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 Post subject: Suspension
PostPosted: December 28, 2007, 10:06 pm 
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Hooooooooooo

Now I understand!!! :cry:

I thought your suspension was similar to the picture at the start of the thread.

I agree that the spacer on top of your modified strut will at least be a good start in the right direction.

Another thing that could help this issue is to mod the tie rod end of your current steering arm by reaming a 1/2" hole more towards the spindle ".i.e. steering ratio" and mount a striper bolt and Hiem joint under the arm. This would allow further spacing the rod end lower to match the rack, as long as you do not get carried away. This will help correct the steering attachment on both Up-Down and Fore-Aft planes.
One other thing to be careful of, once you change the steering arm ratio you will probably have to add stops inside the steering rack to prevent over travel.

Someting else to think about, It might be better to look at swithing the steering arms from side to side and putting the rod end attachment point in the front, for front steer. It would require a different rack but in the long run it may be your best solution if the new rack had more travel per steering wheel rotation it would correction your slow steering at the same time.

I do not think there is an easy answer, it looks like a lot of work will be needed to put the steering right. You need LOTS of luck on this one. Dave W


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 Post subject: Re: Suspension
PostPosted: December 29, 2007, 12:17 am 
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davew wrote:
Hooooooooooo

Now I understand!!! :cry:

I thought your suspension was similar to the picture at the start of the thread.

I agree that the spacer on top of your modified strut will at least be a good start in the right direction.

Another thing that could help this issue is to mod the tie rod end of your current steering arm by reaming a 1/2" hole more towards the spindle ".i.e. steering ratio" and mount a striper bolt and Hiem joint under the arm. This would allow further spacing the rod end lower to match the rack, as long as you do not get carried away.

It turns out that the "correct" vertical position of the -center- of the Heim is almost at the same height as the top surface of the present steering arm. Acad helped me determine that the center of the Heim is also not located on the steering arm though.

Luckily I can temporarily bolt a short piece of 1"x1" angle iron to the original steering limit pad that is located under the steering arm. By using a machined "cone spacer" under the Heim I can adjust the height of the Heim to set the angle of the steering link in relation to the A arms. Once the settings are correct I will weld the angle to the steering arm.


This will help correct the steering attachment on both Up-Down and Fore-Aft planes.

One other thing to be careful of, once you change the steering arm ratio you will probably have to add stops inside the steering rack to prevent over travel.

Supposedly I won't have the condition where the steering link and steering arm get near the zero angular condition. But that is something I will definitely check out on the 1st temporary set up.

Something else to think about, It might be better to look at swithing the steering arms from side to side and putting the rod end attachment point in the front, for front steer. It would require a different rack but in the long run it may be your best solution if the new rack had more travel per steering wheel rotation it would correction your slow steering at the same time.

I considered that but it didn't fit in with my minimum changes criteria.

I do not think there is an easy answer, it looks like a lot of work will be needed to put the steering right. You need LOTS of luck on this one. Dave W

Yes, especially so since I want to do the minimum changes that will correct the major two problems (bump steer and camber correction). By the way, my caster is ~6° and the KPI is ~12°. Scrub is ~ 1.7".

I've been running computer simulations with suspension programs and Acad for many months now and I think/hope I have come up with the minimum changes that will work.

One thing for sure, I've learned a lot about suspension geometry etc because of this exercise. The suspension programs themselves may give quick results but they aren't always the results that are practical for the particular parts you already have on the car.

Hint to anyone getting ready to design/modify your suspension with a suspension program:
The most important step is making absolutely SURE that you have made correct measurements of your steering components. +/- 1/8" error is NOT going to cut it. Even a 1/16" error can affect the results.

Get a nice flat floor to carefully measure the car. The floor doesn't have to perfectly level, but it does need to be FLAT. You may want to load the drivers seat with stuff that is equal to your weight. (Roll the car back and forth to let the suspension settle.)

Measure the car at least twice to make sure the numbers are the same each time. If not, measure it again till the numbers always come out the same.

I can get quite different Ackermann angle error vs: steering input plots by changing critical dimensions .050" when you go beyond the sweet spot. Holding the placement of the parts to that tolerance is going to be fun since there are so many variables.

Luckily I don't think anyone who is not driving at 10/10s is going to notice the difference though.

By using Acad to draw out Ackermann lines, front views of the A arms etc you can get a better appreciation of what the suspension parts are doing as you exercise them. You should see all the construction circles you end up with to do one simple Ackermann plot in Acad. I have used the program for many years and it does give VERY accurate results. By the way, I am NOT an AutoCad salesman. :lol:

Again, if all this works correctly I'll publish what I've learned etc on my website. With pictures yet!

Nice "talking" to you Dave. (Even though it is a monologue!)

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Completed building GSXR1000 CMC7, "Locouki"
Website: http://projekt.com/locouki/


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 Post subject: suspension
PostPosted: December 29, 2007, 11:03 am 
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Will it sound like a pretty good plan.

"steering input plots by changing critical dimensions .050" when you go beyond the sweet spot. Holding the placement of the parts to that tolerance is going to be fun since there are so many variables. "

That is one of my pet peeves in the building process. You've got to spend time setting up frame and the suspenion points, and correcting any weld distortion. With a fully build car it going to be a lot more difficult. I agree with your .050". I alway try to hold the dims to a tolerance of 1mm.

One suggestion is to mark your frame and set up points on the floor so you always have the same reference dims, and can re-set everything in the same location.

Your base line suspension settings should be pretty good, except for the scrub which sound a little on the high side. You might run your program with the the front lower control arm angled down 3 to 5* and see if that will help reduce it. "but will probably raise the roll center" That change improved both my camber gain and scrub but raised the roll center about 1 1/2", but I'm not using a ant-roll bar.

In fact those settings are very close to my final setup with a Toyota van spindle, but I've got about 5/8" scrub and about .7- .9* camber gain per inch of travel and almost equal gain with roll.

I think you have your winter project, good luck Dave W


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PostPosted: January 1, 2008, 11:31 am 
What is the ball joint from?
I'm familiar with the 1st gen Rx-7 and it looks to me like you have the standard joint pressed into that 3-bolt adapter. On an RX-7 the joint is pressed into a hole in the lwr arm.

Ackerman
Your donor had insufficient ackerman from the factory. See here: http://www.gforceengineering.net/products.htm
These turn-in spacers are sandwiched between the bottom of the strut and the steering arm. They will rotate the arm towards the vehicle centerline giving you more ackerman. Maybe you could fabricate something like this instead of making a new steering arm. I bought some for 15" wheels and fabricated some for 13" wheels for my autocross car. They not only help turn-in but the thickness of them raises the frt roll center.

Steering rack location
Could you switch your spindles (put left on right side) so the steering arms are in the front and then remount the steering rack further forward?


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PostPosted: January 1, 2008, 6:12 pm 
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Rotus7 wrote:
What is the ball joint from?
I'm familiar with the 1st gen Rx-7 and it looks to me like you have the standard joint pressed into that 3-bolt adapter. On an RX-7 the joint is pressed into a hole in the lower arm.

OK, thanks very much for the info. That will make it easier to change the joint itself if I need to. When I take the steering arm(s) off for my mods I'll hopefully be able to find out exactly what parts I have.

I've found out the Mustang adapter is also used on the Bobcat and Pinto of various years.


Ackerman
Your donor had insufficient ackerman from the factory.

When I 1st drew up the RX-7 w/b, track and steering arms etc in Acad it seemed that the Ackermann point was forward of the rear axle line. I assumed that Mazda "corrected" the apparent error with the cross link and pitman/idler arm setup they were using. Maybe not?

See here: http://www.gforceengineering.net/products.htm
These turn-in spacers are sandwiched between the bottom of the strut and the steering arm. They will rotate the arm towards the vehicle centerline giving you more ackerman. Maybe you could fabricate something like this instead of making a new steering arm.

I plan on slightly modifying my stock arms with short pieces of angle iron. That will be a lot easier than fabricating new arms.

I bought some for 15" wheels and fabricated some for 13" wheels for my autocross car. They not only help turn-in but the thickness of them raises the frt roll center.

R/C, that's whole other story of the stock CMC front end. CMC should have made my spindle tube another 1" or so longer to help with camber gain. By raising the upper A arm mounting point on the spindle I can get the R/C -above- the ground. :-)

Steering rack location
Could you switch your spindles (put left on right side) so the steering arms are in the front and then remount the steering rack further forward?

I'm pretty sure that the steering arms themselves could be swapped around but I really wanted to do the minimum amount of changes to correct the geometry.

_________________
"My junk is organized. At least is was when I put it wherever it is." -olrowdy
Completed building GSXR1000 CMC7, "Locouki"
Website: http://projekt.com/locouki/


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PostPosted: January 3, 2008, 7:41 am 
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Here is a list of 1997 and older 3-bolt joints that use a nut on the stud versus a pinch bolt. It seems nearly every mfg has used a 3-bolt joint on something, in either the upper or lower position. A heavy domestic's upper may be the perfect rx7 lower.

If the nut is metric, it may be a 90s full size GM fwd lower.

While Moog lists tie rod dimensions, they don't include balljoint dimensions, there is no good way to know without measuring both parts.

K8142
K5303
K5295
K5273
K9601
K9603

It is unlikely that these guys are making their own flanged cups.

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PostPosted: January 7, 2008, 11:59 pm 
Same strut with a different solution



http://www.whartonindustries.com/MacPherson.htm


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PostPosted: January 8, 2008, 12:34 am 
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Rotus7 wrote:
Same strut with a different solution

http://www.whartonindustries.com/MacPherson.htm

The upper alum looking cone spacer under the Heim is how I plan on raising my upper Heim mounting on the strut. That's much easier than lowering the inner mounting points.

By changing the spacer length I can adjust the R/C and chamber change in dive. The caster will change slightly but I don't think it will be enough to matter.

I've ordered the Heims etc and I'm waiting for them to arrive.

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"My junk is organized. At least is was when I put it wherever it is." -olrowdy
Completed building GSXR1000 CMC7, "Locouki"
Website: http://projekt.com/locouki/


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