SAI Mod or Not?

[2FAST4U]

Over on the other forum there is much discussion on the SAI modification to the SN95 spindles. My question is, is the modification necessary if I plan to just drive my Roadster on the street? I have no intention of racing it. Just lots of driving around SoCal.

Thanks

[frankeeski]

Bill, I am guessing that you will be running manual steering. In my MkIII with a 18:1 Flaming River rack, the bump steer is very manageable. Remember that bump-steer is not just a matter of SAI, it is also the effect of the manual rack not absorbing the pot holes and other obstacles in the road. I have often said that a steering dampener would be a great addition to a FFR steering system. Much like the ones we use for off-road trucks. The problem is that most trucks use a steering box and not a rack. There really is not an easy solution to add a steering dampener to a rack and pinion. I think you'll be fine. Start with what you have and adjust accordingly.

[tirod]

The SAI mod is meant to correct that specific angle drawn thru the ball joints to the contact patch. Normal cars run about 9 degrees and the angle goes thru the center of the tire tread patch.

The kit uses adapters for the strut spindles and they wind up with an SAI of 18 degrees, which results in stiff steering - you literally have to lift the suspension when turning. That's why F5 got with a vendor to make a modification kit, and why they offer complete spindles that need no adapters - both of which have an SAI of about 9 degrees.

It's really more for street driving as the mod reduces steering effort and puts the angle back where common practice dictates. The worst case scenario is to ignore the SAI, use a depowered rack, and extremely wide tires, which would make it nearly impossible to manually turn the wheel at rest - as many builders report. They then assume that is normal and add power steering back in to "fix" it, when in reality the car industry knows better and would never use an SAI much over nine degrees. That is essentially what most makers did in the day with economy sedans - even the GT350 Mustangs had stock spindles with 9 degrees. Shelby just added different pitman and idler arms to speed up the ratio.

Rack and pinion steering does have much more kickback, it was common knowledge driving early import sports cars to hold the wheel without hooking your thumbs over the spokes, or suffer the consequences.

[Mike N]

The worst case scenario is to ignore the SAI, use a depowered rack, and extremely wide tires, which would make it nearly impossible to manually turn the wheel at rest

This is where I started, and it was horrible except at highway speeds. Then I added a FR manual rack and bump steer kit from Breeze with offset bushings and things were night and day better. It really transformed the car. Later I added the SAI mod from Whitbys and although it made a noticeable difference it's not something that you couldn't easily live without for street use. I did measure and shim the bump steer on my front suspension both before and after adding the SAI kit and I really don't think it made it any better at least not using my measuring techniques. I used a laser and not a proper bump steer gauge.

[2FAST4U]

Thanks all, especially Tirod, for a great explanation on the merits of doing the SAI mod.

Given the information provided, I'll go with both the SAI mod and bumpsteer kit when the car gets here and I start assembly.

Thanks
bill D

[Someday I Suppose]

Bill, really no reason not too, with the MKIV you already have the inboard mounting locations and only need the spindle adapters from Whitby to get it done. I'm running around 4.5 degrees of caster on mine, steering in a parking lot is heavy but its a pleasure once your rolling. Tempted to try a little bit more as well.

Scott

[Jim Schenck]

Bill,

If you are running pin-drive width on the front end I would check with Jeff at Whitby and make sure that kit will work. It is possible that your upper control arm length would be to short.

[2FAST4U]

Thanks Jim

[rich grsc]

Thanks all, especially Tirod, for a great explanation on the merits of doing the SAI mod.

Given the information provided, I'll go with both the SAI mod and bumpsteer kit when the car gets here and I start assembly.

Thanks
bill D

Dont waste $$$$ on a bumpsteer kit. You dont know if you have a problem till you drive it. Dont spend Money fixing an issue you may not have!

[2FAST4U]

Excellent point Rich.