MARTY: Suspension and Steering Modifications

Overview

The DeLorean's suspension and steering has been extensively modified to improve adjustability, performance, and mechanical robustness. During the initial build, the primary goal was improving the handling characteristics of the vehicle. We modified the frame to install coilovers on all four corners, allowing for quick tuning of ride height, spring stiffnesses, and damping. Bespoke a-arms were designed and fabricated for the front suspension to allow for adjustable static camber. We also modified the frame to package a performance steering rack.

As we started to perform dynamic drifting transitions at speeds and forces far higher than the original vehicle could achieve, we saw accelerated wear of the stock spindle components, especially the wheel bearings. At one point, we were swapping front wheel bearings after every test trip! Thus, in the second stage of development, we designed custom uprights to improve mechanical reliability and also accommodate the new Brembo brake-by-wire system.

Stage 1: Coilover Conversion and Customized Steering Rack

When MARTY was first tested with the stock suspension, in early 2015, it exhibited extreme limit understeer. This greatly restricted peformance driving of any kind, and hindered starting the drifting research.

Measuring and analyzing the stock suspension geometry, we confirmed that there was pronounced camber loss on roll, which exacerbated significant positive static camber. This analysis also showed that this behavior could be greatly improved by lowering the ride height to operate on a different part of the camber gain curve and adjusting the static camber. This is likely near the height that Lotus originally intended, but poor spring manufacturing quality led to a common problem of DeLoreans riding much heigher than designed.

To implement these changes, we converted the front and rear corners to use coilovers instead of the stock springs and dampers. This allowed us to precisely control ride height, as well as adjust spring rates to control roll stiffness and the front/rear proportion of lateral weight transfer. Custom mounts were fabricated for the front and rear strut towers to adapt the spring seat to a clevis for the coilover's upper rod end bearing. Adjustable-length tubular steel upper and lower control arms were fabricated for the front, and allowed static camber to be set precisely. We use double-adjustable Penske 8300 coilovers, with remote reservoir, in the front, and Penske 19.75" 7500 coilovers in the rear.

Cognizant of the reportedly high failure rate of the DeLorean's front lower link, we replaced it with an A-arm design, and removed the front sway bar - they were too skinny anyway compared to our new spring rates. We also designed this change to free up room and allow us to extend our steering range by installing an aftermarket Woodward performance steering rack with a longer stroke and higher ratio. With more steering range, MARTY would be able to drift at larger angles for a given speed. The stiction/backlash of the steering system was also greatly reduced compared to the 37+ year-old original, allowing us to improve the closed-loop servo response. It turns out the DeLorean suspension uses a 7 degree (aka 1.5" / foot) taper, so to connect the rack to the original front steering knuckle, we used tie rod ends from the first-gen Toyota MR2. Similarly, for the A-arms, we were able to find aftermarket Mopar performance balljoints that fit well on the original steering knuckle with just a little bit of hand reaming.

Stage 2: Custom Uprights

Stage 1 was completed in August 2015, and allowed the vehicle to achieve significantly higher levels of lateral grip. When combined with the rapid maneuvers encountered in automated drift testing, this led to accelerated wear of the stock spindle components, especially the front wheel bearings - which are the same as the ones used on the original Fiat 124 and Yugo GV. This was initially treated as a wear item that required frequent service. At one point, we were swapping front wheel bearings after almost every test trip. Although we did get quite proficient at this odd, highly-specific task, improving the robustness of the assembly was noted as a top priority for the next big upgrade.

This major overhaul occured in summer 2016, when we added the second battery pack and integrated a prototype Brembo brake-by-wire system. As part of the latter, we had to incorporate new, bigger, calipers and rotors on all four corners. This combination of goals led to the design and fabrication of custom uprights to accommodate the new brake equipment, and utilize contemporary performance car hubs - namely those from the S550 Mustang - which are far more mechanically robust.

The front uprights use a modular design that is highly adjustable. The bolt-on upper and lower ball joint bosses allow for the quick addition of static camber adjustment plates if needed. The steering arm can also be swapped out to adjust the steering ratio and bump steer behavior. Finally, because we were sourcing and fitting new, much bigger brakes at the same time, we wanted the caliper mounting bosses to be quickly swappable in case of last-minute design changes. (Fortunately, there weren't any.) This bolt-together design also reduces machining cost and time. Since the replacement, we have not had any issues with bearing wear.

The rear uprights were designed to be much more robust than the originals, as cracking was once observed in the stock cast aluminum version. The new upright mates with the original trailing arm and suspension links, though we swapped out the bottom link for an adjustable version. It also has tapped mounting bosses for the new Brembo calipers. During the electric drivetrain swap, we used the same Porsche 930-style outer CV joint as on the original DeLorean, in order to mate with the stock rear spindle. This style of CV joint is commonly used in the aftermarket community, and so is the S550 Mustang hub, and we were thus able to find a suitable splined stub axle at The Drive Shaft Shop.

Wheels and Tires

With the changing suspension design, MARTY's wheels and tires have gone through a series of evolutions. Originally, the DeLorean had 14in wheels in the front, and 15in wheels in the rear, both with a 4x100mm bolt pattern. In the current configuration, MARTY uses wheels with a 5x4.50in bolt pattern. The front wheels are size 16x7, with a +38mm offset; the rear rear wheels are size 18x8, with a a +45mm offset.

The current tire configuration, that was used in all experiments for this thesis, uses the 225/45R18 Bridgestone Potenza S-04 in the rear, and 205/45R16 Bridgestone Potenza RE-71R in the front. We are fortunate to have Bridgestone as a tire sponsor, as we can go through a set in as quickly as 2 minutes of aggressive high-speed testing!

Design, Development, and Fabrication Insights

Throughout the development process for the suspension, rapid physical prototyping was critical to accelerating progress. A few examples are highlighted in the following slides.

Contributors

We owe a special thank you to the incredible Aaron Sellars, who led the overall design and fabrication process for Stage 1, and did most of the welding himself. He taught us many of the techniques shown here, and helped us get comfortable with the practical side of working on cars. The skills he taught us were critical to succesfully completing Stage 2, which was largely led by the Stanford team.

On the Stanford side, the team that developed Stage 1 in the Fall of 2015 consisted of Jon Goh, Tushar Goel, and Mike Carter. Stage 2 was developed in Summer 2016, by the team of Jon Goh, Tushar Goel, Mike Carter, and Joe Sunde.

Thank you to the team at Renovo, who were always ready to share their automotive experience and insight, and let us use their shop while the drivetrain was being worked on simultaneously.