Push rod on upright suspension - an examination

[Andi76]

Piolas Books - Technical Analysis. But these are now rarities and for the books from 1998-2005 in particular you sometimes have to pay up to 200 dollars for one of them. 1999, 2003 and 2004 are extremely expensive. After 2006 it gets a little cheaper, around 80-150 dollars up to the years 2011/2012. Are occasionally offered on EBay and Amazon. I recommend EBay, it's usually cheaper there and you can get a bargain! These old books are now almost an investment! I have bought all the books since 1994 (1994-1997 are only available in Italian and are no longer available anywhere - worth about 500 euros per book) and my collection is worth over 5000 euros!

Incredible! Now it's time to do the forum a service and scan them all

Do the forum a service by stealing the authors' published works? Buy the book(s). I did years ago. What you ask is actually a crime.....

Despite the emojis, you obviously didn't realize that he was only joking.

[Farnborough]

Good find.

The kinematics of it, is to put it on or as close to the king pin axis for the least "bump steer" effect.

Is that known as "bump steer" ?

My understanding of that description classically denotes the steering rack ends proscribing a difference in arc to the one in which the upright is notionally completing.

Hence the steered wheels accumulate more toe in or out as the suspension travels from full droop to full bump. The chassis effectively giving directional shift with the wheel held still throughout that compression event.

Mounting the push-rod at a difference to upright axis SHOULD give rise or drop to suspension height through turning the steering, but not alter toe etc.
Castor and akerman would then be in their purest implementation to alter those effects without being "poluted" by an unquantified overlay and amount of pure "bump" steer.

It will have on effect on the steering becuase it produces a torque around the kin pin axis if it is offset to the axis. The driver will feel it in the steering. In Formula 1 the travel is not great, so that maybe whey they can get away with the amount of steering this push rod on upright can induce if it's off-axis.

It doesn't fit the description of "bump" steer as commonly understood.

Which is, the location of the track rod end on the upright follows a different arc of travel through suspension movement (lowest to highest) than the arc of the end point of the steering rack .... this giving a change in toe dimension through that travel when the two are connected together. This is undesirable for any amount of suspension movement.

The mount of push-rod etc cannot change this if its been designed correctly. This characteristic of push-rod location would fight the dimension of the steering rack length, but it couldn't change that in tension or compression, making toe stay as designed throughout the movement. Bump steer would not happen.

Pusrod location would change the suspension, and therefore chassis height, if mounted on the upright in position not classically "optimum" as this location can move relatively to the other end of push-rod location when the steering is turned. Certainly would change feel in chassis, but not give bump steer.

Effects like this can be easily observed in kart chassis. With no suspension, strong caster and extended outboard wheel centres, they lift the inside of the chassis into negative roll when the steering is turned .... to largely facilitate lifting inside rear wheel and reduce traction there (solid rear axle, no differential) and reduce understeer effect.
This is more accomplished in F1 (the locking differential) by degree of unlock going in to corners and making the steering more free, lessening the tendency to push the front straight from having high locking used in traction.

I thought ... unsure of accuracy ....that the Torro Rosso chassis with James Key designing, brought some elements of changes in suspension height through this geometry. The further inward and external to the wheel mounted top wishbone outer location "seemed" to facilitate this. APPEARED to drop the chassis and hence front wing toward the track surface on inside corner edge in enhancing a ground effect relationship for the wing in improving turn in performance. Thought also this was copied by MB team on their chassis.

They're now moved away from this with raised wing and design constraints on upright to wishbone mount geometry, as I understand that.

[maxxer]

Darn and here i am thinking he is busy scanning them all from 1994 and up.

[Andi76]

Darn and here i am thinking he is busy scanning them all from 1994 and up.

Sorry to disappoint you...
but you are welcome to come by and take over the job

[SiLo]

Piolas Books - Technical Analysis. But these are now rarities and for the books from 1998-2005 in particular you sometimes have to pay up to 200 dollars for one of them. 1999, 2003 and 2004 are extremely expensive. After 2006 it gets a little cheaper, around 80-150 dollars up to the years 2011/2012. Are occasionally offered on EBay and Amazon. I recommend EBay, it's usually cheaper there and you can get a bargain! These old books are now almost an investment! I have bought all the books since 1994 (1994-1997 are only available in Italian and are no longer available anywhere - worth about 500 euros per book) and my collection is worth over 5000 euros!

Incredible! Now it's time to do the forum a service and scan them all

Do the forum a service by stealing the authors' published works? Buy the book(s). I did years ago. What you ask is actually a crime.....

I was joking bud, I don't actually expect someone to sit and scan 5000 pages just for the forum.

[PlatinumZealot]

Is that known as "bump steer" ?

My understanding of that description classically denotes the steering rack ends proscribing a difference in arc to the one in which the upright is notionally completing.

Hence the steered wheels accumulate more toe in or out as the suspension travels from full droop to full bump. The chassis effectively giving directional shift with the wheel held still throughout that compression event.

Mounting the push-rod at a difference to upright axis SHOULD give rise or drop to suspension height through turning the steering, but not alter toe etc.
Castor and akerman would then be in their purest implementation to alter those effects without being "poluted" by an unquantified overlay and amount of pure "bump" steer.

It will have on effect on the steering becuase it produces a torque around the kin pin axis if it is offset to the axis. The driver will feel it in the steering. In Formula 1 the travel is not great, so that maybe whey they can get away with the amount of steering this push rod on upright can induce if it's off-axis.

It doesn't fit the description of "bump" steer as commonly understood.

Which is, the location of the track rod end on the upright follows a different arc of travel through suspension movement (lowest to highest) than the arc of the end point of the steering rack .... this giving a change in toe dimension through that travel when the two are connected together. This is undesirable for any amount of suspension movement.

The mount of push-rod etc cannot change this if its been designed correctly. This characteristic of push-rod location would fight the dimension of the steering rack length, but it couldn't change that in tension or compression, making toe stay as designed throughout the movement. Bump steer would not happen.

Pusrod location would change the suspension, and therefore chassis height, if mounted on the upright in position not classically "optimum" as this location can move relatively to the other end of push-rod location when the steering is turned. Certainly would change feel in chassis, but not give bump steer.

Effects like this can be easily observed in kart chassis. With no suspension, strong caster and extended outboard wheel centres, they lift the inside of the chassis into negative roll when the steering is turned .... to largely facilitate lifting inside rear wheel and reduce traction there (solid rear axle, no differential) and reduce understeer effect.
This is more accomplished in F1 (the locking differential) by degree of unlock going in to corners and making the steering more free, lessening the tendency to push the front straight from having high locking used in traction.

I thought ... unsure of accuracy ....that the Torro Rosso chassis with James Key designing, brought some elements of changes in suspension height through this geometry. The further inward and external to the wheel mounted top wishbone outer location "seemed" to facilitate this. APPEARED to drop the chassis and hence front wing toward the track surface on inside corner edge in enhancing a ground effect relationship for the wing in improving turn in performance. Thought also this was copied by MB team on their chassis.

They're now moved away from this with raised wing and design constraints on upright to wishbone mount geometry, as I understand that.

I think you are taking bump steer as a gramatical definition. I suggest thinking of it as a mechanical definition.

It means that steering will get induced in the wheel by virtue of the suspension compressing/extending (bump). Normally a (non-parallel plane to control arm) tie rod will cause this... But forget about it being strictly a tie rod.. Now that you have introduced a another eccentric rod on your upright, excepting this is a compressible push rod.. It it will generate an eccentric reaction force into the upright that the driver (OR a force from the toe change with camber if the suspension is so configured) will have to counteract.

This is probably why this design didn't come about ealier. Engineers didn't have the tools to calculate around its effects in quick enough way?

[Greg Locock]

Both the pushrod and the stabilizer drop link (not in f1) can be used to steer the front suspension, especially if they attach to the spindle, but also, due to compliance, even if they attach to the arms. For historical reasons we don't call it force based roll steer, but that is what it is.

If done correctly it can be used to help centre the steering from full lock. If not, it can make the steering go overcentre, such as in the first generation X5. The effect is powerful enough to need to be considered when selecting an EPAS. Because it is related to roll it can cause a lightly damped coupling between roll and steer, which again the X5 suffered from.

If you put the true steering axis of the suspension in end view (ie on a double wishbone superimpose the two outer balljoints), any moment of either rod about that axis will generate a steering torque. This all rather old hat, Rover worried about this in the 60s if not before. If you look at a Bronco for example you'll see the drop link has a substantial inclination in side view for this very reason. On some designs I've been asked to specify different sta bar connection points for different suspension tunes, happily that proved to be an over refinement.

[bigpat]

I wrote a question to Gary Anderson ( Jordan designer of the beautiful 191 to the 198) about this geometry, and he answered that the offset pushrod pickup was a means to lower the front end with steering lock applied, mainly as a means to reduce understeer, as the aerodynamic centre of pressure would migrate forwards. This lowering would also aid a little with forward weight transfer onto the front tyres also.

Bump steer is of little influence in an F1 car as you are talking about a total of 10-12mm of wheel movement at the front end, not 100-200 total travel of a road car, or 50-75mm of a racing sedan or touring car.