why Front steer....... [Archive]

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mayuri

11-12-2009, 09:25 PM

Hi fellow petrol heads http://fsae.com/groupee_common/emoticons/icon_wink.gif !

why do we really go for front steer in race cars?

The reasons i think are :

1. Packaging and less space in the cockpit left us with no other option (packaging) http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

2. The car will go for understeer on bumps and that would result in stability of the vehicle post bump ride.

Please do put in if the [points i put in are right and make sense. Also i wish to know ,why does the car actually go for understeer in bumps.

L B0MB

11-12-2009, 10:03 PM

Thats how its done in F1

Do you need any more reason than that? :P

mayuri

11-12-2009, 10:42 PM

Yae i am looking forward to any more reasons, if any :P

why does the car actually go for understeer in bumps...

Mike Cook

11-12-2009, 10:48 PM

maybe I'm misunderstanding this....

Have you ever driven a forklift?

mayuri

11-12-2009, 11:54 PM

why does the car actually go for understeer in bumps in case of FRONT STEER...?

.. http://fsae.com/groupee_common/emoticons/icon_smile.gif tecHnical answers...

ed_pratt

11-13-2009, 01:42 AM

i think he means, steering arm ahead of the front axle centreline rather than behind the front axle centreline.

mayuri

11-13-2009, 02:19 AM

front steer means steering arms ahead of the front centreline.

Mike Cook

11-13-2009, 03:46 AM

ok, I wasn't sure if thats what he meant.

For FSAE cars that steer a lot, it is easier for a given ackerman geometry to steer more with the steering arm in front of the upright.

MalcolmG

11-13-2009, 05:08 AM

really? I've found that having the steering arm behind the upright is much easier, as the connection point to the tie rod is further inboard (assuming you're using pro-ackerman steering) and therefore is less likely to interfere with the wheel at higher steering angles.

mayuri

11-13-2009, 07:17 AM

Racing cars generally will run on lower steering angles and the radius of the turns are also large so connections of the tie rods would actually not interfere with the wheels ...

AxelRipper

11-13-2009, 07:23 AM

man... i was hoping this was a topic on a rear-steering car.... super maneuverable!

ed_pratt

11-13-2009, 07:43 AM

Mayuri

I'm completely in agreement with Malcolm on this. I have found that having the steering arm behind the axle centreline makes life far easier when trying to achieve a large pro-ackermann steering geometry.

Yes, generally circuit race cars will have low steered angles, but FSAE cars are not general racing cars. To even look at getting around the hairpin you're looking at 30deg of steering on the outside wheel, which depending on how pro-ackermann you go could result in as much as 45deg(or more) on the inside wheel.

ed

exFSAE

11-13-2009, 08:29 AM

Originally posted by mayuri:
why does the car actually go for understeer in bumps in case of FRONT STEER...?

.. http://fsae.com/groupee_common/emoticons/icon_smile.gif tecHnical answers...

The car won't necessarily go to understeer over bumps...

Bobby Doyle

11-13-2009, 09:44 AM

Originally posted by exFSAE:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by mayuri:
why does the car actually go for understeer in bumps in case of FRONT STEER...?

.. http://fsae.com/groupee_common/emoticons/icon_smile.gif tecHnical answers...

The car won't necessarily go to understeer over bumps... </div></BLOCKQUOTE>

Right, depends on your geometry.

Mayuri - Think about it for a bit. Assuming you cannot achieve perfect zero bump steer, would you rather have a front steering geometry that has oversteer or understeer tendencies in bump... http://fsae.com/groupee_common/emoticons/icon_wink.gif

mayuri

11-13-2009, 09:50 AM

ohkay!!

According to ed_pratt we should go for rear steer as there are chances of the tie rods ball joints interacting with the wheels in larger steering angles in case of front steer...but that could be checked!

Then what are some of the good reasons why we go for front steer in our FSAE cars ?

exFSAE

11-13-2009, 10:19 AM

Originally posted by mayuri:
ohkay!!

According to ed_pratt we should go for rear steer as there are chances of the tie rods ball joints interacting with the wheels in larger steering angles in case of front steer...but that could be checked!

Then what are some of the good reasons why we go for front steer in our FSAE cars ?

Packaging and component placement.

Either way it's not like one's blow out better than the other. They're just different. Take a look at both arrangements. Pick the one that'll be easiest and still meets all your functional requirements. Shouldn't be a big point...

BillCobb

11-13-2009, 11:21 AM

May I quietly suggest you are missing 2 of the biggest reasons for front steer high cornering performance car? (There are actually 3 or more)...

Tim.Wright

11-13-2009, 11:24 AM

We were just discussing this at work the other day. The gneral consensus was that we'd all heard a good reason for front steer but couldn't remember it!

I remember seeing a good argument for it somewhere and I remember it being related to compliance in the axle somwhere.

Milliken says something interesting on p835 of RCVD. To paraphrase, he says that the control arms and the steering arm all deflect under load. If they deflect by different amounts (likely)then you will get some rotation of the hub, i.e. it will toe in or out under load. The direction of the toe depends on where the compliance is. If it is well ahead of the axle line you will get toe in on the outside wheel. Generally, I think you want toe out on the front under load as it gives a stable (understeer) response.

Also in Milliken (P713) he says that you want the tie rod in the lower-front or upper-rear quarter of the wheel. Though I think this is more relevant to road cars with rubber mounted suspension.

Apparently this is because you will get some compliance in the upper and lower control arms under load. Since one will be in tension and the other in compression, they will deflect in opposing directions and you get some additional camber angle at the hub. If your tie rod is rigidly mounted in either the upper-rear or lower-front quarter of the wheel, you will get toe out (on the outside wheel)as the as the hub cambers out.

This seems to assume the control arms are elastically mounted on bushings and the steering arm is rigid. I belive this is common on road cars. On a race car, I don't think there should be massive differences in the delfections of the control arm and the steering arm so I don't think you can generalise as to which quarter you should put the steering rack.

However you will struggle to pass the template rules if its in any of the upper quarters!

Elsewhere on this forum there is a post I belive by Geoff from RMIT (I might be wrong)on this subject. I think he said something like if you have positive caster there might not be enough room to package the steering pickup on the hub in the lower front or upper rear quarter as that area would be full of ball joint. This is also a fair point. Some kinpin offset could help there but then your scrub radius changes.

So I know I haven't answered your question, but I don't think there is one definate answer. Especially when compliances are involved, it all becomes messy.

Although you can avoid compliances by making the susupension out of 100% metal http://fsae.com/groupee_common/emoticons/icon_wink.gif

Tim

Bobby Doyle

11-13-2009, 11:28 AM

I believe the Milliken book gives a good explanation on how compliance plays a role depending on rack location. The compliance is also linked to what I mentioned earlier with oversteer/understeer characteristics. It is also easy to think about how the steering link is loaded, ie tension vs. compression. A "front steer" setup puts the more heavily loaded link in tension.

Edit - Ah, Timo beat me to it. http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

Tim.Wright

11-13-2009, 11:57 AM

I am still a little confused as to the point in Milliken though. A member loaded in tension shouldn't be any different in stiffness to a member loaded in compression shouldn't it?

Assuming you are no-where near the buckling condition in compression that is.

Tim

Bobby Doyle

11-13-2009, 04:09 PM

Originally posted by Timo:
I am still a little confused as to the point in Milliken though. A member loaded in tension shouldn't be any different in stiffness to a member loaded in compression shouldn't it?

Assuming you are no-where near the buckling condition in compression that is.

Tim

Tim, I don't believe the Milliken book talks about tension/compression. I merely added that on as a general observation. I should have been more clear. And no, the modulus remains the same (although I believe aluminum's modulus increases VERY slightly in compression). If I have a chance I'll look through Milliken tomorrow.

Tim.Wright

11-14-2009, 02:31 AM

Originally posted by Bobby Doyle:
Tim, I don't believe the Milliken book talks about tension/compression. I merely added that on as a general observation. I should have been more clear. And no, the modulus remains the same (although I believe aluminum's modulus increases VERY slightly in compression). If I have a chance I'll look through Milliken tomorrow.

Yes you're right. But what he did say was that changing the steering position will change the direction of the compliance fron US to OS etc.

For a given A-arm geometry, moving the rack from the front to the back will change the loading from tension to compression but the direction of the compliance shouldn't change should it? If the steering arm is 'softer' than the control arm it should still toe out in either case.

Tim

Bobby Doyle

11-15-2009, 08:28 AM

Originally posted by Timo:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Bobby Doyle:
Tim, I don't believe the Milliken book talks about tension/compression. I merely added that on as a general observation. I should have been more clear. And no, the modulus remains the same (although I believe aluminum's modulus increases VERY slightly in compression). If I have a chance I'll look through Milliken tomorrow.

Yes you're right. But what he did say was that changing the steering position will change the direction of the compliance fron US to OS etc.

For a given A-arm geometry, moving the rack from the front to the back will change the loading from tension to compression but the direction of the compliance shouldn't change should it? If the steering arm is 'softer' than the control arm it should still toe out in either case.

Tim </div></BLOCKQUOTE>

Right. Again, don't have the book with me, but I believe he was referring to the effects of camber compliance (from your suspension, not your wheels) on the steered angle of the wheels. If you think about it this way, you can see that a front-lower/rear-upper mounted rack would create an US response (assuming the control arm deflection is greater than the steering arm deflection). Now, with an SLA FSAE car, is this compliance sufficient to create an undesirable response? I don't know.

Tim.Wright

11-16-2009, 10:34 AM

Right, got ya.

Now, with an SLA FSAE car, is this compliance sufficient to create an undesirable response? I don't know.

The suspension I designed for 08 certainly was! I called it 'Active-toe-out' but I think the enduro marshals used the term 'dangerous'.

Tim

Sooner_Electrical

11-30-2009, 12:37 PM

The suspension I designed for 08 certainly was! I called it 'Active-toe-out' but I think the enduro marshals used the term 'dangerous'.

This is clearly just a case of bad branding... it is still a passive system. If you had gone with "Auto-toe" or at the very least "Dynamic-steererizer" you probably would have made it to design finals :-).

Bemo

03-10-2011, 11:48 AM

You brought up a 1,5 year old thread just to yell at someone?!

Canuck Racing

03-10-2011, 12:56 PM

AxelRipper

03-10-2011, 07:25 PM

Originally posted by adii:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Member

Posted November 13, 2009 10:23 AM Hide Post
man... i was hoping this was a topic on a rear-steering car.... super maneuverable!

Any views or opinions expressed by me may in no way reflect those of Kettering University, it's students and administrators, or our sponsors.
what do u knw about idiot??? </div></BLOCKQUOTE>

What do I know about it? Well I happen to have done some time on a golf course, and all of the mowers are rear steer for maneuverability. This allows the vehicle to be able to steer within its axis. Buckminster Fuller, arguably one of the brightest minds of the 20th century, realized this and actually used it for his Dymaxion car back in the 1930's.

The inherent issue with the rear-steering car, or all wheel steer cars even, is stability at high speeds. The steering becomes twitchy. However, with the speeds that FSAE sees at competition this may not be as big of an issue, the same way that aero may or may not be effective at competition speeds.

Next time, do some research before calling someone an idiot on the internets.

TMichaels

03-11-2011, 04:02 AM

Look at his former posts. He is a troll. Don't feed trolls.

Regards,

Tobias

Drew Price

03-11-2011, 09:43 AM

http://www.saabsunited.com/upload/images2009/09/saab_to_concentrate_all_activities_in_trollhattan/made_in_trollhattan_by_trolls.jpg

BillCobb

03-11-2011, 08:18 PM

Two points for Bobby!. Would you rather pull on a chain or push it? And when you are braking, given the elasticity and dive characteristics of the steered suspension, which way would the wheel steer in either case? You also get a nice straight shot for a steering shaft in the case of a N/S front engined car without the complexity and cost of multiple U-joints, good crash collapse options and a good solid gear attachment at the #1 bar.