What is the SAI and scrub radius on Type 1 ball joint beam?

[Evil_Fiz]

These questions will apply to a 1970 convertible Karmann Ghia.

The intention is to run 17" x 7" - 7.5" wheels with 205 - 215 width tires, but it is all speculative at the moment. I have searched for the information I need but have had no luck finding it. I need to know the Sterling Axis Inclination (SAI,) tire diameter, and scrub radius of the factory wheel and tire combination. My intention is to lower the car using 2.5" drop spindles but maintain scrub radius unchanged. These numbers will help me calculate the offset and backspacing needed based on the final tire diameter I select. The beam is original width with adjusters and I plan to reshape the wheel arches as needed to accommodate the changes.

I want to maximize the handling characteristics of the BJ beam. I welcome any suggestions or considerations I may be overlooking.

Thanks,
Emil
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“…It's not just about what's interesting. It's also about what's helpful, and it's helpful even if it helps just one other guy working on a Ghia.”
kiwighia68

See my build on TheSamba at:
The K_R_A_K_E_N_N : a 70 Ghia Convertible reinterpreted

[rugblaster]

You would need to change the gearbox to a swingaxle on the rear to clear the fenders inside......narrowed front beam on the front. I don't know what changing the spindles would have to do with the relationship of the ball joints vis a vis the steering axis inclination. Call Joe Martin in Johnson City, Texas @ 512.766.0128 and ask him what he used.
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[modok]

If you do what you are saying you will greatly limit your turning radius due the the rim hitting the trailing arms when you turn.
It's not practical because of that.

[jeffrey8164]

I’m inclined to agree. The stock suspension is the limiting factor. Even with my measly 5.5” wheels with 195/50s I had to readjust the steering stops as they rubbed on the sway bar.
I put a quick steer kit on mine too, and that made it necessary for longer stop bolts.

Did the terms “SAI” and “Scrub Radius” exist when the Type 1 was designed and manufactured?

I’ve seen quite a few hybrid installs with 914 and 911 spindles and brake components with custom made a-arm or air-ride set ups.

You could always go live axle in the rear. Then you don’t have to worry about steering because you won’t be able to anymore.
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[theKbStockpiler]

You can measure the scrub so that shouldn't be so hard. SAI is part of a alignment so I would try to reference a Hunter manual or the like for that. I believe that involving a lot of positive camber in the front end ;like inexpensive german cars tend to do, is a cheap method to reduce scrub radius.
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[esde]

17x7 on a Ghia, with a stock width beam, and dropped spindles
If only it was that easy. 17's are a PITA. If you want to maximize the handling of the balljoint beam, get some really good 15" tires and shocks. Every car with 17's I've driven rides like a skateboard and handles terrible.
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[oprn]

I put 17" wheels on my daughter's '92 Jetta.

Mistake! Double the price or more for tires, harsh ride, because of the reduced air volume temp changes make a big difference on tire pressure. Constantly fiddling with tire pressures and wondering if they are leaking or not. When one does leak it's hard to tell if it's flat or are you sitting on a soft spot because of having so little side wall. Increased wheel height reduced the already marginal braking power into the scary zone.

Need I go on?

[FreeBug]

If you put some chalk on the ground, little bits spread over a foot diameter circle, then roll one front wheel onto it.

turn the wheel both ways through the full steering arc.

roll the wheel back, or better yet, jack it up and remove it paying careful attention not to smudge the chalk marks.

You will see concentric circles on the tire which will help you "see" or "deduce" their center of rotation.

The distance between this spot and the center of the tread is you scrub radius.

Also, with a bit of math, some inch-to-mm-and-back conversions, and knowing the ET values for your wheels you can work this out, but only if you know what's going on with your spindles and discs, some types of each can offset the wheel.

what you might want to do is get a full alignment, to have your starting values, something to get back to if you mess up.

I have experienced FEW suspension or steering modifications which actually improved the handling or ride, it's all mostly done for looks, and makes the whole car worse to drive. " I can't be seen on the streets without rake!", uh, ok, whatever, I guess I get it because I can't be seen on the streets without hoe.

But just because I haven't seen anything I like in modded suspensions doesn't mean it can't be done...

[Evil_Fiz]

Thank you one and all for the wisdom and advise. Since we have a conversation going let me add some additional comments.

- The car is a custom build in progress. Currently completely torn apart and in the early body work stage.
- This WILL NOT be a track car (Street/highway only.)
- The car will be Subaru powered. No power plant selected yet but most likely 2.5 naturally aspirated.
- My goal is the best handling possible without sacrificing stability and predictability at highway speeds.
- The plan so far is to reshape the wheel arches to tightly follow the wheel diameter (1" - 1.5" gap.) This should? aid in wheel fitment and selection.
- Modest, tasteful flares are an option if needed to fit the correct wheels.
- Appropriate Front and rear sway bars as well as a kafer bar will be installed.
- Front beam-to-body connectors will be installed.
- Rebuilt VW steering box will be used.
- HD tie rods and ends will be installed if needed to handle the bigger tires.
- Four wheel disks are planned (Wilwood Dynalite front, CB Performance kit rear.)
- Rear will be lowered to achieve a flat stance, I am shooting for 4" ground clearance at the rockers
- This is a "Function First" build but form is important.
- I would like an "as close to stock" turning radius as possible. Pushing the wheels out is possible if it will not sacrifice performance.
- I have read through the STF Ghia road course thread but street specific info is still needed.

What I hear from the responses so far:
- 17" rims do not play well with my goals
- Fitment considerations are many
- The data I seek is available and sources have been provided

...more questions
- What would be the best shocks for the task? Ride quality is secondary but I don't want a skateboard either.
- Would 16" rims be a suitable choice or are 15" rims the only real option?
- What would be the lowest recommended side wall profile?
- What would be the narrowest, suitable tire width to provide good handling and traction given the best/most suitable tire compound is selected? Staggered wheel width, front vs rear, is acceptable.

I hope we can continue the discussion, I recognize that there is much to learn.

-----
Emil
_________________
“…It's not just about what's interesting. It's also about what's helpful, and it's helpful even if it helps just one other guy working on a Ghia.”
kiwighia68

See my build on TheSamba at:
The K_R_A_K_E_N_N : a 70 Ghia Convertible reinterpreted

[Wreck]

I run 16x6 and 16x7 with 195/55r16 and 205/50r16's . I had lowered spindles but removed them because the LH tyre rubs on lock . it depends how much you want to modify the front wheel arch shape to how far out in the wheel arch and how low you want the car. Also how much of a turning circle .

My car is not low because of the standard height tyres but I have full steering lock and suspension travel ,which I value over looks .

Shocks depend on the budget . I aim to fit some double adjustable AVO's in the future .

[modok]

[oprn]

He is going water pumper so will be heavier than stock.

[oprn]

He is going water pumper so will be heavier than stock.

[modok]

yeah, and a ghia has a heavy nose, but it's still no heavier than a smart car
I'm just saying why run a wide wheels and tires? because you had to, but now you don't have to now, if you can get so many sizes of tire. 175/50R16 now exists, and not just exists, "ultra high performance" yokohama, from japan
only weighs 16 lbs!
What a strange size, I wonder what it fits?

[earthquake]

Doesn't scrub radius also depend on the offset of the wheel too?

eQ
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[Evil_Fiz]