Vixen S2 bump steer
Discussion
Hi, bear with, loading graphs not easy.
This first graph is bump steer measured on standard Vixen suspension. Single wheel articulating. The other remained at full suspension extension. For reference, the full wheel travel is 150mm, 6 inches. The track rod is level at 90mm travel, so a little over half way travel. The chassis tube is 195mm clear of the ground at that point. So you might consider this at a possible 'normal' running height.
As you can see, it is not good, ideally the graph should show a straight level line, no change in wheel alignment with travel. This is single wheel, so if you were compressing both wheels (under braking for example), the wheels toe out significantly.
Would be interested to hear views. Fixes?
Nic
This first graph is bump steer measured on standard Vixen suspension. Single wheel articulating. The other remained at full suspension extension. For reference, the full wheel travel is 150mm, 6 inches. The track rod is level at 90mm travel, so a little over half way travel. The chassis tube is 195mm clear of the ground at that point. So you might consider this at a possible 'normal' running height.
As you can see, it is not good, ideally the graph should show a straight level line, no change in wheel alignment with travel. This is single wheel, so if you were compressing both wheels (under braking for example), the wheels toe out significantly.
Would be interested to hear views. Fixes?
Nic
Just for interest. Although part of the system. Graph below shows wheel camber change with suspension travel. No steering rack connected. If you connect the rack it is completely different as the bump steer moves the wheel and adds the castor angle into the mix, affecting the camber.
Vixen S3 here, which is effectively full race on the road, (and IMHO as good a handling car as our Tamora, yet only 181BHP), zero bump steer no issues with camber. It has all been said before, rework the chassis and get the shorter rack that was fitted to the Thurner design (different rod arms on each side) that TVR 'price' compromised the car by fitting the Herald rack. A@
The list is not endless...on a standard Vixen a proper UJ, alloy rack mounts and mounting the column to the shell helps.
The list is not endless...on a standard Vixen a proper UJ, alloy rack mounts and mounting the column to the shell helps.
Edited by Adrian@ on Monday 9th January 12:59
NicBowman said:
Hi Adrian. Where can I get a rack like that? Thx. Has anyone actually measured the bump steer in that set up?
I am no longer sure if the Triumph rack is even played with these days (as they are replaced with a Ford item I think) I shortened a batch of Triumph inner racks a good few years ago (I build my own racks), but not theses days. I would speak to Nigel Reuben racing (he did Steve's car recently) No to the measurement recently BUT IMHO you need to keep to figures of what is possible on the road with everything you are running ... simple things like OE 16" open length shocks allow for travel you would be in a ditch to ever find that droop on the road. A@I have never seen it on a piece of paper.
The Thurner MK3 SWB car is Herald wishbones and short steering arms and short rack, Elva rear upright with a two piece lower wishbone (rather than a one piece set up from Elva) with Banjo diff conversion. The Vixen gets what it gets, and is positive camber on steel braced tyres out the box (simply bringing into negative is a chore).
Am I already seeing the imported version of the rack on your car?
A@
The Thurner MK3 SWB car is Herald wishbones and short steering arms and short rack, Elva rear upright with a two piece lower wishbone (rather than a one piece set up from Elva) with Banjo diff conversion. The Vixen gets what it gets, and is positive camber on steel braced tyres out the box (simply bringing into negative is a chore).
Am I already seeing the imported version of the rack on your car?
A@
Edited by Adrian@ on Monday 9th January 16:49
I suspect. I tried to get a rebuilt one, but I was sent one with a bent track rod! Sent it back. This one came from Moss Europe. Hard to tell if rod lengths are exactly the same, part number is the same though! Quality seems good though.
To make bump steer better, needs rack mounted lower, with longer rack and shorter track rods.
To make bump steer better, needs rack mounted lower, with longer rack and shorter track rods.
Nic, as Adrian@ suggests the chap who can build you a suitable rack with unequal length arms is Nigel Reuben.
The other part of the equation is then mounting the rack at the required point, as the existing chassis mount will not allow the arm length needed on the O/S. Therefore the mount has to be moved further inboard.
Nigel modified mine when he rebuilt the chassis and fitted the modified rack, so he will be able to advise.
Your data is really interesting and it would be great to see a comparison on paper. Perhaps that's something you can share when you have made the modification and fitted an unequal length rack. Your data is the only such example I've ever seen.
From a driver perspective I can say Nigel's mod made a really positive difference driving on the road. It is noticeably less nervous and reduces bump steer to levels of a modern car. On the track there is less benefit as by definition tracks are smooth and don't have pot holes. However using the kerbs is less frightening.
Moto
The other part of the equation is then mounting the rack at the required point, as the existing chassis mount will not allow the arm length needed on the O/S. Therefore the mount has to be moved further inboard.
Nigel modified mine when he rebuilt the chassis and fitted the modified rack, so he will be able to advise.
Your data is really interesting and it would be great to see a comparison on paper. Perhaps that's something you can share when you have made the modification and fitted an unequal length rack. Your data is the only such example I've ever seen.
From a driver perspective I can say Nigel's mod made a really positive difference driving on the road. It is noticeably less nervous and reduces bump steer to levels of a modern car. On the track there is less benefit as by definition tracks are smooth and don't have pot holes. However using the kerbs is less frightening.
Moto
Here is my reality, that I have pretty much nothing standard (Koni alloy body shocks are not picking up in OE position and open lenght is 10.5 springs are 8 with helpers, both wishbones are different from OE, steering rack, alloy mounts, actual rack position, UJ, shaft, column mounting dash bracket. A@
Hi,
Thanks Moto. For discussion, why different lengths on either side? The suspension is mirror image and the rack centrally mounted. Weird…
I am surprised at how little actual data I have seen, as you say, never seen a graph. It only takes a couple of hours, three pieces of wood, a tape measure and some chewing gum. The gum isn’t needed, but I quite like gum.
If I make a change I will remeasure.
Thanks Moto. For discussion, why different lengths on either side? The suspension is mirror image and the rack centrally mounted. Weird…
I am surprised at how little actual data I have seen, as you say, never seen a graph. It only takes a couple of hours, three pieces of wood, a tape measure and some chewing gum. The gum isn’t needed, but I quite like gum.
If I make a change I will remeasure.
Nic, In answer to the rod arms being different, the rack is not central, the pinion housing needs to stay as is and passenger side rack mount modified with an extender/longer passenger side rod arm.
On my car I have a one piece column shaft rather than a three piece (TVR extend a Triumph item with a tube/sleeve) so it cleared the engine, as sorting how the column was attached to the dash/through the shell/down past the engine, (the shaft 'just' touched the engine and fed back through the steering wheel) was the final part of the jigsaw that 'was' bump issues on extreme cornering.
From your pictures ...Q. are your top wishbones standard? if so, the top ball joint lock nuts need to be slim not full nut (If you get stuck for a couple, I still have a box of them. Use the full ones welded onto the rear track adjuster shaft! cut in half and re built on the shaft as a turn nut) as I did mention that OE spec is positive camber, and you will need them, and still might need to rework the wishbone/nut to get to negative). A@
On my car I have a one piece column shaft rather than a three piece (TVR extend a Triumph item with a tube/sleeve) so it cleared the engine, as sorting how the column was attached to the dash/through the shell/down past the engine, (the shaft 'just' touched the engine and fed back through the steering wheel) was the final part of the jigsaw that 'was' bump issues on extreme cornering.
From your pictures ...Q. are your top wishbones standard? if so, the top ball joint lock nuts need to be slim not full nut (If you get stuck for a couple, I still have a box of them. Use the full ones welded onto the rear track adjuster shaft! cut in half and re built on the shaft as a turn nut) as I did mention that OE spec is positive camber, and you will need them, and still might need to rework the wishbone/nut to get to negative). A@
Edited by Adrian@ on Wednesday 11th January 07:33
Hi Adrian
They are full nuts, as supplied. I did think that half nuts could be needed. I just got sufficient adjustment with them. I would appreciate some half nuts, perhaps pm me.
The upper wishbones are standard. The second graph on the thread shows the camber vs travel. You can see that there is a large change from positive at full extension to negative at full compression. So, both tracking and camber are massively affected by the chosen ride height. The full camber change is around 7 degrees! So, honestly, setting it 'exactly' is a bit up for discussion, as it shifts so much naturally.
I am more of the view that big wall tyres are not so sensitive. Luckily.
The track rod lengths must remain the same imho, as bump steer is dominated by track rod length. Otherwise each wheel would bump steer a different amount?
Needs software analysis ideally.
Best
Nic
They are full nuts, as supplied. I did think that half nuts could be needed. I just got sufficient adjustment with them. I would appreciate some half nuts, perhaps pm me.
The upper wishbones are standard. The second graph on the thread shows the camber vs travel. You can see that there is a large change from positive at full extension to negative at full compression. So, both tracking and camber are massively affected by the chosen ride height. The full camber change is around 7 degrees! So, honestly, setting it 'exactly' is a bit up for discussion, as it shifts so much naturally.
I am more of the view that big wall tyres are not so sensitive. Luckily.
The track rod lengths must remain the same imho, as bump steer is dominated by track rod length. Otherwise each wheel would bump steer a different amount?
Needs software analysis ideally.
Best
Nic
Hi, anoraks only….
More work. I have measured the camber and bump steer in the standard setup, as seen. I checked the new rack vs the old and found the track rod length is not the same, new one is 180mm, old 170mm, so put the old rack on and remeasured bump steer expecting some difference, to find it almost identical. So, measured the suspension and did a graphical analysis of the suspension travel and bump steer. Get a life you may say. Just to add, I did this kind of thing for a living so I am qualified.
First, camber measured on the car vs graphical.
I have to say I am pretty pleased. The two plots follow the same curve. Forget the actual values, that is adjustment, just look at the shape. So, graphical model not terrible so far.
Then I plotted the movement of the steering arm.
You can see it makes an arc of a circle. This is the circle that the track rod needs to follow to steer without bump steer. If the circle centres don’t match, then you get bump steer. To find the exact steering rack position you need to solve the puzzle of best fit circle to those points. A bit complex, so I cheated and did it by trial and error on MS Excel.
I then plotted my best fit, alongside the results if you put the rack where it actually is mounted.
Getting excited now! The blue curve is the standard, as modelled. This compares pretty well to the measured (see up the thread). Big positive wheel movement with travel, not good. The green line shows the best I found so far. Much lower values, generally under a degree.
I am sure it can be even better. So, magic numbers. Standard rack is mounted 70mm above the lower wishbone centreline (lwc, to save me typing). The steering rack ball joint is 50mm inside the lwc. The new version is 65mm and 35mm. So, moves down by 5mm and needs longer track rods by 15mm.
Getting the rack lower is easy, longer track rods not so…. I will look at best option with current rack. Will add later.
Nic
More work. I have measured the camber and bump steer in the standard setup, as seen. I checked the new rack vs the old and found the track rod length is not the same, new one is 180mm, old 170mm, so put the old rack on and remeasured bump steer expecting some difference, to find it almost identical. So, measured the suspension and did a graphical analysis of the suspension travel and bump steer. Get a life you may say. Just to add, I did this kind of thing for a living so I am qualified.
First, camber measured on the car vs graphical.
I have to say I am pretty pleased. The two plots follow the same curve. Forget the actual values, that is adjustment, just look at the shape. So, graphical model not terrible so far.
Then I plotted the movement of the steering arm.
You can see it makes an arc of a circle. This is the circle that the track rod needs to follow to steer without bump steer. If the circle centres don’t match, then you get bump steer. To find the exact steering rack position you need to solve the puzzle of best fit circle to those points. A bit complex, so I cheated and did it by trial and error on MS Excel.
I then plotted my best fit, alongside the results if you put the rack where it actually is mounted.
Getting excited now! The blue curve is the standard, as modelled. This compares pretty well to the measured (see up the thread). Big positive wheel movement with travel, not good. The green line shows the best I found so far. Much lower values, generally under a degree.
I am sure it can be even better. So, magic numbers. Standard rack is mounted 70mm above the lower wishbone centreline (lwc, to save me typing). The steering rack ball joint is 50mm inside the lwc. The new version is 65mm and 35mm. So, moves down by 5mm and needs longer track rods by 15mm.
Getting the rack lower is easy, longer track rods not so…. I will look at best option with current rack. Will add later.
Nic
Edited by NicBowman on Saturday 14th January 18:57
Hi, clearly this is more interesting to me than anyone else! No big surprise!
Ok, lowered the rack 5mm, took out the mount and pulled it down to the chassis with an exhaust clamp. Need to measure exactly how far, 5mm is close though. It is mathematically sensitive to the height, but also seems to be sensitive in real terms.
Very pleased with the results. The lowered rack is not perfect but substantially better. Total bump steer under 2 degrees. Result!
Still got to measure the effect of a modified rack with longer track arms.
Nic
Ok, lowered the rack 5mm, took out the mount and pulled it down to the chassis with an exhaust clamp. Need to measure exactly how far, 5mm is close though. It is mathematically sensitive to the height, but also seems to be sensitive in real terms.
Very pleased with the results. The lowered rack is not perfect but substantially better. Total bump steer under 2 degrees. Result!
Still got to measure the effect of a modified rack with longer track arms.
Nic
Hey Nic, good stuff - although I've read it 3 times to really get what the data is saying. I'm a bit slow on the uptake 
Your 5mm drop in height presumably is from removing mounts and not lowering the chassis rack plates? Therefore is your plan to mod the chassis as a permanent solution.
Keep it coming.
Moto
Your 5mm drop in height presumably is from removing mounts and not lowering the chassis rack plates? Therefore is your plan to mod the chassis as a permanent solution.
Keep it coming.
Moto
Hi Moto
Yes, bit complex but it is as complex as it has to be. I have also measured changing height and track rod length, but height dominates.
The height was dropped by removing the rubber mount and strengthener and bolting down with exhaust clamps. So, custom solid mounts could do that easily.
I suspect the optimum for ease of mod will be standard rack with custom lower mount on the same chassis plate. It is significantly better with this, although not perfect it should be good enough.
Nic
Yes, bit complex but it is as complex as it has to be. I have also measured changing height and track rod length, but height dominates.
The height was dropped by removing the rubber mount and strengthener and bolting down with exhaust clamps. So, custom solid mounts could do that easily.
I suspect the optimum for ease of mod will be standard rack with custom lower mount on the same chassis plate. It is significantly better with this, although not perfect it should be good enough.
Nic
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