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Don’t buy drop spindles?

2369 Views 51 Replies 21 Participants Last post by  Shaun @ Street or Track
Thoughts? I think someone else figured out the geometry.
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I found other issues with the drop spindles I tried...namely that by moving the spindle pin location upward, you also move the steering arm location downward in relation to the wheel....probably not a big deal for most people, but with my setup it became a dealbreaker. It severely limits you in tie-rod placement vertically.

Note: I am not talking about any particular drop spindle, just drop spindles that achieve their drop by moving the spindle pin upward.
 

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Every few times I open up RCVD, its always amazing to me how "old" the principles are. There's been some additional insight but the base ideas and concepts are nothing new. Any time I look at the information compiled by Doug Milliken or any of his contemporaries, its mind boggling how much extra work had to be done prior to widespread availability of CAD. You guys were nuts and we've got it easy now, especially with something as complex as suspension geometry.
You should have seen us carving a solid block of 6061-T6 for a spindle upright design.
To get to that point there were years of suspension design drawing and mockup.
Yeah. Those were the days.

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My take on what’s being said is not so much relocating the pin to make the car sit lower but actual height of the spindle itself. My understanding of short and tall spindle is in it’s relationship to the upper control arm mounting location. In a performance situation you want the ball joint pivot location to be higher than the mounting point of the control arm. As the suspension compresses it pulls the top of the wheel in following the arc of the control arm for negative camber gain. In the case of the Mustang it’s not so when stock. To get the negative camber gain you can put a taller spindle or lower the mounting location. It comes down to that relationship of the ball joint to shaft location.

In the case of MM, I see it that he’s using a taller spindle to make the ball joint higher in relationship to the shaft while Shaun has shortened the spindle but has also changed the shaft location to accomplish the same basic idea. To me Shaun has taken it to the next level in engineering. That’s my impression of what’s going on.
 

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My take on what’s being said is not so much relocating the pin to make the car sit lower but actual height of the spindle itself. My understanding of short and tall spindle is in it’s relationship to the upper control arm mounting location. In a performance situation you want the ball joint pivot location to be higher than the mounting point of the control arm. As the suspension compresses it pulls the top of the wheel in following the arc of the control arm for negative camber gain. In the case of the Mustang it’s not so when stock. To get the negative camber gain you can put a taller spindle or lower the mounting location. It comes down to that relationship of the ball joint to shaft location.

In the case of MM, I see it that he’s using a taller spindle to make the ball joint higher in relationship to the shaft while Shaun has shortened the spindle but has also changed the shaft location to accomplish the same basic idea. To me Shaun has taken it to the next level in engineering. That’s my impression of what’s going on.
Kind of. I should have put the disclaimer in my first comment too but I'm not a suspension whiz by any means. Wiring, engine tuning, and tire kinematics i've got down pretty well but chassis and suspension stuff I'm still catching up with. That being said, there's a few things going on from my limited understanding:

Its not just the upper control arm (UCA) that's dictating camber change but also the lower control arm (LCA) or LCA + strut rod in our design. You're correct in that the arcs of these are going to dictate kinematic (moving) camber change but its the difference in them that's the important part. As an example, with the stock-height springs and the LCA in an "OE" configuration, it's pointing down. With the LCA pointing down, the amount of lateral change in the lower ball joint (LBJ) was less than the lateral change than the upper ball joint (UBJ), mainly because of their length difference. That's why moving the UCA inboard points down slightly via the "shelby drop" were able to improve the camber curve for a mostly-stock geometry. Instead of an initial outward movement in the UBJ, it's now an inward movement, so the overall camber change is improved. It's possible to have the UBJ point lower than the inboard point, and still have negative camber curve through travel from static.

The spindle location itself isn't going to have a whole lot to do with any of the above, nor the bump-steer qualities. That's all dictated by the inboard and outboard points of the control arms. It will however change how the forces act on those points which the driver is likely to feel. Additionally, changing the tire centerline via wider tires will affect the scrub radius, which will change how the car feels driving and mainly under braking. Also there are advantages and disadvantages to moving the outboard points closer together. How much of the disadvantages are quantifiable, i don't know, I've not played around with that aspect as much.

Also as an aside as an interesting conversation point, all of the designs discussed up until now are "short spindles", their points all land within the inner wheel diameter. There are designs however like the 350z and 370z that have a "long spindle" where the UBJ lands outside of the tire. Totally irrelevant to the conversation at hand but just an interesting tidbit I thought of when yous tarted talking about long vs short spindles!

350z front geometry:
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I am lucky to have our shop in the suburbs of 'Motor City' and been fortunate enough to get to know some very talented engineers. The particular gentlemen that we hired for the spindle geometry project has over 40 years OEM experience in vehicle dynamics. Everything from family sedans to sports cars, Formula SAE to race cars. When he tells me the tire is everything and getting a wider/usable contact patch on the ground is gold, I do as I'm told and listen.

First we measured the entire front end of the car. All the pickup points, suspension link lengths etc. We already had a lot of this data so not too bad.

Then, using suspension design software we were able to start developing a new spindle that would fit inside a 17" wheel, use our existing control arms AND improve the geometry.

We then had to build an entire CAD model to figure out packaging:


View attachment 877662

We then had to sit down with our suspension engineer and CAD guy to tweak packaging and geometry. Having the geometry and CAD model was one thing but they have to play nice together. This back and forth to figure out the best geometry and what would actually fit took lots of time, many meetings and several years...

View attachment 877664

Once the geometry would fit inside the wheel, we then had to stress test the part through computer simulation. Again, this was another round and round exercise. If a certain area wasn't up to the loads we had to modify the design and if that modification affected the geometry would have to go through the process all over again...

View attachment 877665

My real world testing has shown that as well as fitting the wider tires up front, we are indeed using that extra contact patch. Recent trips to Road America have shown we are using all the contact patch of the 275's on our '66 and not the outer half we used to with the regular height spindles. The car is more balanced, has less understeer and is very fun to drive really fast.

The '66 drives perfectly straight even at 160mph on the race track and the 70 drives just fine around town.

I am very proud of this project. It was a big gamble and took a ton of time and money to figure out.
Does your design do anything in the steering game such as reduce bump steer?
 

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Kind of. I should have put the disclaimer in my first comment too but I'm not a suspension whiz by any means. Wiring, engine tuning, and tire kinematics i've got down pretty well but chassis and suspension stuff I'm still catching up with. That being said, there's a few things going on from my limited understanding:

Its not just the upper control arm (UCA) that's dictating camber change but also the lower control arm (LCA) or LCA + strut rod in our design. You're correct in that the arcs of these are going to dictate kinematic (moving) camber change but its the difference in them that's the important part. As an example, with the stock-height springs and the LCA in an "OE" configuration, it's pointing down. With the LCA pointing down, the amount of lateral change in the lower ball joint (LBJ) was less than the lateral change than the upper ball joint (UBJ), mainly because of their length difference. That's why moving the UCA inboard points down slightly via the "shelby drop" were able to improve the camber curve for a mostly-stock geometry. Instead of an initial outward movement in the UBJ, it's now an inward movement, so the overall camber change is improved. It's possible to have the UBJ point lower than the inboard point, and still have negative camber curve through travel from static.

The spindle location itself isn't going to have a whole lot to do with any of the above, nor the bump-steer qualities. That's all dictated by the inboard and outboard points of the control arms. It will however change how the forces act on those points which the driver is likely to feel. Additionally, changing the tire centerline via wider tires will affect the scrub radius, which will change how the car feels driving and mainly under braking. Also there are advantages and disadvantages to moving the outboard points closer together. How much of the disadvantages are quantifiable, i don't know, I've not played around with that aspect as much.

Also as an aside as an interesting conversation point, all of the designs discussed up until now are "short spindles", their points all land within the inner wheel diameter. There are designs however like the 350z and 370z that have a "long spindle" where the UBJ lands outside of the tire. Totally irrelevant to the conversation at hand but just an interesting tidbit I thought of when yous tarted talking about long vs short spindles!

350z front geometry:
View attachment 877810
True the lower arm will have some effects on camber. However they’re usually longer arm than the upper arm. It’s arc of travel is going to be far more gradual and less camber changes one way or another. I was just mentioning the upper as the meats and potatoes of it.
 

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Does your design do anything in the steering game such as reduce bump steer?
I think I read somewhere that his shortened arm and relocated outer tie rod hole does do this....but I can't remember where I read that now.

As far as camber gain goes....its not only LCA/UCA lengths and angles. As a random example: A shorter spindle will also affect roll center:

A shorter spindle will effectively move the outer pivot points of the spindle...pulling down the outer pivot of the UCA, which to some degree undoes some of the effect of a shelby drop(or looking at it a different way allows for more of a shelby drop than the standard 1"). So without re-doing the shelby drop with only the change of a shorter spindle your roll center would effectively be raised. Of course...there are other ways to get around that, such as using a taller ball joint stud, which would effectively keep your outer pivot points where they were...but everything you change effects something else and has to be considered.

Beyond that, spindle height isn't the only thing that affects camber gain under cornering either. SAI for example does as well, an increased SAI will help high speed stability and return-to-center...but at the same time affects the camber as you turn the car, reducing negative camber on the inside tire and increasing positive camber on the outside tire(good for the outside tire, bad for the inside tire), but that's not the end of the story either, because increasing caster can fight this tendency as well and you end up with a rule of thumb that you want 1/2 the caster as your SAI. IE, 4 degrees of caster for 8 degrees of SAI, 5 degrees of caster for 10 degrees of SAI, etc.

Anyway, the point is that none of this exists in a vacuum, there is always more than one variable acting on a car's suspension so what might work perfectly for one car may be completely different for another leading to 2(or more) wildly different approaches that both work.

I am certainly no suspension expert and I have done a LOT of reading on the various factors included, but I am left with the feeling I know just enough to understand I don't know much of anything regarding suspension design and that if I can think of changes to the roll center, etc, there is certainly something else I am missing on a given design that might counteract such a change....or that such a change might not even be bad.
 
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Yes, bump steer was reduced 50% over the 70-73 spindles we took off the development car.
With that dropped spindle, you would also get a lowered ride height and still have greater suspension movement over say someone who used lowering springs correct? I know you mentioned the bump steer being 50% reduced. Is that in reference to someone using a 70 spindle on a 65, or is that any of the spindles regardless of year?

Thanks!!

Chris

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a rule of thumb that you want 1/2 the caster as your SAI. IE, 4 degrees of caster for 8 degrees of SAI, 5 degrees of caster for 10 degrees of SAI, etc.
I'm no engineer, butt I've read in a couple of places that for a road race car you want around the same degree for the caster and SAI.

I love this thread.
 

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I'm no engineer, butt I've read in a couple of places that for a road race car you want around the same degree for the caster and SAI.

I love this thread.
That would make it impossible to get the required caster in a classic mustang then, since the stock spindle has 8ish degrees SAI, but I don't know...maybe the road racing vintage mustangs are able to get that extreme a caster angle? Its quite possible though what I read with that rule of thumb applied to higher speed curves though. 🤷‍♂️ I figure that if you end up changing your spindles you are going to end up playing around with multiple different alignment settings until you find something that behaves the way you like anyway.
 

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A lot of terminology being thrown around! SAI is steering axis inclination. The angle between the upper and lower ball joints with the steering wheel straight. Does this angle change with camber, or is it a fixed angle of the spindle? The drawing below suggest to me that it might change with camber? I'm just curious and it has nothing to do with your average VMF member buying a few basic tools and aligning their car themselves. People throwing fancy terms around might confuse the rest of us and back in the day I aligned hundreds of cars without knowing most of this stuff. All I can say is that I am thankful for CAD and modeling programs but always curious. I could do rise run and diagonal calculations on multi pitched roofs before they had fancy calculators but arbitrary points I have trouble wrapping my head around.

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I found a site with simple definitions! Definitions and Explanations of Suspension Alignment Terms
 
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I can get 8° caster pretty easily.

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I don't doubt that aftermarket suspensions can achieve it, but how about stock suspensions? I know there are some out there who road race on stock control arms and non-adjustable strut rods. I guess its pointless to ask that question though since they probably aren't competitive outside of stock classes.
 

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A lot of terminology being thrown around! SAI is steering axis inclination. The angle between the upper and lower ball joints with the steering wheel straight. Does this angle change with camber, or is it a fixed angle of the spindle? The drawing below suggest to me that it might change with camber? I'm just curious and it has nothing to do with your average VMF member buying a few basic tools and aligning their car themselves. People throwing fancy terms around might confuse the rest of us and back in the day I aligned hundreds of cars without knowing most of this stuff. All I can say is that I am thankful for CAD and modeling programs but always curious. I could do rise run and diagonal calculations on multi pitched roofs before they had fancy calculators but arbitrary points I have trouble wrapping my head around.
I found a site with simple definitions! Definitions and Explanations of Suspension Alignment Terms
Care must be exercised for this particular angle. If i remember correctly, the way SAI is measured is from the kingpin axis (steering axis) to the tire centerline, so it it always a fixed value even if camber dynamically changes. SAI can sometimes be used interchangeably with Kingpin Inclination angle (KPI) but I think there's a difference there, just having a hard time with specifically what at the moment or rather clearly conveying why they're different.

Also, that's not a bad site for the basic terminologies. I'll have to dig around because I've got another somehwere too. Regardless of the source, they should all be derived from "SAE J670 – Vehicle Dynamics Terminology". That's the document that establishes the common definitions so everyone's on the same page. It gets more complex, and if someone's interested in reading it, you'll have to....ahem....search around for a copy that's not behind the SAE paper paywall, but they're out there.

Slope Parallel Font Diagram Symmetry


It's still boggling the amount of work to do all of this w/o CAD accurately, but it's possible. It'll take so many more checks/steps drawing everything in 2D since it's really a 3D problem but even just using the FVSA (front view swing arm) and SVSA (side view swing arm) you can get some approximations, which is normally "good enough" for napkin math. There's so much variance in compliance and other factors that having an estimate within ~10% is usually fine. Having a grasp on the above gets to be important when you start talking to the guy designing the suspension and doing all the n-line analysis (line from the contact patch to the IC), etc. etc. to get some rough numbers on "whats happening".

This might get a little too far into the weeds...but....Here's a good illustration on what's all going on in a corner, and shows those "n-lines" i keep talking about. One of the nice things about n-lines is that as the illustration shows, the roll center (RC) migrates as soon as you get any suspension travel, or in the case of some cars like circle-track cars that are non-symmetrical the RC is never on the car center line. Because of that, thinking about the RC as the "point around which everything acts" makes it a bit more difficult to start doing some basic hand-calcs on what's going on. In contrast, working with the n-line approach, the acting forces can be slid along the n-line to the car centerline and then used to calculate roughly how that's impacting the car dynamics.

Rectangle Slope Font Line Parallel



The FSAE forums could be....interesting at times....and there were some personalities (as we all have them) but this post by one of it's more notable members is a good discussion on using n-lines and a FAP vs focusing on the RC: RC vs FAP

The basic part just boils down to "don't loose the forest for the trees" and then also "there's always 18 ways to solve a problem".

I'm no engineer, butt I've read in a couple of places that for a road race car you want around the same degree for the caster and SAI.

I love this thread.
Also w.r.t. kingpin angles (SAI) moto-iq has a good article on the subject: MotoIQ - Kingpin and Scrub
 

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