Hi Everyone, I'm a new member here, although long time reader. I have a long term Mk1 Golf project on the go (project thread on Pistonheads here if anyone's interested: https://www.pistonheads.com/gassing/topic.asp?h=0&f=47&t=1944677) Part of the project is going to be an in depth study into every aspect of the suspension, you could say this is my 'specialist subject' as I design suspension for my day job. As part of the process I've measured, modelled and analysed the standard front suspension and steering geometry of the Mk1. I thought it would be of interest for Mk1 owning members of Club GTI, so will share the results here. Firstly, I'll explain the process for building the model, and the assumptions I've made: Hardpoint locations are built up from a combination of part measurement, body jig diagrams, and measurements from my own shell. Some amount of error will be introduced by having to estimate certain things (where are the centres of the ball joints, bushes, top mount etc.) I don't have a reference for the standard ride height, so I set the static ride height with the wishbone level to the ground The geometry analysis needs to be done using the laden radius of the tyre. This is a function of tyre construction, inflation pressure, camber angle etc. I have done my analysis around a 195/50 15 tyre, with an assumed static tyre stiffness of 200N/mm, and for my target sprung corner mass. The tyre laden radius that I'm using for my analysis is 276mm Wheel offset used is +38 (Standard Mk1 golf offset) Static geometry settings are set close to the workshop manual figures for a GTI. Static Toe per side: 0.00deg (Book spec +0.08deg) Static Camber: +0.29deg (Book spec +0.33deg) The static camber in particular will have a big influence on the Ground Level Lateral Offset (aka Scrub Radius) Static Toe will have very little influence on this analysis. Here's an overview of the model: This is a table of the main static geometry values: The two columns to the right are work in progress for my new setup. The Castor angle I'm getting for the baseline is slightly higher than the book figure (1.83deg), this could be due to the ride height I've picked. Hopefully the metrics on the left are familiar for most people, I can explain them in more detail if people want. Unfortunately I can't go into detail about where the target numbers come from, or what the blacked out metrics are. (Although I will say that they relate to track rod loads and steering feel) Here are some graphs: This shows the effect of moving the top mount rearwards on Castor angle (perhaps useful data if you have adjustable top mounts, although be aware that this will be affected by tyre/wheel diameter and ride height) Bump steer; the values given are the gradient taken over a +/-10mm range about 0mm ride height. The value of bump steer is therefore quite sensitive to the curvature of the graph. For example, if you took the value for the standard car at a wheel travel of +40mm (Equivalent to lowering the ride height by 40mm) the value would increase to -15.7deg/m. All setups are of a toe out tendency into bump, this in an understeer behaviour (the outside wheel will Toe out when the car rolls) Without starting a bump steer debate, values up to about -8.5deg/m are common on road cars. What level can be acceptable depends on many factors, a big one being roll stiffness. (The less the car rolls, the more bump steer required to deliver the same amount of toe out per g of lateral acceleration) I have seen values higher than -16deg/m on some very well regarded sports cars. Also, whilst it's not immediately obvious from the graph, bump steer is highly sensitive to Castor angle. The big difference between the green and red curves is +1deg castor angle. (Increasing castor angle lowers the track rod end relative to the steering rack, reducing bump steer) (EDIT: I've labelled the 'Top In/Top Out wrong on the graph. The bottom right is the bit we're interested in, where we want negative camber in bump) This is the one that you can see I'm struggling with for my new setup. This is one of the big downsides of a McPherson strut suspension, the inability to gain much negative camber in bump. There are a few ways to improve it, for example increasing the King Pin inclination. However, as you can see from the table at the start, I'm pretty close to the upper limit I want to go to. There's still plenty of work to be done here, so I'm not concerned at this stage. This graph isn't that useful until I measure the steering rack ratios (which I will do and update this) to give Steering Wheel angle vs Toe angle. What can be seen is that using a Toyota Starlet (or Sera, or Paseo) steering rack will slightly increase the steering ratio in front lock. This is for 2 reasons: The rack is slightly wider (~587mm vs ~580mm) The rack needs to be spaced off of the mounts slightly so the pinion can be tilted back to line up with the hole in the bulkhead. (I've estimated 25mm for now) There are a few threads with more detail on this swap on Club GTI. Both of these change the length and plan view angle of the track rod, affecting steering ratio. This will also knock on to the Ackerman, which I haven't calculated yet. (I will update when I have) Note that the Polo knuckle that I'm using has the track rod end further outboard than the Mk1 Golf knuckle. Therefore the effect on steering ratio and Ackerman is less that if I was using the Mk1 knuckle. Finally camber change vs toe. This shows the benefit of increasing castor angle, you gain negative camber with toe angle when steering. (Blue to red is 2.0 -> 3.6 deg Castor). This camber gain is being counteracted by the positive camber change caused by the king pin inclination. I'll gain as much as I can here whilst keeping the other metrics in my table under control. Adding Castor also increases castor trail, which increases the lever arm around the steering axis that lateral contact patch loads act on, causing a moment around the steering axis. This is reacted by the track rod, and ultimately you feel it as torque through the steering wheel. However, as always there is a trade-off, the obvious one being steering weight. What is not so obvious is the interaction between the mechanical trail (defined by the geometry), and the pneumatic trail of the tyre. For smaller slip angles, the actual contact patch of the tyre trails behind the geometric centre of the wheel. This is known as the pneumatic trail, and is added on top of the mechanical trail. (Not my picture, but you get the idea) As you approach the limit and the tyre gives up it's lateral force generation, the pneumatic trail decreases (contact patch moves forwards), and therefore our total trail decreases. If the driver is to be able to feel this through the steering wheel, the ratio between the pneumatic trail and mechanical trail needs to be kept below a certain level. Imagine if we had loads more mechanical trail than pneumatic (like the image above!). Losing the pneumatic trail component won't reduce the steering wheel torque that much, and we'll struggle to feel that we're approaching the limit of the tyre. There's also other things to consider, such as what happens to the pneumatic trail when we accelerate? But that's for another day haha! I'll leave you with some pictures comparing the Polo 6N2 knuckle to the Mk1 Golf knuckle. I'm sure the keen eyed among you will immediately see the benefits (and downsides depending on how you look at it) of using this knuckle! It's not a straight swap, but I think it's worth it. Hopefully you guys found this useful! Thanks, Joe
Interesting write up! actually never seen the polo hubs used on a mk1. Aside the geometric benefits of the polo knuckle, does it also have a stronger hubs than the original mk1 knuckle? I am currently in the progress of converting my golf mk1 with mk2/3 golf knuckles. Mostly because of the stronger hubs (mine broke on track..) but also because of some geometric benefits (after some modifications) also intrigued by your bumpsteer remark, that some race cars run quite a lot of bumpsteer. Also without really wanting to start a bumpsteer debate, i always thought you want as close to zero as you can. But I can see it being beneficial as long as the bumpsteer is in the right direction.
Thanks guys, hopefully the info helps some people out. To be honest, I don't think the Polo hub or upright will be much stronger (or stiffer) than the Mk1 part. The section around the bearing is slightly beefier, and the bearing is about 1mm larger diameter inside and outside. So it will probably be a few percent stronger, but nothing like as strong as a Mk2/3 knuckle and hub. (Mk1 Bearing Inner diameter 34mm, Polo 35mm, late Mk2/Mk3 40mm) On the other hand, it's only 0.2kg heavier than the Mk1 knuckle, and doesn't need a ball joint extender (I know people are happy running them, just I'd rather not). The driveshaft spline is also the same size, so should work with Mk1 shafts. On the topic of bump steer, I should say that I'm referring to road cars, race cars are not my area of knowledge. When I referred to sports cars, I was talking about some high performance German cars that I've seen data for . Bump steer itself is not desirable, as it will of course introduce some instability when driving straight, although only when over a certain level. (My daily driver has about -6deg/m at the front, and you'd never know). What it's useful for is modifying the toe (and therefore slip angle) when the car rolls in a corner. (Roll Steer) Just view it as another tool for tuning the handling balance of the car, and don't stress that it must be zero. So whilst race cars aren't my area of specialism, I don't see why that logic wouldn't also apply.
Race cars get away with little bump steer, by running very stiff, and with very little suspension travel. It's not ideal, but its usually a compromise caused by other issues
