loved all your posts jmac. you should write a book :)

I know this doesn't answer the question - but is there a particular reason you actually want to lower the rear?

Generally if the axle centreline is lower than the front leaf eye bolt (or nominally the point at which the front linkage setup pivots around) then as the tyres push forward they try and lift the rear (a sort of scissor jack action) whilst the body (due to inertia) is trying to move backward (or stay in the same place vs the road, but backward vs the axle). Set up like that, it has a double whammy and actually _'plants' the rear tyres down harder.

If you lower it, the higher the axle centreline gets, the less of this effect there is, and the less grip off the line (axle tramp notwithstanding in either case for sake of discussion)

You could (on some cars) lower it to the point that the axle centreline is higher than the front spring pivot point, and when it gets to that point, as it tries to push forward, it actually 'unloads' the rear, and compromises downforce/grip. It can make for a fantastic burnout setup, but not so much a drag race one.

Of course if it was circuit racing, then typically you aren't accelerating at massive g forces for most of the circuit (except off the start line and maybe out of an extremely slow turn) and reducing the difference in height between axle centre line and front leaft pivot point can have some bonuses in terms of cornering consistency, esp mid corner, but for a drag car? One of the popular 'how to make your car handle' type books (might even be the exact title) covers this in some detail, but I think it potentially might favour the even stevens height setup for circuits, and it either just briefly mentions the higher front leaf eye/lower axle centreline (vs the car body/road) for drag racing, or it might not discuss it at all. In any case, it's an important consideration, and the higher the engine output, the more it will become crucial to it's capabilities as far as drag racing goes.

Essentially it is the opposite (but same principle) as some of the 'anti dive' front suspension relocation kits/procedures to stop certain cars (when handling is more important than super soft ride / comfort, or just to improve the car's performance capabilities) - cars which would tip nose down massively underbraking, causing instability, and possibly bottoming out, but not solely that. the revised geometry resists the nose tipping forward/down as much under hard braking and at the same time actually increases downforce as that resistance/force used to stop it nosing over will have to have it's equal opposing force, and that force goes down through the wheels/tyres that push it back up....


Aside from the geometry issues of that nature metnioned thusfar, there is a second related issue - whenever you fit lowering blocks, it moves the axle away from the leavesm and that gives the axle more leverage on the springs, and the springs less on the axle and furthermore, the lowering block itself can potentially allow the spring to flex more in the central area (it's all relative). Which essentially means even if you didn't lower it to the point the axle is too high, then it can still have the effect of increasing the occurance and severity of axle tramp, So even more to lose.

On those setups where it is actually possible to do so, the 'safer' way (as far as good launch traction and no axle tramp) to lower the rear is to relocate the front leaf spring eye bolt higher in the chassis. This will lower it, but nstead of reducing the amount of downforce on the rears under acceleration, it increases it, so you get it lower and retain (or even marginally improve) the traction off the line. Problem in this case is that due to the way the front leaf is attached, there's not a massive amount of room to shift it higher in the chassis/bracket. But every little bit counts.

On centuras (I know, totally different car) they were originally made for a 4cyl - the overseas car the body was based around and locally produced, and to fit the hemi 6 in them, it had to fit a long way forward, with a lot of weight in front of the front wheel centreline. This meant the rears had little downforce/load since the front/rear weight distribution was far from ideal. Ironically with all else being equal (and this relates to the earlier mention of it) they are only a couple of hundred pounds lighter than an aussie charger, but with identical engines, the centura is an incredibly potent burnout machine, and in stock form ( i,e, base spec motor, not the hi-po ones from the pacers or rt chargers, with big 4 barrels or triple webers respectvely), with stock tight convertor, relatively the charger would struggle to do a burnout at all with hi-po tyres. And on a related note, the charger would (all elze being equal) tend to win a drag race, purely due to traction off the line.

The charger is leaf sprung, the centura 4 link, but even with the 4 link it lacks downforce. So the 'go' for drag racing them is to relocate/drill (or use new brackets) the rear lower trailing arm bolt boles lower (further away from axle centreline) on the diff housing, and raise the front bolt of the lower trailing arms. This effectively does the same as raising the front eye bolt on a leaf spring rear.

Short version - the fact that it might be hard to lower the car might actually be a surprise and it might actually be quicker as a result of not being lowered...

I unbolted the caliper today, and sat the shock on the front side of the diff. Looks like it will work. This (in conjunction with lowering blocks) will allow enough room for me to rotate the caliper back and get the precious clearance to lower my ride.

1200 Coupe them offset and angled to reduce tramp.

1200 Sedan has them nearly straight up/down for maximum shock action.

Here is a pic of what I'm running now


You can see how close the handbrake cable sits to the shock absorber.

Here is how close the caliper sits to the part which Pigdog has chopped out.


This was taken with the suspention compressed. (But the rear end is still at stock ride height)

If I move the shock to the front of the axle tube (remember the shock is held to the spring pad by a bit I made since the ute diffs have no provision for shoch mounting) I'll then be able to redesign my caliper mounting bracket to rotate the caliper rearward. And if/when I fit lowering blocks, there will be greater clearance between the handbrake cable retaining arm and the leaf spring. This should enable me to further rotate the caliper downwards, and get even more clearance on the bit pig dog removed.

The Main query of this thread tho, was will there be any drawbacks to moving the shock. But now that I think about it, it may actually reducae axle tramp by shifting the damping effort to the side of the axle which will be trying to lift up. Although long term I'm love to build some caltrac bars for it.

would only be using it as a park brake for short periods of time like warming the car up and such when it cant be in gear to hold it still

hydraulic handbrake is not really for a park brake, holding pressure in the system would be ok, as long as there is no weeping anywhere in the system... probably not a risk worth taking.

wonder what the coefficient for thermal expansion of brake fluid is, might release when the fluid cools after a hard drive

im supposed to be running a drum handbrake...but ever since i changed from drums i parked the car in gear to keep it in the one place. will be looking into making the drum handbrake fit when i change from the BW78 to a H190
or i'll see how a hydro handbrake goes

so you couldnt change the handbrake cables to make it work at the front of the diff?

because my calipers are different to yours. The arm which holds the handbrake cable outer is already perilously close to the shock absorber. And my handbrake cables run around the back of the diff. What kind of handbrake are you running pigdog?

why dont you just mount the caliper at the front if you are going to redesign the caliper bracket?

i just cut the inner guard out to clear my calipers
as you can see in this pic