Bumpsteer and Roll Centre Correction. This has been a big item on the list for quite some time now, and it’s going to be interesting to see how it will effect, what in my mind is already a well handling car. This post is fairly in depth and fairly nerdy… if that tickles your pickle stay tuned. I chose to start with the Noath Engineering Roll centre correction and track rod flip kit. A lot of the off the shelf ones I don’t like the look of and I’ve heard numerous stories of them failing. The main reason for this kit was it was tried and tested on Nige’s golf, and they use really high quality materials. If you look at the angle of the wishbone it gives you an idea of the roll centre, before and after the kit. OEM at ride height Noath Kit at same ride height With roll centre you are limited by the size of the pins you can use in the BBJ, for interference with the wheel, especially on 15’s. Now onto the track rod flip kit, I was thinking about fitting the kit and leaving it at that, but curiosity got the better of me and I wanted to work out exactly what the factory bumpsteer was, in comparison to the kit. At this point I want to just reference Nige’s website for those who haven’t seen it, a lot of what I’ll be going through here, he has covered. www.pinderwagen.com So, bump steer. What is it? and how do you measure it? Bump steer to put simply, is unwanted toe change through a cars suspension travel. The changes can be pretty crazy, and this can cause unpredictability in handling – especially on track, where you will naturally use more of the suspension travel due to the forces. Bumpsteer is effected by the arc created by the lower ball joint mounting point and the track rod mounting point, and their relationship to each other. – its almost like a push and pull effect. How do you measure it? There’s a few ways, you could take it to a specialist, who will no doubt charge you an arm and a leg. There’s a Longacre gauge, which are upwards of £600. Which again isn’t feasible for us ordinary folk. And where is the fun in not learning along the way!? Taking inspiration from the PinderWagen, I used 2 sheets of flat wood. I bought these from B&Q for £10 – much more respectable! Before you do anything it’s important to take some initial measurement. With the front bumper and front wing removed, it gives great access to the suspension. This is how I measure ride height, using a bolt in the hub and a block of wood on top of the inner wing. It’s also a good idea to measure the front sill to floor height at this point. Next up you need to remove the front spring, this makes life a lot easier when jacking the suspension up and down. Next up is to drill out the bolt pattern so it can be mounted to the hub. The wood was cut into 2 pieces, one will be bolted to the hub, the other will pivot off the floor, and the distance between the leading edge, throughout the suspension travel is where we will measure the bumpsteer. There’s a few ways to measure it, you can use a steel ruler – time consuming and has ability for inaccuracies. Vernier calipers are another option, much more accurate but again a bit of a faff, measuring and remeasuring. I chose to go for a dial gauge, they are fantastic, measuring 1mm through one full rotation of the dial, they are also very inexpensive, this was £8 on amazon. The gauge needs mounting to the wood, I used an old GoPro mount – this worked out really well. on the other side of the wood, mirror the length of the gauge with a bolt. This will remain constant and make any measurement visible on the gauge. With everything ready to go, and the front wheel off, I lowered the car back down on the jack so it was the same height as the sill to floor height we measure previously. I don’t have 2 jacks and axles stands are too high, so I used the standard scissor jack to keep the height of the car consistent. My main jack I used to raise and lower the suspension. The standard set up was then measured. The best way I found to measure this accurately, was to jack the suspension up to ride height. The dial gauge was then zeroed. I then jacked the suspension up to full compression, released the jack slowly and filmed the dial gauge on my phone. With the results filmed on my phone, I could sit inside and plot everything on a graph. The straighter the line, the closer it is to zero bump steer. To explain the above: X axis – Shows toe change in 1mm increments, toe in to the left, toe out to the right. (each small square on the graph is 0.05mm) Y axis – Show suspension travel in 10mm increments, from full extension, through ride height to full compression (each small square is 0.5mm in travel) The suspension was jacked up and every 10mm a dot was plotted on the graph, with a line of best fit to see it visually. What we can see in this standard set up is we gain toe in as the suspension extends from ride height and gain toe out under compression. In driving terms, toe out gain under compression isn’t necessarily terrible, certainly more favourable than toe in, which I think is why the car felt pretty good anyway. – But we are still seeing over 3mm of toe out at 40mm compression, which is significant. So how does the Noath Kit compare? Well, as we mentioned earlier, the relationship between the LBJ and the track rod is what effects bumpsteer, so if you were to just fit the extended ball joints and keep the track rod as standard the bumpsteer would be horrendous, quite literally unmeasurable! I’ll also point out at this point that bumpsteer is bespoke to each car, there’s no one size fits all. Noath doesn’t sell this as a ‘bumpsteer kit” the idea of flipping the track rod is to get the bumpsteer back towards oem, as best they can for a bolt on kit. This graph shows the Noath kit, as it is, as you can see its actually worse for bumpsteer than the oem set up. So although we have corrected the roll centre, that has now had a direct effect on bumpsteer and made it worse. The red line on the graph shows the track rod end lowered 5.5mm using washers, this was to see which way I need to move the track rod. As you can see the redline is worse still. With that data collected I had to work out a way to move the track rod up. I could cut down the misalignment spacers Noath provided, but there was a casting on the track rods which would need to be ground down. Going over the graphs again, I’d worked out I’d need more than just a trim of the spacer. The only way would be to trim the hub to get the desired height. This caused another issue, the Noath kit comes with a tapered bolt which is great for a direct bot on kit but if I had to trim the hub the bolt would no longer sit correctly in the tapper. So I decided to drill the hub out to M14, and use an M14 rose joint instead. These were from McGill Motorsport, ordered on Friday afternoon, arrived Saturday! I ordered their highest quality joints, misalignment spacers and rubber boots, costing £45. With the hub drilled out, I took 6mm off the hubs. The material I’ve taken off the bottom I’ll add back on the top to retain the strength in the arm, I’m just waiting for the steel to arrive. After the material was taken off and cleaned up, I cut one of the misalignment spacers down to 4mm and assembled. I checked the articulation of the joint and there was still plenty of room and no binding in the joint. I then plotted another graph: The above results I was absolutely chuffed with! Red line is: Modified hub with M14 Joint, 4mm misalignment spacer Blue line is: Modified hub with M14 Joint, 4mm misalignment spacer and 2.3mm washer Green line is: Modified hub with M14 Joint, 3mm misalignment spacer Looking at the data, the red line seems best for me, although green looks best on rebound, Red has the most usable area in zero bump, with up to 0.5mm of toe change over 82% of suspension travel. The reason the graph arcs to toe out at the extreme ends of travel is because the track rod is effectively too long, the solution is either a wider rack, or fit rack spacers. That will be version 2 of this project and for now I couldn’t be happier! For those wondering I did some back to back test, and each data point was accurate to within 0.1-0.2mm, I’ll settle for that with a DIY setup at a fraction of the cost! Now I've just got to repeat on the other side.
Yep exactly i used a paint pen and then scored a line with the vernier calipers, cut along that line and cleaned up with a flappy wheel. Ill be adding material back on top once the metal arrives
Is adding the material even necessary? It's solid steel right? I also thought about mounting these pens, same as clearly on your picture the lower wishbone is pointing up towards the outsides. I didn't realise mounting the track arm on the lower side would also be a "necessity". Having said that, I don't really know how much the effect is when 3 mm more toe-in occurs. Other than that, I have some doubts about the longer pens as it creates an arm on the hub. As my car is also track-only, I was afraid it might break on track when hitting the curbstones too often. I know some guys weld the pens to the hub carrier as well, but even then. However, it gives a bit more trust knowing that Nigel is running these things on his car, without a doubt his car is faster and sees more track meters than mine
Im not sure i understand what you mean about the LBJ pins? i dont know if adding material to the hub is needed, but im not an engineer, id rather add on what ive taken off and most likely be able to add more strength than it had from factory. Better over engineered than under engineered!
Cope, why is toe out with damper movement more desirable than toe in? How much droop and compression do you think you have, on track?
Yeah with the pens I mean the longer LBJ shafts True that. Thanks to this topic I now am looking for some spare hubs to modify...............
On compression especially I think if anything toe out would be better as it should help the front of the car steer into the corner. Maybe not so much on extension. Honestly, I'm not sure. All I can do is go off photos, it will be track dependant for sure. The most aggressive would be through the foxhole at the Nurburgring, thats the only place where I've heard a chirp on the front wheel as the suspension is at full compression or very very close to.
I was thinking about it too, but I came to the following; In a corner the suspension on the outer side is compressed the most. Toe-in will mean more steering angle, toe-out will mean less steering angle. The inner suspension will not be compressed, but extended. And for the inner wheel it is the opposite as above; toe-in will mean less steering angle, toe-out will mean more. Looking back to your graphs, the first one indicates toe-out on compression, toe-in on rebound, the second mirrors it: toe-in on compression, toe-out on rebound. Then the third one only has toe-out, but looking at my theory above, wouldn't it be preferred to have a little toe-in when compressed (second graph)? Of course, you want as little changes that are not your input (steering wheel), that's logical. And my next question is; you measured everything with the wheels straight. Is the geometry and the behavior of bump-steer different with the wheels turned in or not?
i dont think it would. If you imagine going through a right hand corner, if the outside wheel toes in on compression it would effectively add steering lock, which would make positioning the car much more unpredictable. If it had toe out it would need a touch more lock which is much safer. But bear in mind we are now talking about fractions of a mm, in reality i dont think youd feel that through the wheel. Especially on a 30 year old mk2 which will no doubt have some chassis flex etc etc. yep steering was was kept straight ahead. Yes there is chance for some movement through all parts of the suspension and any bushes that have slight flex. But what we are talking about isnt that, we are talking purely about bumpsteer (the relationship between the LBJ and track rod end, and the arch they share) suspension geo is all swings and roundabouts, you change one thing and it effects another, but we have got to be realistic, its a DIY built and set up car, not F1, there will always be compromises but its about getting things as good as you can with what you have. in the words of Micheal Dunlop “you can only piss with the dick you've got”
