Triumph TR4A-250 Chassis Assembly, Repair Sections and Attachments
CHASSIS ROT & FATIGUE
The IRS chassis was a compromise which enabled Triumph to go independent without spending too much. Regrettably this left a few development needs for customers to discover. Designs exist for beefing up lower fulcrum brackets and differential mountings which were not made public as a massive recall would have been financially devastating for the cash strapped company in the mid 1970s.
Originally, these chassis were given only a coat of chassis black enamel. Modern paint or galvanising procedures would have controlled corrosion but fatigue problems were inevitable, so should be faced at the first opportunity. The differential mountings can be tackled from underneath although a popular route in the past was through the rear floor area, which may explain some unusual welding visible when the carpet is lifted. The pins could be re-welded and hopefully reinforced as well but the only proper solution is with the body off the chassis and to replace the pins and associated fixings completely with heavier gauge, reinforced units, which fully box the pins. The trailing arm chassis legs collect water and fatigue too, so should never be repaired: replacement is essential. This requires removal of the upper and lower cruciform (or breast) plates to permit full seam welding.
The corrosion at the rear end of the chassis can be rampant, hence the need for ill. No. 21 (RR1324). Fatigue to the differential bridges also quite frequently necessitates complete replacement. At least the replacement bridges carry all the necessary strengthening modifications. The side braces, ill. No. 6, collect water and rot. After replacement, don’t forget to drill water outlet holes at their bases. Cars can suffer from fatigue to the gearbox mounting brackets and should be replaced as required. A little extra reinforcement here does not go amiss. Don’t for get that the 3/8" washers under the four fixing bolts should be hardened, not mild steel which soon wear and allow gearbox movement.
The front suspension chassis brackets, ill. No. 3 fatigue and are easily damaged in accidents or even kerbing. These must be carefully inspected and replaced if showing any untoward signs. Either way, fitment of the reinforcement plate (Ill no. 36 or 37) is recommended and adhere to the 25-ft/lbs. torque clamping the lower fulcrum brackets to the chassis. Finally remember, your chassis is very old and may have had an unhappy previous life. Accident stress might not make itself known for thousands of miles and may be hidden by paint or underseal.
Strengthening Lower Wishbone Arm Brackets
The lower front inner wishbone brackets (illus. no.3) which are welded to the chassis, are considered weak points of the front suspension as they are often found to be cracked or even broken away from the chassis. This is usually caused by running into pot holes or hitting kerbs. These brackets are easily replaced by a competent chassis repair workshop, and represent a considerable improvement to the earlier TR2, TR3, TR3A and TR4 design. It’s a good idea to periodically check these brackets, and have them replaced or reinforced if they are cracked or broken, as this condition presents a severe safety hazard.
We have followed a design from Triumph themselves and produced a strengthening kit which comprises three plates that are welded to the inner wishbone bracket and the chassis.