Thread: Distance between rear axle and frame?

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On a 32 frame with a "C" notched frame and a Ford 9" rear with coil over shocks, what is the minimum distance allowable to keep the axle from slamming into the frame?

I dont know if anyone can give you a definitive answer on that all cars are not created equal . Sure they can get you in the ball park but just beware that you may need more or less space.

I'm not a chassis man but I would imagine there are a few variables, shock location, angle, travel, spring rate. A little more information would be helpful.

I normally recommend a minimum of 4" clearance on a rear axle. Under normal usage, the rear axle will move up and down about 3" above and below its "neutral" position. for potholes and speed bumps you need the extra inch over that.---Brian

On the other year cars we do, the minimum we shoot for is 3-4 inches. It seems that is quite generous in the real world, as the rear seems to only go 1-2 inches on bumps.

Don

Ralph, to partly answer your question, you should consider the entire rear suspension system. The shock will typically use 2/3 of its stroke in the collapse direction. Thus, if you have a 3' travel shock, it should collapse 2" from the nominal height. The other 1/3 of the travel is for the extension direction.

So, when you set up your shocks/axle/frame, you will take the 2/3 of the shock travel and add a 1" safety margin to determine the distance from the top of the axle tube to the frame BUMPER (unless you want to risk hitting axle onto frame, you should have a rubber bumper in the system). This will give you the ride height of the car (assuming the proper tire diameter). This should also give you an idea of where the shock/coilover brackets will go. You wont like the ride height if you are like most of us.

If you want the car to sit lower so that the frame slides on the ice at North Pole, you have two choices...smaller tires or raise the frame notch. Smaller tires make a 32 look funky so we just raised our frame notch by having new frame sides made by a CNC laser outfit and cut the rear portion of the existing frame off and welded the new part on. Of course, we had to raise the trunk floor a bit to match the frame.

In summary, use a 1" buffer between full collapse on the shoock and the frame bumper and you should be OK.

mike in tucson

Thanks all, it's actually on a friends 32 roadster, It has about 4 inches clearance now, but he wants it to sit lower, I told him we could notch the frame radically (it already has a C notch) and lose some height, but I'm not sure he wants to go that route. I'm putting a 37 banjo axle under mine which is mated to a 40 rear crossmember that I just flattened out to drop the rear of the car about 5 inches, and if that's still too high then I'll use a reverse eye spring to gain another inch or so.

See, I told you that you would not like the ride height.

A third way (that may be tough in Alaska) is to dig four holes and sit the tires down in the holes....looks low and cool but doesnt drive too well.

mike in tucson

Here is my 2 cents-I have 2" of clearance between everything, wheel well opening and frame. I use the rubber stop that is Incorporated in the coil over as my stop. I set the ride height where it has 1 1/2" of travel before it makes contact with the stop, the stops are generally tapered for a progressive interference. Then I use a coil spring that gives me a compromise between ride quality and travel. I use the longest shock I can get in there for the dampening. I have never bottomed out any of my cars using this method. Maybe I have just been lucky on the bumps I hit
Ken

That is why I said I thought 3-4 was more than generous. On my T I had shocks originally that were too long, and only had about 1.5 inches of travel. I used the car that way in the beginning and they never bottomed out, even over RR crossings and hitting rough bumps at high speed. I just pulled those shocks off and replaced them before Billetproof because with two passengers in the car it reduced the travel to less than I liked.

I agree that the figure Ken uses is probably correct in the real world, and that we don't need much more travel than that. But being the over-engineers we all are, we put a whole bunch of room in there we probably don't need.

I have also watched my front end dip over RR tracks and bumps, and it probably only moves 2 inches max., if that. On my '27 I have a frame tab on each side to mount the F100 shock mounts, it sits directly over the spring and is only about 2 inches away from the top of the spring. In many years of using it the spring never came up and hit that tab, the paint in that area is still perfect.

Don