Power Steering & Caster

[OB6]

This isn't roadster-specific, so perhaps I should have posted in a different section. Anyway, can someone explain why power steering allows for more caster? I understand (and have experienced) the benefits of greater caster, but to be honest I'm not sure why you can dial-in more with PS. Maybe a stupid question.

[edwardb]

More caster = more effort to turn the steering wheel. Only so much effort is manageable without the boost that power steering allows.

[GoDadGo]

More caster = more effort to turn the steering wheel. Only so much effort is manageable without the boost that power steering allows.

I'm at 4 degrees with the manual rack and am fine for cruising; however, I don't think I ever would want to attempt an Auto-Cross event with the manual rack even though back road curve carving is quite fun and manageable.

[OB6]

Ok thanks Paul. It didn't make sense to me that it would be a physical limitation (other than the driver's strength :))

[Jeff Kleiner]

Power steering doesn't actually allow more positive caster but it assists with the increased steering effort that positive caster creates. The geometry of the inclined steering axis means that positive caster of any amount is always using the vehicle's weight to try to bring the wheels back to center; with more caster the driver has to overcome more of that force to steer. Without power assist the effort required to steer the car when in the higher caster ranges (generally >3 to 4 degrees) becomes excessive and unmanageable. Power steering allows us to get the benefits of more positive caster...increased straight line stability, return to center and camber gain when turning...without the downside of excessive steering effort.

Jeff

[OB6]

Power steering doesn't actually allow more positive caster but it assists with the increased steering effort that positive caster creates. The geometry of the inclined steering axis means that positive caster of any amount is always using the vehicle's weight to try to bring the wheels back to center; with more caster the driver has to overcome more of that force to steer. Without power assist the effort required to steer the car when in the higher caster ranges (generally >3 to 4 degrees) becomes excessive and unmanageable. Power steering allows us to get the benefits of more positive caster...increased straight line stability, return to center and camber gain when turning...without the downside of excessive steering effort.

Jeff

Awesome, thanks for the detail Jeff. I'm always trying to figure out "why"

[Hoooper]

I'm at 4 degrees with the manual rack and am fine for cruising; however, I don't think I ever would want to attempt an Auto-Cross event with the manual rack even though back road curve carving is quite fun and manageable.

You should just go try it. Unless the driver is borderline bedridden turning a +4 caster manual rack quickly enough for autocross is not even noticeable effort. My experience from doing an autocross with +4.5 caster on 275 section (front) R comps, couldnt tell a difference in effort between it and driving my daily which has a quicker rack and PS.

[GoDadGo]

You should just go try it. Unless the driver is borderline bedridden turning a +4 caster manual rack quickly enough for autocross is not even noticeable effort. My experience from doing an autocross with +4.5 caster on 275 section (front) R comps, couldn't tell a difference in effort between it and driving my daily which has a quicker rack and PS.

I'll try it the next time we have a PCA event at NOLA Motorsports.

[Jeff Kleiner]

I'll try it the next time we have a PCA event at NOLA Motorsports.

The worst that can happen is you’ll be slow :p

Jeff

[Hoooper]

The worst that can happen is you’ll be slow :p

Jeff

Exactly. And even if you are slow you will almost certainly still have a whole pile of fun

[edwardb]

Power steering doesn't actually allow more positive caster but it assists with the increased steering effort that positive caster creates. The geometry of the inclined steering axis means that positive caster of any amount is always using the vehicle's weight to try to bring the wheels back to center; with more caster the driver has to overcome more of that force to steer. Without power assist the effort required to steer the car when in the higher caster ranges (generally >3 to 4 degrees) becomes excessive and unmanageable. Power steering allows us to get the benefits of more positive caster...increased straight line stability, return to center and camber gain when turning...without the downside of excessive steering effort.

Jeff

Now there's a first. Jeff's response is longer than my usual wordiness. :p Very good explanation BTW.

[Jeff Kleiner]

Now there's a first. Jeff's response is longer than my usual wordiness. :p Very good explanation BTW.

:D :D :D

Jeff

[CraigS]

One of the things I thought about when I first had a manual steering FFR was safety. I noticed a minor, no accident but sure got my attention, situation in a parking lot where another car did something unexpected. Then I started thinking about the unexpected maybe in a neighborhood at typical 30-40 mph. Could be a car or a kid and a ball. I am old and far from athletic, so upgraded to PS as soon as I could. One advantage w/ the MkIVs using the FFR spindles is that the SAI has been reduced from the old cars w/ Fox spindles. Caster is looking at a side view and seeing the upper ball joint is to the rear of the lower BJ. SAI is from a front view and seeing that the upper BJ is toward the center of the car compared to the lower BJ. Although caster and SAI have other effects on handling, considering steering effort only, they have very similar effects. So the SAI being reduced in the new suspensions would make them somewhat more manageable regarding manual steering effort.

[John Ibele]

One of the things I thought about when I first had a manual steering FFR was safety. I noticed a minor, no accident but sure got my attention, situation in a parking lot where another car did something unexpected. Then I started thinking about the unexpected maybe in a neighborhood at typical 30-40 mph. Could be a car or a kid and a ball. I am old and far from athletic, so upgraded to PS as soon as I could. One advantage w/ the MkIVs using the FFR spindles is that the SAI has been reduced from the old cars w/ Fox spindles. Caster is looking at a side view and seeing the upper ball joint is to the rear of the lower BJ. SAI is from a front view and seeing that the upper BJ is toward the center of the car compared to the lower BJ. Although caster and SAI have other effects on handling, considering steering effort only, they have very similar effects. So the SAI being reduced in the new suspensions would make them somewhat more manageable regarding manual steering effort.

The whole steering mechanism is really an engineering marvel.

The presence of caster means the wheels tend to stay pointed in the direction you point them.
Achieving it through the tilting of the steering axis away from vertical is really the genius part:

The wheels will go where you point them, but they like straight ahead a little bit more.
The spindle remains right on the steering axis. You wouldn't want a car with varying wheelbase, or wheels going in and out of the wheel well when they turn.
You have both wheels 'lean into' a turn. This increases pressure on the inside (apex) side of each tire effectively reducing its radius, and decreases it on the outside edge of each tire, increasing the effective radius there. This turns the tire into a conic section, and each tire then does what cones do: roll smoothly around a corner.
That work you do to lift the frame when you turn the wheel? You're actually lifting the frame on the inside wheel (because you're pointing this spindle slightly down) and unweighting the outside one, which helps counteract the centrifugal forces which weight the outside tire and unweight the inside one when you're going through a corner.
... not the full list ...

In other words, caster without caster angle would be about as much fun to drive as a shopping cart. Way wordier than Jeff's note but I couldn't help myself.

There's not much intelligent out there that really dives into the design choices and impacts of caster angle specifically, but GRM's short writeup (https://grassrootsmotorsports.com/articles/what-camber-caster-and-toe-handling-basics/) is pretty good for those that haven't seen it.

[CraigS]

For the first 7 years of owning my FFR MkI and the MkII, both w/ the old system w/ Fox spindles, I was still working as a Lexus tech. So I used the latest Hunter alignment equipment at least once per week. Of course most of my cars were Lexus models but I had time to simply look at any other car spec I wanted to. I was also slowly but surely upgrading my MkII to newer FFR front suspension. What I found was that around +8 deg caster (+ or- 1deg) was near a universal spec in cars that I considered to be known as good handling. I am also an avid autocrosser so I like to try things and I like to talk to others doing the same. I tried up to +10 but came back to about +8. I am retired now but the web is better than ever. And that # is still valid.

[MB750]

If anyone wants an extreme example of too much caster, look at those old motorcycle choppers from the 70's with the front wheel about 6' in front of the frame.

Rolling straight was a cake walk. Turning at all was a chore. Those things had the turning radius of an aircraft carrier. Super stable on the highway though.

[Ted G]

Maybe that is why my Big Wheel was so hard to steer.... specially with my feet on the pedals.

:o