help me understand caster
#16
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100% ditto on everything fatbillybob says; just adding a more direct correlation for you. I run about 4.5 deg caster in my 924, always been quite happy with feel and all in my tail-heavy (2%) car. Sure, it's heavy in the paddock - but I prefer to race on the track, at speed. When I'm going at it with the rest of the crowd out there - the steering effort is just one on a long list of things that I ignore.
#17
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Originally Posted by RedlineMan
At your Service, Larry Darling!
Scrub Radius - The distance between the extended line of the steering axis, and the centerline of the tire contact patch.
Scrub Radius - The distance between the extended line of the steering axis, and the centerline of the tire contact patch.
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Larry Herman
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Larry Herman
2016 Ford Transit Connect Titanium LWB
2018 Tesla Model 3 - Electricity can be fun!
Retired Club Racer & National PCA Instructor
Past Flames:
1994 RS America Club Racer
2004 GT3 Track Car
1984 911 Carrera Club Racer
1974 914/4 2.0 Track Car
CLICK HERE to see some of my ancient racing videos.
#18
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Here's another perspective on what the Larry and John show has demonstrated. Increased caster = increased self centering action i.e. the car wants to go straight - excellent for high speed stability. As L and J have explained, the reason the car is more stable is that the tire is being twisted. That 'twist' which creates the self centering force is scrubbing the tire. How much? That's what John showed in his scrub radius post. So, more caster = more self centering = more scrub. A GT3 calls for about 8 degrees of caster. At 9, the tire rubs on the fender liner- at least mine does.
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Just one small point, Larry--with great respect!
King Pin Inclination and Caster are not the same thing. KPI is the inward or outward tilt of the steering axis when viewed from the front. Caster is the forward or rearward tilt of the steering axis when viewed from the side.
Another downside of excess caster is toe change. Caster changes camber (as already mentioned). Any time you change camber, you change toe, hence tire wear can be a problem. This is why toe is always set last, every other adjustment you make will affect toe to some extent.
Scrub radius is the distance between the centre of the tire and the place where an imaginary line drawn through the steering axis would meet the ground. Typically in stock steups, this line extended outside of the centre of the tire. This makes the steering more stable during bumps as there is a smaller moment on the steering. Increasing the offset moves this line to the inside of the tire centre making handling more responsive and turn in crisper.
King Pin Inclination and Caster are not the same thing. KPI is the inward or outward tilt of the steering axis when viewed from the front. Caster is the forward or rearward tilt of the steering axis when viewed from the side.
Another downside of excess caster is toe change. Caster changes camber (as already mentioned). Any time you change camber, you change toe, hence tire wear can be a problem. This is why toe is always set last, every other adjustment you make will affect toe to some extent.
Scrub radius is the distance between the centre of the tire and the place where an imaginary line drawn through the steering axis would meet the ground. Typically in stock steups, this line extended outside of the centre of the tire. This makes the steering more stable during bumps as there is a smaller moment on the steering. Increasing the offset moves this line to the inside of the tire centre making handling more responsive and turn in crisper.
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And...
King pin Inclination is more universally refered to as Steering Axis Inclination. SAI is the lateral angle formed by a line drawn through the upper and lower ball joints (McPherson strut through the strut bearing and ball joint). It is yet another way engineers have discovered to increase stability at speed. It also has the concurrent effect of reducing steering effort by reducing scrub radius.
King pin Inclination is more universally refered to as Steering Axis Inclination. SAI is the lateral angle formed by a line drawn through the upper and lower ball joints (McPherson strut through the strut bearing and ball joint). It is yet another way engineers have discovered to increase stability at speed. It also has the concurrent effect of reducing steering effort by reducing scrub radius.
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Originally Posted by DGaunt
Just one small point, Larry--with great respect!
King Pin Inclination and Caster are not the same thing. KPI is the inward or outward tilt of the steering axis when viewed from the front. Caster is the forward or rearward tilt of the steering axis when viewed from the side.
King Pin Inclination and Caster are not the same thing. KPI is the inward or outward tilt of the steering axis when viewed from the front. Caster is the forward or rearward tilt of the steering axis when viewed from the side.
Originally Posted by RedlineMan
And...
King pin Inclination is more universally refered to as Steering Axis Inclination.
King pin Inclination is more universally refered to as Steering Axis Inclination.
#22
Burning Brakes
Does anyone know the maximum caster which can be set on a stock '86 944/951? I know 2.5 to 3.0 is what it calls for--I'm just wondering the max if both eccentrics are set to max positive. Also, where would the adjusting tab on the eccentric be positioned for this max?
Just curious since I recently converted my951 to manual steering and thought some of the hard steering effort was due to caster being too much but when I measured it I came up with +5 or so. Just want to know if that's possible or if my measuring was off?
Thanks--
Just curious since I recently converted my951 to manual steering and thought some of the hard steering effort was due to caster being too much but when I measured it I came up with +5 or so. Just want to know if that's possible or if my measuring was off?
Thanks--
#23
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Originally Posted by Larry Herman
One of the down sides of too much caster is the weight jacking that it produces. As you increase the caster you actually start lifting the front of the car as you turn the wheel.
Interesting discussion. I've speculated that your comment must be true also for a car chassis, but don't recall ever seeing it discussed by the Smiths and Van Valkenburghs etc.
On my son's small cadet chassis kart, the rear tire will be an inch off the pavement at a rest when the wheel is at full lock...
#24
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John, you are right. KPI was the original term used when vehicles had solid axles and actual kingpins. When I was trained in the late 80's KPI and SAI was pretty much used interchagably in SLR (short-long arm) suspensions. On Mcpherson struts, SAI is the preferred term and as SAI functionally serves a similar role to caster (like, corner jacking and camber change in turns) and is a necessary part of Mcpherson packaging, caster has been reduced in strut cars, giving over some of its duties to SAI.
Time to shut up.
Time to shut up.
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Thanks Dave, that's even more helpful information. I just ran across an excellent article written by Roger Jackman on Scrub Radius on the H&R website. Click here to read. I thought that it explains even more than we discussed, and is worth reading.