JBC Suspension Design
The stock Datsun 510 is equipped with a recirculating ball steering box system which leaves a lot to be desired. After 40 years nearly every box is worn out and loose. This can be anything from annoying during daily driving to downright scary in a high performance situation. Upgrading the engine, suspension, brakes, and everything else in a 510 can quickly make the steering a weak point in overall driving experience. JBC set out with the goal of bringing a higher performance rack and pinion steering system to the well deserving 510 chassis. This was accomplished while improving the steering geometry and correcting for popular suspension modifications such as lower ride height, roll center spacers, increased caster, and increased camber.
Steering geometry is a fickle beast. Even the slightest changes to a precisely engineered steering setup, such as the stock geometry, can make a car behave unpredictably. The 510 has a very narrow track width. This meant that finding an OEM steering rack with the exact width was not possible. And even if one was found, a steering rack moves in a straight line whereas the arm from the steering box swings in an arc which yields different geometry progressions. After sorting through various OEM options, the rack to be used was decided to be the steering rack from an AW11 Toyota MR2, a nearly identical unit can be used from an AE86 Toyota Corolla. These racks are quite narrow, available in manual and power steering options, and are re-buildable. There is the added benefit that Quaife offers a quick steer gear set for the manual version. The main purpose of the project was to create a nearly bolt in kit for the rack and pinion. Since we were already redesigning so much of the suspension, we decided to take the opportunity to modify the suspension geometry to correct for popular modifications and optimize for performance driving.
In order to find the optimum positions of each suspension point, JBC created a full CAD model of the stock 510 front end. While this took a lot of time and diligence, this allowed us to try many more point combinations than would ever be practical with fabricated setups.
While a model is a great idea; it is worthless unless it is proven to match reality. This is why we verified our model. We assembled a model that consisted of a stock 510 front end. We then assembled that identical combination of parts in real life. Then on the model and in real life we measured the bump steer of each. The results are shown in the graph below. All bump steer angles were measured as toe angle in relation to the angle of the LCA to the ground.
Satisfied that our model was validated we were able to continue development.
First, the model was tuned to match the other suspension modifications that JBC feels are appropriate for a properly handling car.
1) Lowered static ride height to 4 degrees of LCA to ground level: This is the suspension position in which the wheel is aligned to 0 degrees toe. The alignment height affects the final tie-rod length. The actual ride height of the car will vary based on tire diameter.
2) 32mm roll center spacers: Also called bump steer spacers, 32mm is a typical size space. The height of the spacer affects the final position of the steering knuckle at static ride height and again this translates into altered tie rod length.
3) 30mm shortened TC rod: Shortening the TC rod adds caster angle to the strut: This modification is popular because it increases steering feedback. This has the side effect of lowering the outer tie rod relative to the ball joint which is a very sensitive relationship in the steering geometry.
4) Move strut top back 25mm: This modification has the same effect as shortening the TC rod. Shortening the TC rod more is not feasible due to tire rub on the front of the fender so , the strut top is moved.
5) Shortened steering knuckle: The rack and pinion has a shorter overall stroke than the stock box, this means that the rack will result in significantly less max steering angle if used with stock knuckle, the JBC steering knuckles are shortened to compensate for this difference. This length greatly affects the outer tie rod position relative to the ball joint. The stock arm will still mount in place but the steering characteristics will diminish and the steering ratio will be greatly reduced.
It is important to note that, all of these modifications make a measurable impact on the bump steer characteristics of the setup and any deviations will yield slightly different suspension characteristics than outlined in these results.
It is also important to know that any components that modify the suspension points in ways not outlined here will have negative effects.
Specifically, the ball joints (pivot and height) and TC-rod pivots are intended to remain stock. The TC-rod is less critical, the ball joint can cause drastic changes. Beware that some aftermarket LCA’s on the market use extra large ball joints.
For example, the T3 GTX2 LCA’s have ball joints that are 20mm taller than the stock ball joint.Figure 5 shows the results of using such ball joints. It shows the drastic toe out bump steer induced by using this setup as compared to many other part combinations using te stock ball joint. Not recommended.
Our model allowed us to explore many more suspension combinations than practical to assemble in real life.
Specifically, the JBC suspension design has over 30 iterations tested in model before construction. This meant we were able to take in to consideration as many options and suspension combinations as possible when making decision on the specific points we would use.
A new crossmember was then constructed to accommodate the rack and pinion and locate it in the correct location to avoid popular motors and allow ease of connection to the steering column. The crossmember was made as a “flipped” crossmember to accommodate front sump motors. As well the new LCA mounting hardware is a double shear connection with the ability to be spaced forward and back depending on TC rod length. This prevents overstressing the suspension components by trying to force them into fixed locations.
The LCA inner pivot point was moved from the original location in order to correct bump steer generated by the new inner tie rod points. As well the effects on roll center were considered and the point was raised 30mm from stock.
With the crossmember modeled and all of the suspension points determined. The bump steer characteristics were measured as shown in the chart below as compared to stock setups.
The stock setups shown are for a completely stock 510 at stock ride height. This means these are the bump characteristics designed by the Datsun engineers. When the stock setups are lowered to popular performance ride heights, and other adjustments are made the bump steer gets very… very… bad. Consider the chart below comparing a stock late wagon to that same wagon lowered and with added castor.
This poor bump steer along with the worn out steering boxes is what gives the 510 such undesirable handling. The final JBC setup has less bump steer than the best stock setup, while raising the front roll center and increasing Ackerman.
With the suspension points determined for optimum performance. A crossmember and steering arms were built to hold the critical oints in location while also avoiding popular motor swaps for ease of installation. The resulting components were analysed in FEA software and tweaked to optimize strength within the fitment constraints.