59 replies to this topic

[Froilan_G]

Hi there,

I know what castor is, I know how it can be increased etc. but what are its effects on handling? What exactly does it do? What are the negative things, what are the advantages?
I've heard people say that when they have to use power steering on race cars these days, it's because their cars have so much castor installed.
Why would there be so much castor on Sportscars and F1 cars?

[leegle]

To give more bite as they turn in. By having a large caster angle the outside wheel gains negative camber as it turns and the inside wheel gets positive camber which puts it in the same position as the outside wheel so they deform to give greater contact area. The downside is heavier steering as the effect is that the wheel has to lift the car as it is turned on lock. Hence power steering.

[100cc]

in karting the more caster the easier it is to lift the inside rear wheel, which is what you want.

[Froilan_G]

So... with this camber thing, you actually gain more grip? It's not the case that you remove the grip from the rear and place it on the front as a anti-roll change would give ...

So am I right with saying that with huge castor (which leads to an OS situation because of the extra font grip), you can balance it out by making for instance the rear anti-rollbar softer therefore gaining grip on the rear as well, and thus having more grip overall ?

Regards,

FG

[Ben]

The camber argument is entirely correct (it's called camber compensation) and is simply because up to a point tyres generate more lateral force when they're cambered (camber thrust). But there are other effects of having castor.

Steering systems are in general self-centering. There are two reasons for this, self-aligning torque of the tyre, and castor angle. Self aligning torque is a result of the resultant lateral force vector being behind the geometric centre of the contact patch, this distance is called the pnuematic trail.

The self-aligning torque and the pnuematic trail reduce to zero as lateral force increases (steering goes light at high cornering gs) using castor (steering axis angled back) adds mechanical trail ensuring that the lateral force vector is always behind the point where the steering axis intersects the ground and you always have a self-aligning torque at the steering wheel in the same direction. This is good for making the car feel predictable to the driver.

The other aspect of all this is that having the steering axis inclined has a tendency to lift the sprung mass upward a small amount as you steer, when the steering effort is removed, the sprung mass pushes down and centres the steering. There is a limit to this obviously, because if you're lifting the sprung mass this will increase the steering effort.

As an interesting aside, many people have called F1 drivers whimps when they complain about their power steering failing but think about it, You have power steering so you can increase the castor angle and get more camber compensation without increasing steering effort. So if your PS fails you have a level of castor you would never expect a driver to deal with without the PS in place.

Sorry that was a bit long, but until I read this sort of explanation in Milliken I was pretty lost on this subject as well.

Ben

[MclarenF1]

What effect does the kingpin inclination, along with SLA (short/long arm) have on camber gain? There are so many thing going on with the upright that it is difficult to visualize the effects of all these factors. Also, by the placement of the spindle can have an effect on stearing difficulty. Reading Milliken can be very confusing at times until you see the entire assembly in person, or in a linkages modeling program.

[Yelnats]

I would think castor is an extreamley variable way of getting positive camber. This is because of the vastly different turning angles of the front tires in low and high speed corners. The low speed corners would experience much greater contribution from caster generated camber due to the smaller radius and the larger steering input required to negotiate the corner. Camber for racecars is more reliably derived from static camber designed into the suspension as we see in F1 cars as they sit on the grid.

Camber generated by caster is more suitable for producution vehicles that are usually only cornered hard a low speeds where high speed cornering is best done in a state of sedate understeer and it allows them to drive down the straights with the tires vertical for more even wear.

[Ben]

I'm quite sure it's a variable way of getting camber compensation while cornering. That doesn't mean it can't be successfully exploited if you know what you're doing.

Alan Jenkins, for example, always designed (and may do again?) cars with lots of front castor angle. The Arrows FA17 and Stewart SF1 had the same front suspension geometry with a large castor angle (the actual number wasn't stated) and while performances of note for the FA17 escape me, the SF1 was roundly praised as a very good car mechanically.

I agree that the technique would work better for low speed corners and could have negative effects elsewhere, but then everything else about designing race cars is compromise so it's not really surprising.

Ben

[Terrance W. Peterson]

why use castor In a race car castor like ben said is stablizing in a stright line and it also affects the toe curve . as castor increases it increases the steering effort! It is best to use as little camber as possible to give the driver better feel for less effort! also it could be used to compensate for a design problem to correct camber curve or toe curve.

[Ben]

Agreed, the straight line stability point raises the issue of castor lead. Most, if not all, oval racers will use different amounts of castor between the left and right front wheels to make the car want to turn left.

Ben

[Yelnats]

Of couse castor in a oval race car is an entirely different proposition to F1 as the cornering speeds and curve radii vary little for a given track. This allows the designer to put the exact amount of castor required for a given radius and your point about the differing castor for the inside and outside wheels illustrate just how sensitive a car is to this adjustment.

It's hard to see how a F1 car could contend with curve raddi that vary from 15m to 150m with one very comprimised castor angle. I don't doubt that someone has attempted this but I belive that by the look of most F1 cars, they have enough static castor to contend with any cornering situation and castor derived camber would be superfluous.

[Ben]

Yeah, ok. But the camber changes will be different depending on the corner radius because the steer angle required will be different, so I'm not convinced it would be such a compromise if you did it right.

Ben

[imaginesix]

Caster is called camber compensation because it's most important role is to compensate for the positive (undesireable) camber that occurs due to the geometric effects of kingpin axis inclination (KAI). In a well laid out suspension geometry, caster shouldn't provide any significant camber change (for the outside tire) over different steer angles.

KAI is used to provide some self-centering to the front wheels at low-speed, while trail will overpower it's effects at medium and high speeds. Trail will have a stronger and stronger effect on the centering of the front wheels as the speed increases.

For race cars, low-speed self-centering is not needed, but KAI is still regularly used largely because there is not enough room within the wheel to package the suspension with a vertical kingpin axis without sacrificing other design criteria. As a result, caster is designed in to 'correct' the effects of KAI.

I don't remember why the steering goes light near maximum cornering force, but with mechanical trail in place (as it is in all steered-wheel vehicles), even a tire that is 100% sliding on the road surface should provide some self-centering force.

[Ben]

I explaned in my post above that the steering goes light due to the reduction of pnuematic trail to zero at high cornering forces. The reason you still get feedback through the steering is that the mechnical trail provides a moment arm about the steering axis even when the pnuematic trail is zero or even negative.

I agree that in many cases you would simply use caster to cancel out the KPI, but in the case of Jenkins' designs he clearly used more than was necessery for this and made it work.

A widely discussed philosophy in Formula SAE design is "Minimal KPI (zero if possible), large caster angle, live with the scrub radius". The idea being to get a lot of camber compensation in cornering without requiring large static camber which adversely affects braking stability.

Ben

[Ross Stonefeld]

If Castor is the angle between the top of the upright and the bottom, how is it so easily adjustable? I would think rotating the upright wouldnt allow it to properly connect to the upper and lower A arms as the points move both vertically and laterally with upright rotation.

[jetsetjim]

Ross,

Adjustment depends on the way the wishbones are connected to the car. In the case of an F1 car, adjustability of Castor is not an option, due to the use of flexures. To change it would mean the constuction of new upper and lower wishbones, as the upright pick-up points would have to change position. Generally the castor angle is defined in the initial design phase as a set figure. (Usually between 2 and 7 degrees for an F1 car)

On other cars, where rod-end bearings are used, it is possible to adjust the pick-up points by winding the joints in and out of the legs that attach to the chassis... In effect a relatively simple procedure. What you have to be careful with though is to ensure the upper and lower wishbones are moved together, otherwise the camber changes on the car. Changing the camber shims can usually help. (another side effect is a change in track, so you have to be careful if you are running cars at maximum width. This was a common problem on the F3000 cars)

Hope that helps

[desmo]

Wouldn't it be feasible to use a threaded adjuster even with a flexure arrangement by placing a threaded coupling between the wishbone ends and the flexures? Or given room, even an eccentric on the upright? I'm not saying it's a good idea, just perhaps possible.

[imaginesix]

Originally posted by Ben
I explaned in my post above that the steering goes light due to the reduction of pnuematic trail to zero at high cornering forces.

Yeah, I realised after posting that the steering effort only 'gets light', it doesn't 'go away' as I had been thinking. Thanks Ben.
Interesting insight into FSAE goings-on too. I would have to agree that zero KPI is a beautiful thing.

[AS110]

I like to use a lot of castor because it smells just great!

[Manson]

On the short ovals that I run, I generally run as much caster in the LF as my arms will take (no power steering). It really helps the car turn into the corner and we can reduce stagger. Like 100cc said, it trys to pick up the LR, loosening up the back end. We normally run in the neighbourhood of 3-4" of stagger so 1/2" less helps reduce straightline drag. For my racing, the majority of the time is made up in the corners so the drag down the straights is usually of lesser concern.

[BRIAN GLOVER]

Ben,

You mention the 3rd spring somewhere. I saw a spy shot of Neweys front suspension on the 2000 car. It seems that with the small tirepatch due to about a 3' negativer camber, there would be insufficient rubber on the road for breaking. Is it possible that when the 3rd spring is delfected under breaking transfer, the camber goes more positive?. If this is the case,would downforce also reduce this camber?. Does the straight line static camber reduce rolling resistence? Aslo comparing the smaller camber employed on the Michelin cars, do you think more caster is required in turns over the Bridgestone?. Would scrub radius play more of a roll in the use of Michelins? What about bump steer? I see the tie rod right next to the leading arm on the A arm on some designs, and others show possible bump applications. Would driver preferences have an effect on caster angle, especially sice power steering is illiminated?

I watch for in car camara shots at tire wear in various corners by the color changes on the tread. It very seldom seams that the wear goes all the way accross the tread even in R130 in Japan. I wish I could see a full lap on many different circuits with Michelin and Bridgestone from this view. It would be interesting to see damper properties of the single shock. How does it damp independent deflections on left to right wheels? How does it damp independent deflections under breaking. How is the rate of the tire calculated into the equation? How much does the tire spring rate change with camber? What changes will McLaren have to make to accommodate the Michelins? It is so frustrating that all the new inovations are so secret.
I suppose that is what keeps this forum alive.

These are some of the questions that remain unanswered. Any light that you can throw on it, would be appreciated.

Originally posted by Ben
I explaned in my post above that the steering goes light due to the reduction of pnuematic trail to zero at high cornering forces. The reason you still get feedback through the steering is that the mechnical trail provides a moment arm about the steering axis even when the pnuematic trail is zero or even negative.

I agree that in many cases you would simply use caster to cancel out the KPI, but in the case of Jenkins' designs he clearly used more than was necessery for this and made it work.

A widely discussed philosophy in Formula SAE design is "Minimal KPI (zero if possible), large caster angle, live with the scrub radius". The idea being to get a lot of camber compensation in cornering without requiring large static camber which adversely affects braking stability.

Ben

[Ben]

A third spring simply increases the heave rate independently (apart from some small cross-coupling) of the roll rate, and as such has no effect on wheel camber. That is down to the suspension geometry, and I don't think any current F1 car has a geometry that will make the wheels camber outward with a downward heave motion.


As for bump steer, I see no reason you would want any, anywhere. Scarbs has pointed out that the placement of the steering arm between the wishbones is to interact with the flow off the front wing.

As for damping, that's a lot of questions and I'm knackered, so I'll have to have a think.

Ben

[MclarenF1]

to quote Caroll Smith from Tune to Win
"Sometimes I think that I would have enjoyed racing more in the days of the friction shock. Since you couldn't do anything much to them or with them, I would have spent a lot less time being confused"

My thought exactly.

[Darren]

Originally posted by MclarenF1
to quote Caroll Smith from Tune to Win
"Sometimes I think that I would have enjoyed racing more in the days of the friction shock. Since you couldn't do anything much to them or with them, I would have spent a lot less time being confused"

My thought exactly.

Nope, fiddle room there as well. Nowhere near as much, but you've still got to mount them and get them to move with leaf springs without doing any springing. And then you have to get the right damper setting and the right degree of movement at the mounts. Admittedly, once you've got them right you don't play with them, but setting them up isn't always straightforward.

[Froilan_G]

Ben,

Why do you see no reason for bump steer? Could be convenient on the rear axle, or am I thinking wrongly there?

[Ben]

Bump steer would make the car twitchy and unpredicatable over rough surfaces and is unlikely to make the driver feel confident about the car.

Roll steer on the other hand could be used more predictably and is probably worth looking at more than bump steer.

And before I get a barrage of response to this, you can have roll steer with minimal if not zero bump steer.

Ben

[Ross Stonefeld]

Bump steer is when the suspension moves it gives a steering input of sorts? If thats the case, I cant imagine anyone enjoying that, especially as ben said, on uneven circumstances.

[imaginesix]

Originally posted by Ben
And before I get a barrage of responses to this, you can have roll steer with minimal if not zero bump steer.
Ben

How? Show me. PpPpPpPlleeaaase!
Suspensions never have been able to distinguish bumps from roll, so how could they behave differently in each scenario?
Unless you mean 'steer' as in 'generating a yaw moment' (as in McLaren brake-steer, in which the wheels didn't actually 'steer'). But other that that, I maintain that roll steer is impossible with zero bump steer.

[Ben]

In bump the inboard pick-up for the track rod is fixed laterally and the arm rotates around it. In roll the pickup itself moves laterally as do the wishbone pick-ups. The motions are different and make it possible in my opinion to get roll steer with minimal bump steer.

I will attempt to prove this using some geometry software when the mood takes me.

Ben

[Froilan_G]

I did some looking-up on the subject, and indeed Ben, you are quite right when you say that you can have roll steer without bump steer. But I think it has nothing to do with pick-up point movements and you don't need a computer simulation to prove it.

When a wheel moves in bump, it is geometrically-wise possible to make it move forward as well as upward, and it is also possible to make it move backward in a downward motion (rebound). This is the principle of Roll steer. Because the inside wheel moves in rebound and the outside wheel moves in bump, you can mis-align the rear wheels which will introduce a steering torque.

[BRIAN GLOVER]

Better not drive any rear engine Porsche then. I drove a Boxter the other day, and was alarmed at the bump steer. NASCAR uses it on ovals and in the case of Porsche, where the layout of the car defies logic and physics, they must use anything at their disposal to make the damn thing work, including electronic differentials.
Vette Man.

Originally posted by imaginesix

How? Show me. PpPpPpPlleeaaase!
Suspensions never have been able to distinguish bumps from roll, so how could they behave differently in each scenario?
Unless you mean 'steer' as in 'generating a yaw moment' (as in McLaren brake-steer, in which the wheels didn't actually 'steer'). But other that that, I maintain that roll steer is impossible with zero bump steer.

[imaginesix]

Originally posted by Ben
I will attempt to prove this using some geometry software when the mood takes me.
Ben

LOL! Yes, I sympathise. Remember I have yet to complete my proper critique of the article you sent me some 10 months or so ago. Though I actually still intend to!

Anyways, I see now that you do indeed have a point about roll steer being possible without bump steer (almost).
Enabling zero bump steer requires a very specific location of the steering arm vis-a-vis the upper and lower control arms. Small misalignments will cause bump steer somwhere along the travel of the suspension, no matter how hard one works to eliminate them.
That very sensitivity can be used then to 'misalign' the steering arm with steer angle, causing bump steer, and thus roll steer by way of steering wheel angle.
Geometricaly, I imagine such a layout would require that the steering rack move up and down vertically, or forward and backwards longitudinaly with steering input. Rather than a steering rack then, a steering yoke (like karts have) would provide both the lateral movement required to actually turn the wheels, along with some amount of vertical and/or longitudinal movement to mis-align the inboard end of the steering arm.
Thus, hitting a bump with the wheels pointed straight ahead would cause zero toe angle change, while turning the wheels for a corner would allow some toe angle change from suspension bump/droop.
The problem though is that if the car then hits a bump while turning at the same time, the suspension geometry will be in a position the steer the wheels as a result of the bump, which is the very situation for which one tries to avoid bump steer in the first place.
Besides, allowing toe angle change as an result of steer angle doesn't provide any benefits that couldn't be found through the use of ackermann geometry.

[BRIAN GLOVER]

Thanks Ben
Im pretty dumb. I went back to look at the spy photo and obviously camber control cant be made with the 3rd spring. I recalled a coil/over as the third spring and a Watts type link connected to the lower wish bone attach points. Must have been dreaming. It was past my bedtime.
Are you involved with race car design? What CAD do you use? I am sorry, there is no prize offered after all these questions, whether you get them right or wrong.

b.

Originally posted by Ben

A third spring simply increases the heave rate independently (apart from some small cross-coupling) of the roll rate, and as such has no effect on wheel camber. That is down to the suspension geometry, and I don't think any current F1 car has a geometry that will make the wheels camber outward with a downward heave motion.


As for bump steer, I see no reason you would want any, anywhere. Scarbs has pointed out that the placement of the steering arm between the wishbones is to interact with the flow off the front wing.

As for damping, that's a lot of questions and I'm knackered, so I'll have to have a think.

Ben

[BRIAN GLOVER]

As long as the streering arm radius is the same as the included radius of the A arms, there will be little steer. This will only work for a small percentage arcs in any design, as you say.
10mm front and 18 rear travell in a F1 car, is hardly a concern as long as it is not intentional. This is the question. I think McLaren has tried everything to control push. Maybe Hakkinen is retiring because of it. He steers with the back wheels so he may have less caster than Coolturd . Maybe the Michelins will cure it.
A solid axle in front drive cars or live axles in rear drive cars, it is customary to dial in either roll understeer or oversteer with 4 or 3 links. Roll toe in IRS suspensions is also desirible. Toe in at the rear is for high speed stability.

[QUOTE]Originally posted by imaginesix
[B]
Small misalignments will cause bump steer somwhere along the travel of the suspension, no matter how hard one works to eliminate them.
That very sensitivity can be used then to 'misalign' the steering arm with steer angle, causing bump steer, and thus roll steer by way of steering wheel angle.]

[Ben]

I agree about Ackerman geometry, but roll steer could be useful at the rear to provide a passive steering effect.

Brian: I'm not involved in race car design on any professional level. I'm currently working for a helicopter manufacturer on a year out from Uni (I've done 2 years of a mech eng degree at Birmingham in England).

As for CAD, I have AutoCAD 2000 and Solid Edge V9 from Unigraphics. I haven't used the later much which I need to address. I also use Mike McDermott's wishbone software (www.racetechmag.com) and I do anything else I might need (modelling, etc) using MatLab.

Ben

[imaginesix]

Originally posted by Ben
I agree about Ackerman geometry, but roll steer could be useful at the rear to provide a passive steering effect.
Ben

Then you must be talking about compliance steer. That is a function of lateral loadings, not roll angle. The fact the car must also roll due to these same lateral loadings is incidental.
A kart could have compliance steer, but I sure wouldn't call it roll steer!

[imaginesix]

Originally posted by BRIAN GLOVER
NASCAR uses it on ovals and in the case of Porsche, where the layout of the car defies logic and physics, they must use anything at their disposal to make the damn thing work, including electronic differentials.
Vette Man.

The layout used in Porsches is the most sensible one I know for rear-drive sports cars.
The rear weight bias is desireable for the sake of max acceleration and max deceleration.
Though it does magnify the effects of drop-throttle oversteer, this is a concern that needs to be dealt with in all cars with sporty pretensions. The difference is that with a Porsche you can be sure it has been investigated and controlled to the highest degree, which can't be said for many of it's competitors.
What defies logic or physics?

[BRIAN GLOVER]

Sounds like you should be. Im from the slide rule days of engineering, and I cant even work the spell check on this forum, let alone post a sketch. I plot it out full size on a chalk board.
Do you have a 'feel' for suspension design? IE, do you drive in competition, or can you tell by driving your street car, what alterations could be made?
Reading between the lines, it seems that you have more than the theory down. Some designers just seem to have a gut feel for suspension dynamics, that you cant get from a book. With computers, some college kids can come up with some pretty cool stuff. Years of aviation have made me pretty sensitive with what is happening in the chassis of cars.
"seat of the pants' driving is what Hans Stuk was refering to, when he described how he learned to drive the Audi Quatra. He used the word Poophol. He prefers no padding on the seat.
Press down on the floor with your left foot now. Can you feel the load taken up on your right buttox on the chair?
You asshole is a very sensitive measuring tool in suspension design. Ask M Schumacher.

[QUOTE]Originally posted by Ben
Brian: I'm not involved in race car design on any professional level. I'm currently working for a helicopter manufacturer on a year out from Uni (I've done 2 years of a mech eng degree at Birmingham in England).

[BRIAN GLOVER]

Having the engine behind the axle is not smart. Semi trailing arms is not smart. Mc Pherson struts with their poor camber gain is not smart, especially in a car that costs more than $80 000. Drive a Porsche on the limit and it will bite an inexperienced driver. Even though I enjoyed my stint in my friends mid engine Boxter S, I was alarmed at the axle hop at launch and the ancient 911 feel in the steering.(Bump steer). A turbo Porshe is hard to keep in the lane at speed. It wonders. No siree, I was happy to get back into my Vette. Even if it were the same price, Id opt for the Vette.
The 911 has done remarkibley well over the 37 years of its production and their performence has been very close to Corvettes for all that time. You always see them racing together. It is a question of preference, I guess. I attribute it to tire development.

Originally posted by imaginesix

The layout used in Porsches is the most sensible one I know for rear-drive sports cars.
The rear weight bias is desireable for the sake of max acceleration and max deceleration.
Though it does magnify the effects of drop-throttle oversteer, this is a concern that needs to be dealt with in all cars with sporty pretensions. The difference is that with a Porsche you can be sure it has been investigated and controlled to the highest degree, which can't be said for many of it's competitors.
What defies logic or physics?

[Ben]

Compliance steer and roll steer are different things. You're always going to get some compliance steer but itentionally having roll steer is another matter entirely.

Are we talking about the third spring article I sent you BTW?

As for having a feel beyond the theory, I've never owned a car. I've driven a Formula Student car a couple of times but nothing major. I think I just tune myself in to what I read from drivers and engineers in the business and try and absorb everything. I've been reading Racecar Engineering since I was 13, I hope I've learnt something from it.

Ben

BTW I do own a racing machine of sorts. Its got an 18-speed sequential transmission, slick tyres, a leather seat and aerodynamically faired wheels, plus a TIG welded aluminium spaceframe (www.rooworld.com )

[imaginesix]

Originally posted by Ben
Compliance steer and roll steer are different things. You're always going to get some compliance steer but itentionally having roll steer is another matter entirely.

Are we talking about the third spring article I sent you BTW?

Ben

Yup, that's the one.

I give up trying to imagine how you might find roll steer w/o bump steer, so I wish you luck. Keep me in mind when the time comes to toss your ideas around.

[BRIAN GLOVER]

I did not receive anything, Ben. [EMAIL=bglover@earthlink.net] . i[QUOTE]Originally posted by Ben
Are we talking about the third spring article I sent you BTW?
]

[desmo]

Ben if you're mailing it out anyway, could you cc one to desmo@fineartscrimshaw.com too? Thanks.

[BRIAN GLOVER]

Are we talking about the third spring article I sent you BTW?

I didnt get it. I sent something to you. I have no evidence of it. I wonder where it went. did you get it. This is far too high tech for me.

As for having a feel beyond the theory, I've never owned a car. I've driven a Formula Student car a couple of times but nothing major. I think I just tune myself in to what I read from drivers and engineers in the business and try and absorb everything. I've been reading Racecar Engineering since I was 13, I hope I've learnt something from it.

You could have fooled me.

Ben

BTW I do own a racing machine of sorts. Its got an 18-speed sequential transmission, slick tyres, a leather seat and aerodynamically faired wheels, plus a TIG welded aluminium spaceframe (www.rooworld.com ) [/B][/QUOTE]

[Ben]

I've just spent the last three days have a variety of very interesting conversations with amongst others, Tony Southgate and a Jaguar roadcar suspension engineer. Points discussed relevant to this thread are;

1) You can't get roll steer without bump steer. I was wrong and it appears a numerical error in some software was throwing me off. Incidently, as a result of that discussion I learnt how to design a double wishbone suspension with zero (within a few mm anyway) roll centre movement, more on that when I get it clear for myself.

2) Using caster as described in my previous posts to gain some useable negative camber in excess of compensating for KPI etc is a useful design aim. Tony Southgate told me that he hasn't worked on a car with less than 5 degrees caster recently, and that the Audi Le Mans cars are up around nine degrees. It was also his view that power steering allows you to get away with relatively large caster angles and scrub radii. He concurred with the approach I advocated in an earler post (for FStudent cars anyway).

The article I6 and I were discussing earlier on the thread was just Peter Wright's article from a 1997 edition of Racecar.

Thanks to all anyway, this thread gave me some interesting discussion points, and made me look good in front of one of my heros in racecar design, cheers.

Ben

[jpf]

Congrats, Ben

It's always fun to think of what questions I would ask the experts if I got the chance... I have a feeling it would be harder to decide if I had the depth/breadth of knowledge that you and several others around here demonstrate.

[Froilan_G]

Ben,

Glad to be of assistance by asking my Caster-question !

So I know now what Caster is for, but why do you want a large scub radius (as I kinda understand from Mr Southgate this is a desirable thing?)

[BRIAN GLOVER]

Now I can say that I know someone that knows Tony Southgate. Still no info Ben. Try this one.

[Ben]

I appear to have lost my transcribed copy of the Wright article. I'll have a look for it on my other PC soon.

As for Scrub radii, it's not that you'd want a large scrub radius, it just tends to be a by-product of packaging constraints in this region. Under any split-mu braking event (this could be due to weight transfer increasing the load on one wheel) any scrub radius will cause a large torque that will be fed back to the steering wheel.

If you have a large scrub radius the torque will be large, hence heavy steering. The reason you can live with this on an FStudent car is the light weight which means the forces are small. Power steering allows you to live with it on a larger car such as an LMP900. It's better to live with a large scrub radius if you can, because the alternative tends to be more king pin inclination which is definitely undesirable.

Ben

[imaginesix]

Originally posted by Ben
I've just spent the last three days have a variety of very interesting conversations with amongst others, Tony Southgate and a Jaguar roadcar suspension engineer...

Excellent insight. I'll just have to give your item no. 2 some more thought, I can't make sense of it as of yet.
I've sent the article out to the clamoring masses. so don't worry 'bout it.

...hmmm prolly something to do with the different camber requirements for changing download and lateral loads on aero cars, but what about non-areo... hmmmmmm