Steering Rack Location-Opinions Needed Please

[LafayetteFSAE]

Hey guys,

My name is Will, and I'm a Junior at Lafayette College in Easton, PA. We competed last year at VIR, finished about middle of the pack IIRC...well anyway, myself and another Junior MechE student are doing a chassis/suspension design for a technical elective(since we do not have a team for this year).

One thing I'm wondering about is where some of you guys place the steering rack to eliminate bump steer and keep it out of the way of the drivers shins. On our current car it is hard for tall drivers like me (6'2") to drive it due to my shins hitting the shield that is on the rack.

So we are looking into a floor mounted rack, and we are not sure since our school has always done the rack mounted high up with the tie rod running about 1/2" above the upper a-arm, and basically parallel to that a-arm.

Another thing, I was reading in Milliken that for a floor mounted rack you want the tie rod to attach on the front side of the upright, and the opposite for a high mounted rack. Any truth to this, or does it not matter?

FWIW we will be running a Planar front suspension for next year. We are a very new team, so any thoughts/ideas would be greatly appreciated

Thanks,

Will

[LafayetteFSAE]

Hey guys,

My name is Will, and I'm a Junior at Lafayette College in Easton, PA. We competed last year at VIR, finished about middle of the pack IIRC...well anyway, myself and another Junior MechE student are doing a chassis/suspension design for a technical elective(since we do not have a team for this year).

One thing I'm wondering about is where some of you guys place the steering rack to eliminate bump steer and keep it out of the way of the drivers shins. On our current car it is hard for tall drivers like me (6'2") to drive it due to my shins hitting the shield that is on the rack.

So we are looking into a floor mounted rack, and we are not sure since our school has always done the rack mounted high up with the tie rod running about 1/2" above the upper a-arm, and basically parallel to that a-arm.

Another thing, I was reading in Milliken that for a floor mounted rack you want the tie rod to attach on the front side of the upright, and the opposite for a high mounted rack. Any truth to this, or does it not matter?

FWIW we will be running a Planar front suspension for next year. We are a very new team, so any thoughts/ideas would be greatly appreciated

Thanks,

Will

[LafayetteFSAE]

Some pics from our 07-08 car:

http://i149.photobucket.com/albums/s58/bluestang2002/Formula%20Car%2007-08/DSC00155.jpg

http://i149.photobucket.com/albums/s58/bluestang2002/Formula%20Car%2007-08/DSC00152.jpg

[JamesWolak]

Your going to have a hard time trying to mount that rack on the top side of your chassis with the new template rules (the whole point of the rule was to prevent teams from being able to do it).

You are correct Milliken does suggest that the rack should be on the front side of the upright. But then with traditional rack suppliers for FSAE your steering input would be backwards (you would be turning left and the car would turn right). This can be easily fixed but most teams with lower mounted racks mount it on the backside for simplicity.

The judges (and we have had Claude the last two years) haven't called us out for having it on the backside. What i would worry more about is your bump steer and that you have linear steering inputs. It looks that your input isn't liner on last years car.

But I am just a lowly power train boy though so take this with a gain of salt.

[Mike Macie]

Last year we had a "high nose" setup with a low rear steer rack. Basically our foot pan was a couple inches higher than our lower wishbone frame mounts which left plenty of room for the rack and master cylinders. The RCVD book suggests those locations so that any camber compliance will cause an understeer reaction. I think a low rear steer rack is easier to package, pro ackermann geometry easier(more so with 10's), tie rods are more protected from cones, and you have more options with your pedal assembly.

[LafayetteFSAE]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by JamesWolak:
Your going to have a hard time trying to mount that rack on the top side of your chassis with the new template rules (the whole point of the rule was to prevent teams from being able to do it).

You are correct Milliken does suggest that the rack should be on the front side of the upright. But then with traditional rack suppliers for FSAE your steering input would be backwards (you would be turning left and the car would turn right). This can be easily fixed but most teams with lower mounted racks mount it on the backside for simplicity.

The judges (and we have had Claude the last two years) haven't called us out for having it on the backside. What i would worry more about is your bump steer and that you have linear steering inputs. It looks that your input isn't liner on last years car.

But I am just a lowly power train boy though so take this with a gain of salt. </div></BLOCKQUOTE>

Would you be able to tell me the rule # that pertains to the steering rack?

Also, can you explain more about how our steering inputs aren't linear on last years car?

Another thing I'm concerned about is if the rack is mounted on the floor and attaches to the back side of the upright(so a standard rack w/out modification would work), how can you get the steering shaft down at such a steep angle, I feel that a single u joint would lock up due to the large angle

[RBbugBITme]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Another thing, I was reading in Milliken that for a floor mounted rack you want the tie rod to attach on the front side of the upright, and the opposite for a high mounted rack. Any truth to this, or does it not matter? </div></BLOCKQUOTE>
You shouldn't have enough compliance to worry about this, its for rubber suspensions on road cars.

You shouldn't use just one u-joint and I'd bet with the steering wheel angle you'd be comfortable with it will be impossible. Two high quality u-joints can get you about 60 degrees though. We attempted to do a rack ahead of the pedals/behind the FBH to keep the solid shaft but we couldn't get the ackermann we wanted with our wheelbase so we'll be a high nose low rear steer car as well.

[LafayetteFSAE]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by RBbugBITme:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Another thing, I was reading in Milliken that for a floor mounted rack you want the tie rod to attach on the front side of the upright, and the opposite for a high mounted rack. Any truth to this, or does it not matter? </div></BLOCKQUOTE>
You shouldn't have enough compliance to worry about this, its for rubber suspensions on road cars.

You shouldn't use just one u-joint and I'd bet with the steering wheel angle you'd be comfortable with it will be impossible. Two high quality u-joints can get you about 60 degrees though. We attempted to do a rack ahead of the pedals/behind the FBH to keep the solid shaft but we couldn't get the ackermann we wanted with our wheelbase so we'll be a high nose low rear steer car as well. </div></BLOCKQUOTE>

Ok thanks a lot, using 2 U-Joints makes sense for sure.

[Zac]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by RBbugBITme:
You shouldn't have enough compliance to worry about this, its for rubber suspensions on road cars. </div></BLOCKQUOTE>

Are you sure about this?

[RBbugBITme]

Yes.

[Zac]

That's interesting. From the data I've seen I'd say that the typical FSAE car has enough compliance to make me worry about how it's going to influence my cars performance, but to each their own.

[RBbugBITme]

Most teams don't have access to a K&C rig so the best thing they can do is shoot for difficult stiffness goals on their uprights, good load paths into the chassis, and avoid these "dallara style" steering rack joints. 90% of the teams use pretty much identical spherical bearings/rod ends and similar sized steel control arms so we're all going to be in the ballpark of each other in terms of compliance.

That still doesn't really hit the topic at hand though, did you avoid low rear steer on your car because the compliance was so bad you worried about inducing oversteer in every corner?

[Zac]

You'd be surprised at who can get access to a K&C rig.

I agree with you on the main point of your last post, that there's an upper limit as to what is achievable for the typical team. But from the K&C data that I have on multiple cars(from several teams) I think it's fair enough to say that many teams are either not setting aggressive enough stiffness targets or achieving the targets that they set. There's actually about an order of magnitude difference between a podium finisher and a middle of the pack car.

Maybe I wasn't clear in my last post. All I'm saying is that if you want to use a low rear steer arrangement, that's perfectly fine as long as you understand the other implications it will have. Even if the compliance doesn't dominate your response (in my case it does not) it is more than likely a non-negligible effect.

[LafayetteFSAE]

So what precautions would be need to be taken if we decide to go with a low rear steer arrangement? What harmful effects are there that should be tried to be designed around?

[BillCobb]

The two cv joints need to be at the same in-plane angle along with a phase angle on the shaft for a CV effect. OR, use a Cardan for one joint and a CV joint or an approximate CV joint in the other. These parts are already available for other similar applications. Consider a modified ATV front drive shaft for example...

Another option to consider is a deliberate mis-phasing of the steering shaft in order to produce a non-linear overall steer ratio. That could mean quick on-center, slow off-center, or just the opposite. The BIG question to answer is which way do you want it. But that's another chapter...

[pucksaver]

On a similar topic, where do all of you purchase your steering racks?

[Wesley]

Can't you just flip the pinion/rack arrangement to change the steering direction?

[RBbugBITme]

Yes.

[LafayetteFSAE]

After dropping an extra steering rack that we had laying around our shop into position where it could attach on the front side of the upright, it seems that it gets awfully close to the pedals and would obstruct your ability to drive...after looking through pictures we have from VIR I don't see too many teams going front low, saw a good amount with rear low steering though

[JamesWolak]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by LafayetteFSAE:
Would you be able to tell me the rule # that pertains to the steering rack?

Also, can you explain more about how our steering inputs aren't linear on last years car?

Another thing I'm concerned about is if the rack is mounted on the floor and attaches to the back side of the upright(so a standard rack w/out modification would work), how can you get the steering shaft down at such a steep angle, I feel that a single u joint would lock up due to the large angle </div></BLOCKQUOTE>


I don't mean to be rude but if you don't know about the template rule then you need to re-read your rules. I am not looking up the rule number because its just silly. Its like looking up the restrictor sizes we all know what they are we dont need the rule number. Just search around the forums about the template rule. I think Pat Clark mentions that the rule was created to force a lower placement of the rack. But if you are looking for a rule that states "the rack must be here" there isn't one. You can go with a top mount still but the template rule was created to guide you away from it.

For the linearity of your steering input. I am a nearly dyslectic powertrain guy so i apologize for my poor job at explaining it. Basically the angle from your rack to you attachment on the upright is causing it. That angle is going to change as you turn the wheel. Which will change the rate of your steering angle input to actual wheel angle. I think thinks a poor explanation i apologize.

I don't mean to call you baby ugly either. We have done the same, we even did it last year when we were aware of it but it was minimal and couldn't be avoided with the c-factors available. Sure we could of made our own rack but i big portion of FSAE is weighing time to gain and we found it not to be worth it.

Also they are correct you can flip the rack and that would work. Didn't think about that and because you are on the front side of the upright flipping it wouldn't make the the steering shaft angle isn't going to be any worse then putting it on the backside. Good luck with your small drivers though this might be just as bad in ergonomics as the top mount but then again it could work perfect. I would suggest this if you can pull it off.

I am sure my roommates/teammates are giggling over the fact that i am commenting on suspensions threads.

[LafayetteFSAE]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by JamesWolak:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by LafayetteFSAE:
Would you be able to tell me the rule # that pertains to the steering rack?

Also, can you explain more about how our steering inputs aren't linear on last years car?

Another thing I'm concerned about is if the rack is mounted on the floor and attaches to the back side of the upright(so a standard rack w/out modification would work), how can you get the steering shaft down at such a steep angle, I feel that a single u joint would lock up due to the large angle </div></BLOCKQUOTE>


I don't mean to be rude but if you don't know about the template rule then you need to re-read your rules. I am not looking up the rule number because its just silly. Its like looking up the restrictor sizes we all know what they are we dont need the rule number. Just search around the forums about the template rule. I think Pat Clark mentions that the rule was created to force a lower placement of the rack. But if you are looking for a rule that states "the rack must be here" there isn't one. You can go with a top mount still but the template rule was created to guide you away from it.

For the linearity of your steering input. I am a nearly dyslectic powertrain guy so i apologize for my poor job at explaining it. Basically the angle from your rack to you attachment on the upright is causing it. That angle is going to change as you turn the wheel. Which will change the rate of your steering angle input to actual wheel angle. I think thinks a poor explanation i apologize.

I don't mean to call you baby ugly either. We have done the same, we even did it last year when we were aware of it but it was minimal and couldn't be avoided with the c-factors available. Sure we could of made our own rack but i big portion of FSAE is weighing time to gain and we found it not to be worth it.

Also they are correct you can flip the rack and that would work. Didn't think about that and because you are on the front side of the upright flipping it wouldn't make the the steering shaft angle isn't going to be any worse then putting it on the backside. Good luck with your small drivers though this might be just as bad in ergonomics as the top mount but then again it could work perfect. I would suggest this if you can pull it off.

I am sure my roommates/teammates are giggling over the fact that i am commenting on suspensions threads. </div></BLOCKQUOTE>


Sorry about that, I thought there was a specific rule that was speaking about rack placement. I looked at the template you are talking about, I guess our team passed it last year at VIR so maybe a top mounted rack wouldn't be out of the running. We are only a 3rd year team so there is a lot for us to learn about this semester in preparation for next year. I have plenty of racing/mechanic experience, but my school doesn't offer any vehicle dynamic courses, so Im just trying to read what I can and learn from the forums here which has been a big help so far.

[JamesWolak]

I really have a hard time believing that you passed the template rule but i could be wrong.

[RBbugBITme]

Templates didn't exist at VIR last year so you're right they didn't pass the template rule.

[JamesWolak]

Our car got "pre" templated in Oct 2008 at a SAE meeting so they coulda been floating around at VIR.

[LafayetteFSAE]

By the time I made it down to VIR last year, the car had already went through tech, so I couldn't tell you what the deal was on that...Im going to cut the template out tomorrow and see what the results are.

[Dave K]

If you are concerned about bump steer from rack placement, you just need to make sure to connect your tie-rod and a-arm ICs (if you don't already know about it). Check it:

http://server3.e2etech.com/~ad...mp_steer_drawing.jpg (http://server3.e2etech.com/%7Eadmin4/AdvHTML_Upload/bump_steer_drawing.jpg)

Using this method, a simple ADAMS sim gives us like .05 degrees of bump steer through a 2.5-inch wheel travel.

Compliance isn't something we are concerned about, since we are using solid rod-ends and sphere bearings and whatnot (no rubber mounts).