steer upright? Does the method of drawing a line to the center of the rear axle still apply? If so the steering arms would be angled out from the top view.

Thanks again,

Matt

[Ben Jamin']

Also, how would you achieve ackermann with a front steer upright? Does the method of drawing a line to the center of the rear axle still apply? If so the steering arms would be angled out from the top view.


This is true, I believe the Gillespie book talks about this a bit.


From what I saw on how the template was used last year, with a low mount rack, the tech inspectors start with the slot in the template upwards (and it is a slot not a flap on the template I saw). They pass the template forward with the steering column passing down the slot. Once they get to about half way, they flip the template over in the cockpit so the slot is on the down side, and continue towards the pedals.

According to the rules there is a considerable amount of "stuff" you can take out of the car so that the template can fit though. We got into a bit of a discussion with Mr. Royce at VIR this year on this topic. We brought our 2008 car which did not meet the template rule anyway so it was no problem for us. Also, the way I understood him explain the test is that the plate can actualy move verticaly up and down while it is moving horizontaly through the chassis to clear various protrusions assuming it does not come incontact with any other part of the chassis while doing so.

[Thomas MuWe]

@Matt:

I thought for a long time about this question about the tierod location at the upright.....

I will not give you the answer straight away, but think of this process...

If you steer, what happens?
If this procedure starts, what will happen with the car / especially the suspended mass?
If that happened, what about your wheel position? What is the tierod for?

I hope that helps to see why some tierod locations are better than others for some driving situations....

Best regards,

Thomas

suspension jr08 / jr08evo
joanneum racing graz

[URracing]

I'm also technically considered a little new to suspension design but have been researching it as much as I can, when I can. I read Carrol Smith's books and learned alot from them. While on this subject I'm just curious to see what most FSAE teams are running for scrub radius and kingpin offsets. Some previous members had some uprights designed and when I joined the team we went and had them machined once we found a machining sponsor. I made a quick n dirty diagram to show what I'm talking about (front view):

wheel diagram (http://s268.photobucket.com/albums/jj16/cdnbacon85/?action=view&current=wheeldiagram.jpg)

Please correct any misunderstanding of scrub radius and kingpin offset I may have displayed in this diagram but from what I can gather, I'm pretty sure I'm correct my the terminology here. You'll see our scrub radius is quite tight. From what I read you want this ideally as small as possible. I'm just curious to see what other teams run for scrub radius as well ask kingpin offset. From this diagram our kingpin is zero if the wheel is perpendicular to the ground. We realized the upright/brakes are packaged quite deep into the rim while the wheels that were ordered from Kodiak have quite a deep offset to them.

kodiak wheels (http://s268.photobucket.com/albums/jj16/cdnbacon85/?action=view&current=KodiakWheelgraphic.jpg)

I see they bolted the wheel centres to the outside of the outer shell's mounting lip. Is this structurally sound or are we going to be having issues with these wheels? I guess I figured bolting them between the two rim shells would be the best, no? I heard Kodiak has their issues with some products but haven't heard too much about FSAE wheels failing. I've also heard of failing Kaizer wheels too so I guess it's a mix up just like any other product you buy.

[Adambomb]

Originally posted by URracing:
I'm also technically considered a little new to suspension design but have been researching it as much as I can, when I can. I read Carrol Smith's books and learned alot from them. While on this subject I'm just curious to see what most FSAE teams are running for scrub radius and kingpin offsets. Some previous members had some uprights designed and when I joined the team we went and had them machined once we found a machining sponsor. I made a quick n dirty diagram to show what I'm talking about (front view):

Please correct any misunderstanding of scrub radius and kingpin offset I may have displayed in this diagram but from what I can gather, I'm pretty sure I'm correct my the terminology here. You'll see our scrub radius is quite tight. From what I read you want this ideally as small as possible. I'm just curious to see what other teams run for scrub radius as well ask kingpin offset. From this diagram our kingpin is zero if the wheel is perpendicular to the ground. We realized the upright/brakes are packaged quite deep into the rim while the wheels that were ordered from Kodiak have quite a deep offset to them.


For the scrub radius, it's a tad hard to tell from the pic, but it looks like you have it right. There is a bit of a trade off, you don't necessarily just want it "as small as possible." Think about the aligning moment caused by one of the front wheels as it begins to lock up in braking, and the effect this aligning moment has through the steering wheel. So what's a good value? Well, I don't want to "spill all the beans," this is still a competition you know! Not terribly hard to play around with it though.


Originally posted by URracing:
I see they bolted the wheel centres to the outside of the outer shell's mounting lip. Is this structurally sound or are we going to be having issues with these wheels? I guess I figured bolting them between the two rim shells would be the best, no? I heard Kodiak has their issues with some products but haven't heard too much about FSAE wheels failing. I've also heard of failing Kaizer wheels too so I guess it's a mix up just like any other product you buy.

It should be fine to mount the center on either the outside or inside of the outer parts of the rim, as far as I know. It does change the loading (effective moment) somewhat, but not likely enough to be a problem. We've done that several times with our Keizer wheels; they shipped them with what we thought was the wrong offset, but it turns out they just had the center mounted on the wrong side. I wouldn't bother trying to mount the center between the two center pieces though, notice there's likely a big bead of silicone in there to seal the two halves together. Also, think about the effects on both offset and rim width. You could do it, but personally I'd rather just order the ones I want and not mess with it!