Trophy suspension settings
Fred, I've noted your preference for toe-in before, and would be the first to admit that less toe-out or even some toe-in is a classic cure for instability. I am sure that you would agree that toe settings are also part of the camber, castor and scrub radius equation, which leads me to wonder if your advice is based on less offset than the Trophy runs, i.e less than 45 ET?
Am tempted to wager a couple of pints of Devon's finest ale on it! Otter Bright that is.
Am tempted to wager a couple of pints of Devon's finest ale on it! Otter Bright that is.
i just feel they drive much better with toe in, its suits my driving style too i guess
we run the 240bhp cup with slight toe in (3deg neg camber)
here's a question
what happens to the wheels when you accelerate??? (apart from the obvious!)
we run the 240bhp cup with slight toe in (3deg neg camber)
here's a question
what happens to the wheels when you accelerate??? (apart from the obvious!)
Working on the basis that the Clio runs negative scrub radius (which is reduced if you run less ET) they will at a first pass toe out, but by the same token they will toe in under braking and as it is possible to brake at 1g but acceleration is much more liesurely, I would argue the braking is the more important factor! Hence the argument for toe-out!
The trouble is that there are various other factors including contact patch distortion, camber, castor and KPI which should be fed into the model (normally ADAMS).
The reason that negative scrub radius was first used by Audi in the early 70's was to provide braking stability on split mu surfaces - unfortunately they combined it with a high mounted rack and that led to weird and wonderful bump steer characteristics, nowadays there are better solutions. It does also allow for better load paths for the bearings and top seal of the strut and should reduce torque steer. However modern FWD cars are moving away from the simple stut system, as per the Clio 197, the Peugeot 407, some Hondas and Audis, so it is possible to optimise the geometry. It would be interesting to know what toe they run.
BTW what ET do you run?
The trouble is that there are various other factors including contact patch distortion, camber, castor and KPI which should be fed into the model (normally ADAMS).
The reason that negative scrub radius was first used by Audi in the early 70's was to provide braking stability on split mu surfaces - unfortunately they combined it with a high mounted rack and that led to weird and wonderful bump steer characteristics, nowadays there are better solutions. It does also allow for better load paths for the bearings and top seal of the strut and should reduce torque steer. However modern FWD cars are moving away from the simple stut system, as per the Clio 197, the Peugeot 407, some Hondas and Audis, so it is possible to optimise the geometry. It would be interesting to know what toe they run.
BTW what ET do you run?
George K said:
Very interesting reading, trying to understand all that from a layman's point of view - are you saying that under acceleration, due to the hub being on the outside edge of the wheel, the distortion leads to temporary toe-out as the inside edges of the driving wheels are pulling away more, and so the opposite under braking where toe-in occurs? So how would you counter this to optimise under both circumstances, and can it really have that much of an effect?
Also, what are your opinions on Ackerman with a FWD car like the Trophy? As I've heard lots of different opniions on it's importance and it generally being unecessary in a road car due it affecting the drivability more than the possible benefits to the cornering.
What is KPI and how does the packaging of a FWD car affect the range of possibilities for castor or is the fact that FWD cars tend to have a lot less than a RWD sports car like a BMW M3 more down to the fact that it affects traction in the driven wheels?
all getting very technical - sounds like snow ploughing to stop when your a beginner skyer isn't as good as using toe out to stop??????? I hope you've got strong groin muscles lol :lol:
Cue - great comment!!
Steve, I work near Newbury during my rather luckythree day week and would be happy to meet up somewhere halfway to you one Tues/Wed evening- probably better than boring everyone here on the forum. There are no definitive answers, because FWD asks the front wheels to drive, steer and brake. Just remember that if I knew the solution I would have retired years ago - I just have a few insights into the issues and some myths.
Steve, I work near Newbury during my rather luckythree day week and would be happy to meet up somewhere halfway to you one Tues/Wed evening- probably better than boring everyone here on the forum. There are no definitive answers, because FWD asks the front wheels to drive, steer and brake. Just remember that if I knew the solution I would have retired years ago - I just have a few insights into the issues and some myths.
If anyone is bored by your technical answers they can not read your posts (polite way of saying piss off), but I'm pretty sure a lot of users are more than interested in what you have to say, given your background in the field and the fact you are unbiased and not just saying things that support buying your products.
A three day week! Bliss, something I aspire to one day, not very common for Architects, 9 day week the norm (in hours anyway), that way I can spend the rest of the week restoring classic cars, 1968 Mustang 390 GT first on list.
A three day week! Bliss, something I aspire to one day, not very common for Architects, 9 day week the norm (in hours anyway), that way I can spend the rest of the week restoring classic cars, 1968 Mustang 390 GT first on list.
i find it all very insightful, keep up the good work George... and Fred of course... then there's Yozza...... Nik..... the list goes on.
With some kind encouragement I thought that I would try to put together some explanations, not too long and with some sort of logic. All aimed at the front wheels – the differences between FWD and RWD should emerge as it proceeds. This is a mixture of long time ago student engineering, crumbs from the tables of race car designers (mainly F1) and self taught by reading, analysis and work on touring cars. As they often say the opinions expressed are purely personal. Every now and then somebody explains a new facet and I realise how little I know and how complex the subject is. Anyway, if you’re still awake, here goes.
Contact patch – the area of the tyre in contact with the road. This actually deforms quite considerable under braking and acceleration, so that the effective centre moves away from the geometric centre.
Slip angle – the difference between the direction in which the wheel rim is pointing and that in which it is actually travelling. Caused by sidewall distortion and the tyre tread slipping over the road, but not enough to actually break away. I believe this can be up to 5 degrees.
KPI – King pin inclination. The angle to the vertical of the axis through the top mount and bottom ball joint, when viewed from the front of the car. When this axis is projected down to ground level, the distance between where it touches the ground and the centre point of contact patch is known as the scrub radius – not to be confused with offset, which is the distance from the back face of the wheel hub to the wheel centre line. If it touches the ground outside of the KPI point, then you have a negative scrub radius. This is common on FWD cars with strut type suspension, partly due to packaging constraints and the desire to avoid over-hung bearing loads and because it was found to help braking stability on un-even surfaces and also to make torque steer more manageable. It is often about 11- 13 degrees, which is quite steep. It has the draw-back that as the wheels turn, for example for the o/s front going onto left lock, this amount of KPI will actually reduce camber.
Castor on the other hand is the angle between the same axis, but when viewed from the side. 99.99% of cars (all in my experience) have positive castor so that when viewed from the side the intersection is in front of the contact patch centre. The distance on the ground is known as the trail and when the wheels are not transmitting power gives a self centring effect. The other benefit of castor is that it adds camber as you wind lock on, thus cancelling out the minus effect of KPI.
What a few moments reflection will show is that scrub radius will also produce a turning couple about the KPI/castor axis. Under braking positive scrub radius will try to toe-in the wheels, but power on (only a problem with FWD) will try to toe-out, and when the car is also rolling, so transferring weight to the outside wheel, power will try to increase the amount of lock This is not the whole explanation of torque steer, but should give you an idea of the possible causes. It does not then take long to realise that positive castor induced trail is great when power is off, but when it is on, can create instability. Just think what happens to a super market trolley when the castors are facing the wrong way.
OK, you say, so let’s run negative castor, and so get rid of torque steer. As I explained to Fred the other day braking forces are much greater on road cars than their acceleration forces; so the compromise has to be in favour of positive castor and negative scrub radius. You will now see how blissfully simple RWD cars are in comparison.
Is there a solution? Not completely, but moving away from simple strut suspension does open up the possibilities for cleverer geometry– ref Clio 197 etc. Personally I favour relatively low castor – say 2 degrees, rather than the 4-6 run on some cars, and to get the stability/steering feel by a fairly large scrub radius. The Trophy meets the castor criteria, but I do not have any data re the scrub radius. I tried to measure it in a crude way and came out with a lemon!!
Toe-in or toe-out; It depends on all the above and even the type of construction of the tyre. Conventional FWD and negative scrub radius in general likes a bit to toe-out. Run more camber, or less scrub radius (smaller ET – 37 instead of 45) and you may well need toe-in to compensate. As an aside, years ago we had a source of alloys, which all came with a smaller than o.e. ET – the result was diabolical instability on some cars - cured by machining down the back face of the hub – the reason was that the scrub radius had been reduced too much. See also the logic below re wide tyres
The effect of wide tyres. On a flat surface the centre of the contact patch will be in the centre of the wheel. However most UK roads are un-even and so it is possible for the main contact point to be either in or outboard of the tyre centre line – what i have christened contact patch migration This of course will change the scrub radius; I reckon that in extreme cases it is possible for one wheel to have a positive scrub radius and the other negative. This is why modern cars with wider tyres are so susceptible to tramlining and wandering. My own view is that running some camber – 2 degrees as a norm for road use, will reduce this effect by biasing the load to one side of the tyre; albeit the toe-in/out setting may need to be changed. The only time this will create a wear problem is possibly when the bulk of driving is on motorways, and there are virtually no cornering forces.
Bump steer. Partly because of compromises in design and partly to make cars user friendly, when the front wheels go up and down the toe settings change. In general they go to toe-out on bump – which is the same for the outside wheel in roll – this is meant to make the responses less twitchy. Racing cars do everything possible to avoid it. Lotus handing packages have been known to re-design the steering arms so as to reduce it. Clio settings not known.
Ackermann angle is a geometrical way of arranging the steering arms so that when the wheels are turned, the inside wheel turns more to allow for the smaller radius turn. This is most apparent on full lock. There are lots of opinions as to what is best for handling purposes. My own view is that in most normal conditions the amount of lock applied is so small as to render Ackermann irrelevant. In racing cars I have seen normal Ackermann used, none or even negative Ackermann. There did not seem to be any very clear conclusion – with better simulations things will have moved on in the last five years, but I am not aware of any particular conclusion. The whole discussion is further distorted by the different slip angles side to side and front to rear. I ignore it!
I am sure that I will think of some more points and will do an update in due course. Also someone will have some other facts to add to the story so they should be incorporated to retain some balance.
Contact patch – the area of the tyre in contact with the road. This actually deforms quite considerable under braking and acceleration, so that the effective centre moves away from the geometric centre.
Slip angle – the difference between the direction in which the wheel rim is pointing and that in which it is actually travelling. Caused by sidewall distortion and the tyre tread slipping over the road, but not enough to actually break away. I believe this can be up to 5 degrees.
KPI – King pin inclination. The angle to the vertical of the axis through the top mount and bottom ball joint, when viewed from the front of the car. When this axis is projected down to ground level, the distance between where it touches the ground and the centre point of contact patch is known as the scrub radius – not to be confused with offset, which is the distance from the back face of the wheel hub to the wheel centre line. If it touches the ground outside of the KPI point, then you have a negative scrub radius. This is common on FWD cars with strut type suspension, partly due to packaging constraints and the desire to avoid over-hung bearing loads and because it was found to help braking stability on un-even surfaces and also to make torque steer more manageable. It is often about 11- 13 degrees, which is quite steep. It has the draw-back that as the wheels turn, for example for the o/s front going onto left lock, this amount of KPI will actually reduce camber.
Castor on the other hand is the angle between the same axis, but when viewed from the side. 99.99% of cars (all in my experience) have positive castor so that when viewed from the side the intersection is in front of the contact patch centre. The distance on the ground is known as the trail and when the wheels are not transmitting power gives a self centring effect. The other benefit of castor is that it adds camber as you wind lock on, thus cancelling out the minus effect of KPI.
What a few moments reflection will show is that scrub radius will also produce a turning couple about the KPI/castor axis. Under braking positive scrub radius will try to toe-in the wheels, but power on (only a problem with FWD) will try to toe-out, and when the car is also rolling, so transferring weight to the outside wheel, power will try to increase the amount of lock This is not the whole explanation of torque steer, but should give you an idea of the possible causes. It does not then take long to realise that positive castor induced trail is great when power is off, but when it is on, can create instability. Just think what happens to a super market trolley when the castors are facing the wrong way.
OK, you say, so let’s run negative castor, and so get rid of torque steer. As I explained to Fred the other day braking forces are much greater on road cars than their acceleration forces; so the compromise has to be in favour of positive castor and negative scrub radius. You will now see how blissfully simple RWD cars are in comparison.
Is there a solution? Not completely, but moving away from simple strut suspension does open up the possibilities for cleverer geometry– ref Clio 197 etc. Personally I favour relatively low castor – say 2 degrees, rather than the 4-6 run on some cars, and to get the stability/steering feel by a fairly large scrub radius. The Trophy meets the castor criteria, but I do not have any data re the scrub radius. I tried to measure it in a crude way and came out with a lemon!!
Toe-in or toe-out; It depends on all the above and even the type of construction of the tyre. Conventional FWD and negative scrub radius in general likes a bit to toe-out. Run more camber, or less scrub radius (smaller ET – 37 instead of 45) and you may well need toe-in to compensate. As an aside, years ago we had a source of alloys, which all came with a smaller than o.e. ET – the result was diabolical instability on some cars - cured by machining down the back face of the hub – the reason was that the scrub radius had been reduced too much. See also the logic below re wide tyres
The effect of wide tyres. On a flat surface the centre of the contact patch will be in the centre of the wheel. However most UK roads are un-even and so it is possible for the main contact point to be either in or outboard of the tyre centre line – what i have christened contact patch migration This of course will change the scrub radius; I reckon that in extreme cases it is possible for one wheel to have a positive scrub radius and the other negative. This is why modern cars with wider tyres are so susceptible to tramlining and wandering. My own view is that running some camber – 2 degrees as a norm for road use, will reduce this effect by biasing the load to one side of the tyre; albeit the toe-in/out setting may need to be changed. The only time this will create a wear problem is possibly when the bulk of driving is on motorways, and there are virtually no cornering forces.
Bump steer. Partly because of compromises in design and partly to make cars user friendly, when the front wheels go up and down the toe settings change. In general they go to toe-out on bump – which is the same for the outside wheel in roll – this is meant to make the responses less twitchy. Racing cars do everything possible to avoid it. Lotus handing packages have been known to re-design the steering arms so as to reduce it. Clio settings not known.
Ackermann angle is a geometrical way of arranging the steering arms so that when the wheels are turned, the inside wheel turns more to allow for the smaller radius turn. This is most apparent on full lock. There are lots of opinions as to what is best for handling purposes. My own view is that in most normal conditions the amount of lock applied is so small as to render Ackermann irrelevant. In racing cars I have seen normal Ackermann used, none or even negative Ackermann. There did not seem to be any very clear conclusion – with better simulations things will have moved on in the last five years, but I am not aware of any particular conclusion. The whole discussion is further distorted by the different slip angles side to side and front to rear. I ignore it!
I am sure that I will think of some more points and will do an update in due course. Also someone will have some other facts to add to the story so they should be incorporated to retain some balance.
belting post - keep up the good work. One day I will actually know what it means 
Camber and Toe in/out is where my knowledge ends, never even heard of KPI in that context or Slip angle.... much reading and testing to do next year when the track looms again me thinks...
Cheers again George.
Camber and Toe in/out is where my knowledge ends, never even heard of KPI in that context or Slip angle.... much reading and testing to do next year when the track looms again me thinks...
Cheers again George.
George i've been pondering this thread all weekend and realised that although i nodded and asumed i knew what you were talking about it might as well have been marsian that you were talking, your last thread has begun to clear the fog. thanks
what a great summary
you are so right re RWD its just so easy in comparison
getting rid of the bump steer on road cars makes a great difference on braking and turning - but its a real faff with current track rod end design and steering rack placement
running less than 20mins toe out when you are using 2 deg neg camber helps with the nervousness and tramlining some people get for reasons you've explained
no good news from Sachs re damper rebuilds, they claim they aren't rebuildable - this is of course complete rubbish, I suspect I'm talking to the wrong person, will update everyone when I get the definitive answer
you are so right re RWD its just so easy in comparison
getting rid of the bump steer on road cars makes a great difference on braking and turning - but its a real faff with current track rod end design and steering rack placement
running less than 20mins toe out when you are using 2 deg neg camber helps with the nervousness and tramlining some people get for reasons you've explained
no good news from Sachs re damper rebuilds, they claim they aren't rebuildable - this is of course complete rubbish, I suspect I'm talking to the wrong person, will update everyone when I get the definitive answer
donkey said:
LMAO - who the hell were you talking to? they're race spec dampers, they have to be rebuildable!