|The MGA With An Attitude
REBOUND STRAPS, Length And Suspension Travel -- RS-111
As noted on the prior page, rebound straps on the rear axle serve to limit downward motion of the axle. This is intended to catch the axle when the car goes airborne, or when it may encounter a sudden steering transition that causes excess body roll. The axle must not be allowed to drop so low as to hit end of travel of the shock absorbers or to land on the exhaust pipe.
This begs the question, how long are the rebound straps supposed to be? And what is the effect if they are 1/2 inch longer or shorter? In late September 2013, Moss Motors has just produced another batch of rebound straps for the MGA (hopefully stronger this time). Gordon Fletcher just took delivery of a new set. They measure 8-5/8" long center to center. You may expect them to stretch about 1/8" when installed and loaded, especially if you leave the spring and axle load on them for 24 hours (to hopefully take a permanent set). That would make them 8-3/4" long as installed. I am waiting anxiously for a report on how much the new Moss straps stretch when installed, holding the weight of the rear axle and force of the preloaded leaf springs. Gordon may have that answer soon.
I just measured the 25-year old (good) straps on my car. They are 9" center to center when loaded (with the spring loaded axle hanging on the straps). To my surprise, the hand brake cable does not touch the exhaust pipe (where other people have asked about that problem). So 9" length is fine, as long as the new ones don't stretch more when loaded. With the car sitting on the pavement, nearly full fuel tank and no passengers, the link studs are 7-1/2" center to center. Spring rate is 125 lb/in per spring (250 lb/in for 2 springs). 280 pounds worth of passengers will add 190 pounds to the rear axle load, dropping the tail another 3/4". So the laden rear suspension is expected to have about 2-1/4 inches of downward travel available before it hits the rebound straps. This is why it is important not to have leaf springs sitting an inch or more too tall.
Gordon Fletcher also measured the situation with rebound straps detached and the axle hanging at lowest position (lowest mechanical limit of shock absorbers). This puts the studs 9-3/4" apart (and the axle housing touching the exhaust pipe). The axle must never be allowed to drop this low in operation. So if your rebound straps are around 9-1/2" long center to center, then they should be replaced (before you find collateral damage). New straps should be no more than 9" long center to center (hopefully a little shorter) as installed and loaded.
If the straps were too short it would screw up handling of the car, same as if the leaf springs are too tall. With a little body roll, if the strap runs out of travel and lifts the inside rear tire off the pavement you get sudden transition to severe oversteer. A very bad situation, even dangerous if it happens when you are not expecting it.
Questions still abound, so this is an update on the situation. A few years on, many replacement rebound straps continue to be faulty with insufficient internal fiber reinforcement, stretching to too much and sometimes breaking. See other tech articles in this Suspension section, and a special article in the Faulty parts section. I have finally procured good quality rebound straps that do not stretch. Replacement rear leaf springs also continue to be too tall.
I have recently replaced rear springs on my MGA, and now the rear ride height is substantially higher (actually a little bit too high). With my current travels we have a fully laden vehicle with 450 pounds of passengers, fully stuffed boot space, and a trailer with 40 pounds tongue load at the rear bumper. Even with this maximum loading the rear suspension is still a bit too high. This makes steering a bit twitchy, and can result in undesireable sudden transition to oversteer during sudden steering manuvers (when a rebound strap goes taught and lifts a rear tire). I am unhappily learning to live with this condition, as correct height leaf springs are not available, and re-arching the new springs to be lower is a royal pain and a costly process. I really don't know how other people can handle the car in this situation with a single passenger and no aditional loading. It is on ongoing problem with no easy solution at present.