Ride Height and Tire Pressure???

[High School 69 Charger]

I have noticed a lot of discussion about lowering the front end stance of our cars in the forum but am curious on how that conflicts with the FSM height specification and the note that all suspension geometry is based on this starting point.
So.....   can you truly lower the ride height and still maintain good suspension.   Spec is 1 7/8 with 1/8 tolerance.   How far can you go beyond that without sacrificing suspension operation.
My car is fairly stock including tire size and at Height Spec I would say there is a small 1/4" of clearance from top of tire to the fender opening.    So my tires are not tucked up in the fender well at all.
This could bring lots of opinions.....

And... this also ties into tire pressures.  While the original car specs were for bias ply tires new radials come with their own press and load ratings.   Which is the right way to go on these new tires.
Thanks for any guidance on this.

[randy73]

When radial's first came out, it was a no-no to put a radial on a car not made for them, then later it was standard to have a different alignment for both radial and bias tires. Now radials on a car made for bias is the norm, so I am a little lost on what you are asking or saying and with everyone lowering or raising their ride height, you just adjust the height and take it to a shop you trust and they will align it and set the geometry to get the car to track correctly, I mean now a day's with all the aftermarket suspensions, that are setup and track straight, plus all the guys that upgrade their suspension with newer/different parts and get shops to align with no problem, so I am not seeing what you are concerned about.

just my 

[375instroke]

The factory spec won't change since it's the difference between the inner and outer parts of the lower control arm.  It's angle won't change with wheel or tire diameter changes, and that's all that matters as far as suspension geometry goes.  I think everyone will agree that the stock specs suck.  Keep ride height if you want, but go for -1/8° to -1/4° camber, and as much positive caster as you can get, with perhaps +4° being a goal.

[HPP]

So.....   can you truly lower the ride height and still maintain good suspension.   Spec is 1 7/8 with 1/8 tolerance.   How far can you go beyond that without sacrificing suspension operation.

And... this also ties into tire pressures.  While the original car specs were for bias ply tires new radials come with their own press and load ratings.   Which is the right way to go on these new tires.
Thanks for any guidance on this.

Yes, you can lower the ride height and maintain good suspension geometry. The geometry does not change radically since you are simply putting it closer to one end of the range that the factory designed in. However, it does have some effects, and those can typically be adjusted for. Changes in roll height, changes in dynamic camber and caster gain, changes in dynamic toe all results when lowering or lifting.  To optimize these changes, you may need to modify some of the locations points for some peices, notable the outer tie rod will need to drop in its location to avoid significant change sin bump steer.  However, the caveat with that is you must also step up the springs rates in concert with the lowering to avoid bottoming the suspension out and possibly breaking things. This is also why using a radial tire for such significant deviations from stock are a must. Bias plys do not tolerate a wide range of alignment angles whereas a radial tire will. This ability allows you to alter the angles of the alignment to allow more aggressive settings that will provide greater grip without excessive wear.  How far can you go...my car's ride height measured in factory terms is 0. The control arm pivot is in flat plane with the lower ball joint in my car. I also run a 1.22 torsion bar too.

For tire pressure, start with what the tire manufacturer recommends. The sticker on the body was for bias ply. If you really want to get serious about maximum tread contact, set your tire pressure by taking temp readings across the tread face and adjust pressure up/down to equalize all readings.

[High School 69 Charger]

Thanks for all the input.  Each response gave me more perspective on what the setting impacts as well as what to watch out for.   Thanks again.

[375instroke]

What are your goals, and in what order?  Do you have to run bias ply red line tires for the look?  Do you have to run a 14" or 15" rim?  Do you need the car at stock height, or slammed on the ground?  Some affect things more than others, and you don't sacrifice too much deviating a little one way or the other.  Slammed on the ground hurts handling more than keeping it high in the air at stock height.  Are you using dropped spindles, or just lowering the torsion adjuster?  Lowering with the adjuster lowers the roll center, which is like dropping spring rates several rates lower.  The car will lean over in turns easier at the same torsion stiffness.  Stock tires and alignment specs sucked, so deviating will be offset by better radial tires and better alignment specs like positive caster and negative camber.  Stiffer torsions and bigger sway bars allow the better tires to work more effectively.  Bias tires couldn't take the loads as well and would lose grip easier, as well as ride harsher, so they had to keep wheel rates lower than what they run today.  My '69 is lowered maybe 2" in front, the rear has the original R/T leafs with the front hanger flipped, lowering it an inch or so, 1.04" torsions, 1-1/8" 1970 style sway bar, QA1 shocks, BFGoodrich 245/60-15 on 7" rims front, 275/60-15 on 8" rears, and it rides and feels great.  Far superior to the original suspension.  What are your main goals?  Tires and rims are probably the most limiting factor on what you want to do.  I can't bring myself to run 18" rims on a car like this.