My SPF seems to have somewhat heavy steering effort. Toe is 1/16" in. Castor (non adjustable) is 7 degrees. I assume original Gt40's also had a high castor setting too (for high speed stability?). The steering tie rod ends attach to the upright at an unconventional angle by modern racing standards. Was there a reason for that? Some vehicles like my pickup truck have high castor, but power steering. The late Carroll Smith (of GT40 fame) stated that race cars should have castor between 2 and 4 1/2 degrees. Short of fabricating new a arms, do you just get used to the steering effort?
Castor/Heavy Steering
- Thread starter SwiftDB4
- Start date
Jack Houpe
GT40s Supporter
Dave, toe in or toe out? I had a little extra effort when it was out but now I have adjust to 1/8" in seems very easy to turn.
7 degrees, in my experience, is pretty high. We ran that much on a Trans Am car because of tire wear issues with Good Years, but were able to bring that down when we switched to BFGs. It may be that that was the original setup, but it seems excessively high for a street application. It will make it harder to turn at slow speeds, especially if you have wide tires.
Dave
As Crash 33 said 7 deg. is pretty high, I would run no more than 5. Also You said it is un-adjustable, which I find unusual. If I remember from looking at the SPF chassis it is almost a carbon copy of the original, and that caster was adjustable, Essentially shimming the upper arm rearward creates more caster. As you add caster to that setup you will also add camber to the positive, so taking away caster will give you more negative camber. So that will have to be adjusted also. I would take your car to someone familiar with that suspension setup and I believe they can sort out your issues.
Essentially with positive caster you are lifting the chassis as the wheels go thru the turn arc so that combined with a wide front tire can make for heavy steering.
Good luck
Phil
As Crash 33 said 7 deg. is pretty high, I would run no more than 5. Also You said it is un-adjustable, which I find unusual. If I remember from looking at the SPF chassis it is almost a carbon copy of the original, and that caster was adjustable, Essentially shimming the upper arm rearward creates more caster. As you add caster to that setup you will also add camber to the positive, so taking away caster will give you more negative camber. So that will have to be adjusted also. I would take your car to someone familiar with that suspension setup and I believe they can sort out your issues.
Essentially with positive caster you are lifting the chassis as the wheels go thru the turn arc so that combined with a wide front tire can make for heavy steering.
Good luck
Phil
Ross Nicol
GT40s Supporter
Is the steering heavy when not moving? or when cornering.I was under the impression my heavy steering was caused by a large amount of castor until the learned gentlemen on this forum informed me about scrub radius.I have 9" rims shod with slicks on the front of my race car and the steering is much lighter now after reducing the scrub radius.
Ross
Ross
Phil,
Thanks for the tip of shimming the upper radius arm to change camber. I'll have to look at the SPF to see if it has shims. I'm pretty used to formula cars with fully heim jointed a arms. Also Ross referred to scrub radius. What is that? My steering is heavier at rest, but stiil not as light as I'd like in motion. I have 225/60/15 tires on 8" rims. Other race cars I have owned have much lighter steering, but they weighed less too.
Thanks for the tip of shimming the upper radius arm to change camber. I'll have to look at the SPF to see if it has shims. I'm pretty used to formula cars with fully heim jointed a arms. Also Ross referred to scrub radius. What is that? My steering is heavier at rest, but stiil not as light as I'd like in motion. I have 225/60/15 tires on 8" rims. Other race cars I have owned have much lighter steering, but they weighed less too.
Ross Nicol
GT40s Supporter
Dave
Phil was referring to shimming the upper 'A' arm rearward to increase 'caster' not camber. No doubt you have heim joints for setting up the camber. If you draw a line to ground through your front ball joints and measure from this point to the centre of the tyre, this measurement is the scrub radius. Incorrect wheel offset, wheel spacers, thick disc rotors and poor knuckle/ upright design can all be responsible for causing a large scrub radius. I've reduced mine by 2" and it's made a heck of a difference in reducing the heavy steering.
Ross
Phil was referring to shimming the upper 'A' arm rearward to increase 'caster' not camber. No doubt you have heim joints for setting up the camber. If you draw a line to ground through your front ball joints and measure from this point to the centre of the tyre, this measurement is the scrub radius. Incorrect wheel offset, wheel spacers, thick disc rotors and poor knuckle/ upright design can all be responsible for causing a large scrub radius. I've reduced mine by 2" and it's made a heck of a difference in reducing the heavy steering.
Ross
Dave: The original GT40s ran bias-ply tires, which require different alignment specs, particularly caster/castor. It could be a lot less due to higher rolling resistance. I don't remember exactly what we used on the MkIVs, but I think it was no more that +1 degree. I think the MkIs and MkIIs followed the same pattern. My MkI uses radials, so I run +6 degrees caster for steering centering at speed, but steering is light, even at lower speeds. It is, however, bit heavy in the parking lot, which is not a good environment for a GT40 anyhow. Toe is in 1/8 in. I keep the front tires straight up, or 0 degrees camber.
As for scrub radius--distance between the center of the tire patch and insection of the steering axis at the ground--it can't be changed without changing wheel offset or doing major chassis work. The good news is scrub doesn't need changing, except low scrub is does make low-speed steering a bit stiff. Note that most modern race cars, not including NASCAR, use nearly zero scrub. This reduces kickback forces going into the steering system, although it does improve parking lot steering effort.
As for scrub radius--distance between the center of the tire patch and insection of the steering axis at the ground--it can't be changed without changing wheel offset or doing major chassis work. The good news is scrub doesn't need changing, except low scrub is does make low-speed steering a bit stiff. Note that most modern race cars, not including NASCAR, use nearly zero scrub. This reduces kickback forces going into the steering system, although it does improve parking lot steering effort.
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Dave;
I might have been a little vague on the shimming. Essentially as Ross was pointing out the upper arm is shimmed fore or aft at the inner pivots, and you should have a Heim joint or upper ball joint at the top of your upright that can be threaded in or out to change camber settings. Thanks Ross, you always have the good observations. Also on the scrub radius as Jonathans put up it is essentially the distance created by the center line of wheel mounting drawn in a vertical plane, intersecting with the horizontal line of road surface, and the line drawn thru the ball joints doing the same, the distance between these two points being the scrub radius.. Again as Ross pointed out wheel offsets, mountings such as additional pin drive adapters etc., can increase effort by increasing scrub radius. One thing that is notable these days is the offsets being used on OEM wheels, some at 5 or 6 inches, especially on front wheel drivers as it was found early on that there was a lot of steering force generated by applying power. Most of these front ends are fairy neutral now, and fairly wide tires are the norm. I also wanted to ask if your steer rack is ok, I have seen some racks bind for various reasons, its not frequent, but it happens. I think with all here on the forum you should be able to sort your problem fairly easily.
Cheers
Phil
I might have been a little vague on the shimming. Essentially as Ross was pointing out the upper arm is shimmed fore or aft at the inner pivots, and you should have a Heim joint or upper ball joint at the top of your upright that can be threaded in or out to change camber settings. Thanks Ross, you always have the good observations. Also on the scrub radius as Jonathans put up it is essentially the distance created by the center line of wheel mounting drawn in a vertical plane, intersecting with the horizontal line of road surface, and the line drawn thru the ball joints doing the same, the distance between these two points being the scrub radius.. Again as Ross pointed out wheel offsets, mountings such as additional pin drive adapters etc., can increase effort by increasing scrub radius. One thing that is notable these days is the offsets being used on OEM wheels, some at 5 or 6 inches, especially on front wheel drivers as it was found early on that there was a lot of steering force generated by applying power. Most of these front ends are fairy neutral now, and fairly wide tires are the norm. I also wanted to ask if your steer rack is ok, I have seen some racks bind for various reasons, its not frequent, but it happens. I think with all here on the forum you should be able to sort your problem fairly easily.
Cheers
Phil
The other big difference nowdays that tends to mask some of these geometry effects is the use of power steering in newer cars.
Howard Jones
Supporter
Speed220, Do you know what the "standard" scrub radius was on the orginal MK1's. My GTD is a little heavy at speed but not bad enough to spend a lot of money on. However it would be interesting to compair my car with a orginal cars geometery.
Ross, If your car had more than 2" scrub radius what did you end up with that seams to have made it a lot better.
Thanks guys, I'm trying to find out what would work and be doable with my car. I am pretty sure I can gain (reduce scrub) 3/8 inch or so but not much more by milling the back of the wheels. The point is would it be worth the effort. Srub radus is the only area of interest left for me on my front geometery as I think I have done what I can within my buget. New uprights and a complete redo are not going to happen.
By the way I have set my GTD up with +4 degrees of caster on both sides and -5/8 degree of camber. Getting very even tire wear and eben tire temps on the front with these settings.
Ross, If your car had more than 2" scrub radius what did you end up with that seams to have made it a lot better.
Thanks guys, I'm trying to find out what would work and be doable with my car. I am pretty sure I can gain (reduce scrub) 3/8 inch or so but not much more by milling the back of the wheels. The point is would it be worth the effort. Srub radus is the only area of interest left for me on my front geometery as I think I have done what I can within my buget. New uprights and a complete redo are not going to happen.
By the way I have set my GTD up with +4 degrees of caster on both sides and -5/8 degree of camber. Getting very even tire wear and eben tire temps on the front with these settings.
Howard: I'll have to do some major digging to find the specs. As for caster affecting tire wear, it doesn't. That would be toe and camber. But uneven caster will cause a pull, or left pull for excess positive caster on the right and vice versa.
I'll see if I can find the original GT40 alignment specs.
Tom
I'll see if I can find the original GT40 alignment specs.
Tom
This is maybe only my opinion - but
I would suggest that the original specifications for alignment would have little relevance given that the technology in tires has changed so dramatically over the years.
One key reason that lower caster angles were used back in the day of the bias ply belted tires is that they could not alter their profile to accommodate the higher angles.. The same holds true for camber..
Many drivers shunned the advent of radial ply race tires because they felt that the tires was not giving them the proper feedback of the racing surface. Once the tires were made widely available and teams started actually dialing in the chassis to the tires (read alignment and springs) and drivers learned how to make use of the new tires - Track records started dropping like panties on prom night..
I would suggest that the original specifications for alignment would have little relevance given that the technology in tires has changed so dramatically over the years.
One key reason that lower caster angles were used back in the day of the bias ply belted tires is that they could not alter their profile to accommodate the higher angles.. The same holds true for camber..
Many drivers shunned the advent of radial ply race tires because they felt that the tires was not giving them the proper feedback of the racing surface. Once the tires were made widely available and teams started actually dialing in the chassis to the tires (read alignment and springs) and drivers learned how to make use of the new tires - Track records started dropping like panties on prom night..
You're right, Big. We need a lot more caster and a little more camber. One problem with radials is they don't give as much warning when they are about to lose grip. I experienced this at about 210 mph. Where steering felt a bit mushing before bias ply tires lost grip, I was at full lock with little warning with the new radials. We had to develop a more sensitive back side with radials.
There is an extensive discussion on scrub radius and other factors here.
http://www.gt40s.com/forum/gt40-tec...what-causes-heavy-steering-racing-speeds.html
Somewhere in there I think Trevor Booth said the scrub radius on originals was 4 1/2 inches. I haven't got time to hunt it out now, got to get to work.
http://www.gt40s.com/forum/gt40-tec...what-causes-heavy-steering-racing-speeds.html
Somewhere in there I think Trevor Booth said the scrub radius on originals was 4 1/2 inches. I haven't got time to hunt it out now, got to get to work.
:laugh:
He he he.... Well you got me by a few MPH or so... I think I was at a buck-sixty three wide when I sort of found out...
The problem is (or was) that with the old belted tires, they would respond instantly to steering input if they were going to respond at all..
The radials - well - who knew that you had to initiate your turn well in advance of the corner and wait for the tire to catch up and actually turn you?
So here you are on your brand new radial slicks, you are used to tires that respond immediately, you turn into the corner and nothing, then you instinctively turn in more, and more and you have now run the tire into excessive slip angle where it isn't going to bite into anything but the wall...
Oh the wailing... Virtually gone with the radial tires... No more long wail of the tire where you could judge just how much she had left in her before she would turn you loose... The radials - well the only time they would wail is if they were already gone and you were heading up the hill to meet the folks in the stands... One thing we learned pretty quickly was to not listen to the tires any more but (like you said) put a sharper tune-up on our back-sides to feel the G's as they changed...
Oh the good(?) old days....
Glad we're still here to talk about them... A lot of guys aren't...
He he he.... Well you got me by a few MPH or so... I think I was at a buck-sixty three wide when I sort of found out...
The problem is (or was) that with the old belted tires, they would respond instantly to steering input if they were going to respond at all..
The radials - well - who knew that you had to initiate your turn well in advance of the corner and wait for the tire to catch up and actually turn you?
So here you are on your brand new radial slicks, you are used to tires that respond immediately, you turn into the corner and nothing, then you instinctively turn in more, and more and you have now run the tire into excessive slip angle where it isn't going to bite into anything but the wall...
Oh the wailing... Virtually gone with the radial tires... No more long wail of the tire where you could judge just how much she had left in her before she would turn you loose... The radials - well the only time they would wail is if they were already gone and you were heading up the hill to meet the folks in the stands... One thing we learned pretty quickly was to not listen to the tires any more but (like you said) put a sharper tune-up on our back-sides to feel the G's as they changed...
Oh the good(?) old days....
Glad we're still here to talk about them... A lot of guys aren't...
Randy, somehow I think your prom must have been considerably more exciting and memorable than mine was.....
Howard Jones
Supporter
Wouldn't 4.5 inchs be a HUGE! amount of scrub radius. I was thinking in terms of 1/4's of inches.
Frank C, where are you guys at on this issue (scrub radius) with GTD's ? It would be very helpful to know what the GTD's start out with. Standard granada uprights and the fake knock on 15" hasselbrads (sp?) that came with them back in the day.
By tire wear I mean that when you have a look at contact scrubing on the tires after coming in from a track session the tires look like they are getting good even wear accross the contact patch. That along with temps and pressure gain side to side seam to tell me that I am pretty close to good on the front geometry.
I would like to reduce the effort it takes to drive the car however. I am pretty sure that scrub radius is the fix for this but with only really less than 5/8" to play with I would like to know what a good number would be to to use for a boggie.
My MK1 eyeball tells me that I am at something less than 2" now. Could be as little as 3/4" but I don't think so. I guess I will need to make some measurements and quit guessing.
Frank C, where are you guys at on this issue (scrub radius) with GTD's ? It would be very helpful to know what the GTD's start out with. Standard granada uprights and the fake knock on 15" hasselbrads (sp?) that came with them back in the day.
By tire wear I mean that when you have a look at contact scrubing on the tires after coming in from a track session the tires look like they are getting good even wear accross the contact patch. That along with temps and pressure gain side to side seam to tell me that I am pretty close to good on the front geometry.
I would like to reduce the effort it takes to drive the car however. I am pretty sure that scrub radius is the fix for this but with only really less than 5/8" to play with I would like to know what a good number would be to to use for a boggie.
My MK1 eyeball tells me that I am at something less than 2" now. Could be as little as 3/4" but I don't think so. I guess I will need to make some measurements and quit guessing.
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