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Front and rear track width?
bowood14 - 29/1/14 at 01:00 PM

I am having some new front wishbones made by Phil at Talon, I want to know the best width to have the front should it be the same as the rear or wider I am running Sierra IRS at the moment the front is 20cm narrower overall. 10cm each side. What are peoples thoughts on how wide I should go
thanks Adrian

NigeEss - 29/1/14 at 04:32 PM

99% of people would not notice any difference in handling with a 10 mm difference.
What equal track does help to avoid though is catching the rear wheels on kerbs
having used the front wheel position to clip apexes.

adithorp - 29/1/14 at 04:42 PM

Invariably on kits, the front track is narrower than the rear. 10cm/side sounds a lot though somaybe a bit wider than that would be OK, but I wouldn't go wider than the rear. Only car I can think of ever seeing with wider front than rear was an old Fraser-Nash.

rodgling - 29/1/14 at 05:10 PM

My car is a bit wider at the front, doesn't really cause any issues AFAICT. Andy Bates who set it up commented on this being very unusual, but didn't seem overly concerned by it. Can't remember how much difference there is though.

You can always tweak it a bit by adding spacers, although 10cm spacers are probably a non-starter. 2.5cm is doable (I've done this at the rear to reduce the effect on my car) but in your case that does still leave you with a fair imbalance.

motorcycle_mayhem - 29/1/14 at 06:08 PM

My Westfield's lap times and handling were better with the 'wide track' front. 'Wide track' in this context meant that the track front and rear were rendered identical.
My advice therefore, worth absolutely nothing, would be to have the track widths F = R.

As has been mentioned, hitting the apex with an equitrack front will usually mean the rear is undisturbed when it passes the same point a little wider due to drift.

Helps when you're in a hurry to string the car in the paddock after a track rod bending crash too.

big_wasa - 29/1/14 at 06:11 PM

My rear axle was 4" wider than book so I made the front 4" wider than book. If you read the books it does make a difference to things like scrub, kpi and other things that I chose not to try and understand

Sam_68 - 29/1/14 at 07:06 PM

First off, can we be clear about whether we're talking cm (CENTImetres) or mm (MILLImetres)?

As Nige says, all other things being equal most people wouldn't notice the difference with a variation of 10mm, but 10cm would be a different kettle of spaniels altogether...

All other things being equal, increasing front track relative to rear will reduce understeer, but 10mm wouldn't be enough to notice, and there's probably no point worrying about such things unless you know the car is properly set up and the handling balance spot-on in the first place.

Be aware that there are other potential side effects, though:

Altering front track width using lengthened wishbones can influence bump steer characteristics (and at the very least you will need track rod extensions, of course) and change the position of the geometric roll centre (which in turn can influence oversteer/understeer balance because roll axis inclination affects diagonal weight transfer). How much and to what degree this will cause problems is difficult to say without doing a proper analysis, but the bigger the change, the bigger the likely side effects.

Altering the front track by introducing wheel spacers changes the kingpin offset, which has an influence on steering feel and self-centring.

For what it's worth (and at odds with Adithorp's 'invariably' ), genuine Caterhams are traditionally narrower at the front BUT 'wide track' Caterhams (ie. most of the current high-performance models) and (as Motorcyle_mayhem says) 'wide track' Westfields have equal track front and rear, and the Caterham CSR is 40mm wider at the front. There is no fundamental rule of geometry that dictates the track needs to be wider at one end or the other - you can juggle other factors like roll axis inclination and roll stiffness to make things balance - I think it has simply been a matter of adopting convenient dimensions dictated by steering rack, drive shaft and axle lengths, and perhaps just slavishly copying the 'classic' Caterham dimensions to some extent.

NigeEss - 29/1/14 at 08:09 PM

Oops, didn't read the OP properly !

My comment about kerbs is even more relavant then

drt - 29/1/14 at 08:16 PM

quote:
Originally posted by Sam_68
First off, can we be clear about whether we're talking cm (CENTImetres) or mm (MILLImetres)?

As Nige says, all other things being equal most people wouldn't notice the difference with a variation of 10mm, but 10cm would be a different kettle of spaniels altogether...

All other things being equal, increasing front track relative to rear will reduce understeer, but 10mm wouldn't be enough to notice, and there's probably no point worrying about such things unless you know the car is properly set up and the handling balance spot-on in the first place.

Be aware that there are other potential side effects, though:

Altering front track width using lengthened wishbones can influence bump steer characteristics (and at the very least you will need track rod extensions, of course) and change the position of the geometric roll centre (which in turn can influence oversteer/understeer balance because roll axis inclination affects diagonal weight transfer). How much and to what degree this will cause problems is difficult to say without doing a proper analysis, but the bigger the change, the bigger the likely side effects.

Altering the front track by introducing wheel spacers changes the kingpin offset, which has an influence on steering feel and self-centring.

For what it's worth (and at odds with Adithorp's 'invariably' ), genuine Caterhams are traditionally narrower at the front BUT 'wide track' Caterhams (ie. most of the current high-performance models) and (as Motorcyle_mayhem says) 'wide track' Westfields have equal track front and rear, and the Caterham CSR is 40mm wider at the front. There is no fundamental rule of geometry that dictates the track needs to be wider at one end or the other - you can juggle other factors like roll axis inclination and roll stiffness to make things balance - I think it has simply been a matter of adopting convenient dimensions dictated by steering rack, drive shaft and axle lengths, and perhaps just slavishly copying the 'classic' Caterham dimensions to some extent.


Spot on ^^
I only like to contribute that trackwidth has a correlation with wheelbase...
Various coefficients will have an impact on the dynamic vehicle model.
(FE look at the f1car WB/TW and then a rally car etc)

When talking about TW (theoretically keeping other variables constant) it changes your dynamic weight transfer,
thus increasing or decreasing the load on the outer(or inner) wheel...
This can alter the level of grip.

When looking at your car, it is best to think of 2 'grip' circles... front axle and rear axle.
A change in TW could increase your front grip circle so relatively killing understeer (depends on the corner state; entry,mid or exit) and promoting oversteer...

Advice?
You can never have enough set-up options...
design it, build it, brake it, upgrade it

Sam_68 - 29/1/14 at 08:59 PM

quote:
Originally posted by NigeEss
Oops, didn't read the OP properly !


I'm not sure that you're not reading the OP right, and the OP has written it wrong (if that makes sense?)!

A front track 200mm narrower than the rear is quite a dramatic and unusual difference. 50-75mm difference in track wouldn't raise my eyebrows, but 200?!

And just to be sure the OP is aware of the correct terminology, remember that track is measured from centreline of tyre to centreline - not across outside faces or anything. Can be particularly confusing if you're running different width tyres front and rear. We're all happily talking about track, where, to be fair, the OP has only spoken about width....

Sam_68 - 29/1/14 at 09:04 PM

quote:
Originally posted by big_wasa...it does make a difference to things like scrub, kpi and other things thatI chose not to try and understand


After 25 years trying to get my head round suspension design, I'm gradually coming to the conclusion that you've got exactly the right approach!

craig1410 - 29/1/14 at 09:34 PM

My rear track is standard Sierra width (57.8" off the top of my head) and the front is something like 56" so around 23mm narrower per side. Sorry about the mix of units but the Sierra track was specified in inches and were memorised as such.

This difference makes no practical difference and allows me to use standard Sierra driveshafts on the rear (de-dion back end) and allows me to use a standard escort rack on the front with track rod extensions to eliminate bump steer. A sierra rack body is too wide

So my advice is simple, avoid a radically different track width front to rear (ie. hopefully you meant mm not cm) and make sure that your component choices are compatible with your choice. This will mainly affect driveshafts, wishbones, steering rack and any extensions you might need. If you choose wisely then you may be able to avoid having to source custom driveshafts and/or a custom width steering rack. I should mention that my car is a 4" wider chassis to accommodate my V8 engine.

HTH,
Craig.

bowood14 - 30/1/14 at 12:38 AM

Just confirm as it said in my question it's centimeters not millimeters therefore overall 20cm or 200mm I think the overall opinion seems to be equal. Although I would add MK fronts are wider than rear

Sam_68 - 30/1/14 at 07:33 AM

quote:
Originally posted by bowood14I think the overall opinion seems to be equal.


But do be conscious, in that case, that what you are suggesting represents a substantial change to the car's geometry.

As stated above, the tendency will be to decrease understeer, which, by corollary, means that you will be increasing tendency to oversteer - not necessarily a welcome change, and one that you may wish to 'tune out' by other means.

You will also need to address the steering; 100mm steering arm extenders are quite large, so you may prefer to cut and weld sleeved extensions into the steering arms. You'll need to check and adjust bump steer when it's finished.

What it'll do to the front roll centre location, roll axis inclination and camber gain is anybody's guess, but then I suspect that design of the Locost geometry was more by spit and hope than any sort of proper analysis in the first place, so you've probably got 50/50 chance of making it better rather than worse!

big_wasa - 30/1/14 at 07:25 PM

But I did spend a fair amount of time tuning the bump steer by moving the rack around.

[Edited on 30/1/14 by big_wasa]

drt - 3/2/14 at 02:38 AM

suggestion,

As you're in the process anyway,
you could weld on an extra pair of top suspension brackets.
So positioned that the Wishb run parallel.
Dropping the RC on the ground. (infinite)
If you're running the Sierra's single trailing link than the rear RC will be there too.

(down side, springs/dampers/rollbars have to cope with everything...
plus side, springs/dampers/rollbars have to cope with everything )

Camber comp. will be non existing, but carry some extra static negative camber and will be fine.

Sam_68 - 9/2/14 at 01:30 PM

quote:
Originally posted by drt
...So positioned that the Wishb run parallel.
Dropping the RC on the ground. (infinite)
If you're running the Sierra's single trailing link than the rear RC will be there too.


A couple of points of order:

1) Parallel wishbones only give fixed, ground level roll centres so long as they remain parallel. In other words, they have to be parallel and equal length, or as soon as they move away from the static, parallel position, they will produce a virtual swing axle length and hence the RC starts to move.... obviously equal length wishbones are not practicable on a 'Locost'/'Seven' type chassis, so you're not going to achieve a fixed, ground level roll centre by this means.

In fact, you'll find that the lateral location of the roll centre can vary dramatically with this geometry (obviously, as soon as the suspension moves, your instantaneous centres go from infinity to merely many miles away then start to rapidly converge as the suspension moves further). How bad the vertical roll centre location is tends to depend on the difference in length of the wishbones.

2) The Sierra is a semi-trailing arm, not a pure trailing arm, so its roll centre isn't at ground level. To find the RC of a semi-trailing arm, you project the axis of the chassis pick ups on plan until they intersect the axle line, then on elevation project a line back from the intersection to the centre of the tyre contact patch. Do this for both sides, and where the lines from the instantaneous centres to the tyre contact patches intersect is where your roll centre is located.


[Edited on 9/2/14 by Sam_68]