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MG MGF Technical - Toe vs Understeer

Hi,

I am decided to get my F aligned. Iīm thinking of getting a little toe-in to solve understeer.

Is it better 0 deg. 5 min.
or 0 deg. 10 min toe in?

I dislike understeer a lot, but I donīt want to do a 180š spin everytime I negociate a fast bend! What about motorway stability, is it worse?

Thanks a lot.

Valter.

Valter

Toe-in at the front makes a car more stable in a straight line, and less keen to turn in to a corner. Toe-out makes the steering sharper at the expense of reduced straight-line stability. Neither has a great effect on the car's behaviour on the limit, so toe-out won't cure an inherently understeering car (nor will it make it likely to spin) but it will improve the initial response to steering inputs.

Mike
Mike Bees

.. agree with Mike.

Toe OUT (- angle) will also lead to tyre wear at the inner side of the front wheels.

Suggest to check Rob Bells webside on further explanations. Mine is to confusing :)

Dieter
Dieter Koennecke

Thanks for the explanation, but isnīt it the other way around? Isn't it Toe-in (front wheels poiting to the middle of the car) that gives a sharper steering??

Toe-out=more stability in a straight line, but more tyre wear, that's right, isnīt it?!

Cheers,

Valter.

Valter Fernandes

Valter,

I've got 5mins toe-in on the front (back is standard). I choose this configuration as I had suffered inner tyre wear on numerous occasions. With toe-in, straight line stability at speed is much better. None of the previous lightness when travelling 140km/h and above (only in Germany officer). When I inquired on this BBS about the changes to the dynamics of the car, the general concensus was that it would lead to a greater likelyhood to oversteer. I'm no performance driver but everyone assured me then that in normal fast pace driving, this wouldn't be a problem. That is, the car does not become dangerous. In fact, some of the guys prefered it when on the limit as the understeer had been minimised.

David
p.s. oh, and it did stop the inner tyre wear - thanks guys for the advice!!!
David

I actually do use toe-in at the front, and yes, it does improve stability and turn-in, as well as sharpening the steering response.

The reason for using this is to compensate for the geometric and kinemetric induced toe-out of the loaded outside front wheel: when the suspension is compressed, it tends to toe-out. Similarly, when a large load is placed on the wheel, the tendency is for the reaction of the wheel to 'push' the wheel rear-ward, causing it to pivot around the steering axis, and toe-out. By dialing in some toe-in to the static settings, one doesn't remove these tendency, but does reduce the ultimate toe-out angle of that loaded wheel. What does this mean for understeer? On the loaded outside front wheel, toe-out counter-acts the steering movement which is in the other direction (I guess you could say that by turning into the corner, you are causing the outside loaded wheel to 'toe-in'). So, by setting the front geometry to toe-in, you are enabling the front outside loaded wheel to turn in more into the direction you are turning the steering wheel.
Ultimate understeer probably isn't really effected, but the perception is that the car steers more keenly, and that understeer is resisted for longer.

I run 0 degrees 5 minutes toe-in all round (using rear tie-bar bush compliance washers) and find this to be a good compromise.

Back to the original question, which is best 5 minutes or 10 minutes toe-in, then my preference is for 5 minutes, as this angle shouldn't be too harsh on the outer tyre edge under severe track use. Alan uses 10 minutes toe-in and is delighted with the effect on his road car. It suits his taste- whether it'll suit yours I do not know- try it and see if you can afford to get your car tracked more than once (not too expensive, so a possibility). It's fun to experiment like this, so long as you know a little about the consequences of doing this.

Altering the rear geometry is a different ball game- keeping it standard is a VERY good idea unless you have upgraded the suspension bushes (like Tony).

HTH- and the explainations are understandable!!!!

Rob
(Note to myself, must get around to finishing the geometry explanation page soon!- some already there follow the tyre fast link on my web site- http://www.mgf.4mg.com )
Rob Bell

I like the explanation Rob, i think i had got it just about figured out in my head, but its nice to see it explained well like that.

The toe in setting sure gives an improvement. another thing to consider Valter is to upgrade the tyres. If you change to S02s (i assume S03s are as good as S02s will be hard to get) and use 195s instead of 185s (if you have 15" wheels) on the front and change the tracking to toe in, then understeer pretty much becomes a thing of the past. With these tyres and these settings the car starts to handle how you imagine a small mid engine car should. A lot more neutral, a good turn in with little/no understeer and oversteer on the limit (that is controlable). The car really is a joy to drive like this IMO and it always amazes me how easy it is to put the car just where you want it.

Also if after getting the tracking how you want it you are still worried about motorway stability (which shouldn't really be a problem with toe in) you can invest in a splitter at the front, which reduces front end lift nicely at speed. (and looks the nuts IMO)
Matt

>>>>Altering the rear geometry is a different ball game - keeping it standard is a VERY good idea

Unless your new F came with a 'standard' 5' toe-in on the R rear and 37' toe-out on the L rear, like mine!

8-C
David Bainbridge

So did the car like turning right then. atleast you can get round roundabouts easily
Matt

Yes, although they didn't charge any extra for what I like to call "roundabout autopilot".

Problem now solved - see my 'my MGF has a club foot' thread.
David Bainbridge

Yeah, i saw it.
unfortunatly, my car ended up pretty much like that, very quickly aswell. a combination of london pot holes and repeatedly hammering the kerb at a track day in anglesea
Matt

As Rob said I use 10 minutes toe in each wheel at the front and love the feel of the car. What he didn't say was that I also run the back wheels parallel, with the compliance bushes, on standard 15" wheels and tyre sizes with Yokohama A520's fitted. I find this combination makes the car much more stable in a straight line but it also reacts much faster to any steering input made. Although I haven't had my car on the track even with these settings the car when pushed really hard will still try to understeer. Fitting tyres anything larger than the 185's on the front might well change this.

Personally I find this sensitivity to every small steering input (and every large or small blemish in the road surface as well) exactly what I want in a small sports car. But if I did large mileages on Mways and fast major roads every day I may well find this setup quite tiring and much too twitchy. But regardless of anything else if you haven't already replaced the OE Gyear NCT3's with something better do it first. The difference top quality tyres make is phenomenal. And there is plenty of input on this BBS as to which tyres to change to. Any of them will make you wonder why you haven't done it before.

Alan
Alan

Hi,

Alan, I know it's stupid but I've got the standard tyres on 15īī alloys... But I don't want to spend money just yet, as I bought this month the 57i kit.

I've raced go-karts and I used two diferent setups: one that gives you a lot of fun and the other that is fast on the clock. The first one I don't want it for my F, as it oversteers a lot even entering corners, very difficult in an everyday use on public roads! (You spent more time turning left than turning right, in a right corner...).
The other one is what I want for my F, that is: very sharp steering when entering a corner, without a single hesitation from the front, neutral during the whole corner and ultimate oversteer when exiting!
I'll try the 10 minutes, keeping the rear standard.

Thanks a lot Rob for the long text, very apreciated!

Cheers, Valter.


Valter Fernandes

As far as i can remember the only settings that have resulted the 'fun' rather than fast handling have been due to rear settings. parralel or toe out on the rear and i think people have found on tracks that like this the rear is a little to happy to step out of line and hence slows you down. and more importantly in the F increases your chance of spinning, cos it doesn't like to slide like and MX5.
Matt

Just to confirm what Matt said, I found that having parrallel rear was huge fun, you really felt that the car was adjustable on the throttle, BUT the rear was way too loose on the track. Having toe-in ties down the rear much more.

Not sure I'd recommend it for the road after Dieter's unfortunate experience.

And as Alan says, it really pays dividends to buy the best tyres that you can afford: for tyre recommendations from BBS experiences, follow the 'Tyre' fast link on http://www.mgf.4mg.com

Rob
Rob Bell


Valter / all,

Dunno if it'll help, but I'm now running with parallel all round and I like it lots. Safe and fast.

You do have to work more to start the corner, but there's no drastic oversteer when going at full tilt round a corner.

Have tried toe in at the front and parallel at the rear.
Great 'fun' but so so slow when sprinting - you spend all your time coming out of corners sideways.
Hated it.

It may be that toe in all round is what I should try, but until I have the time to try it properly I've changed back to parallel all round. Safe and fast.

nb. I have S02s all round with 195R15 and 205R25. Also have the compliance washers fitted and lowered via knuckles. After talking with Techspeed, it was they who pointed (hoho!) me in the direction of parallel all round.

Hope this helps,
Paul.
Paul Nothard

Interesting: my negative impression of a parrallel rear was based on having the front toe-in at the time...

5 minutes toe-in all round = good
parrallel all round = good
5 min toe-in front, parrallel rear = bad

Makes some sense I think.
Rob Bell

Valter - "Toe-in (front wheels poiting to the middle of the car) that gives a sharper steering??

Toe-out=more stability in a straight line, but more tyre wear, that's right, isnīt it?!"

No, it's the other way around. Toe-in gives stability, toe-out makes the steering more lively at the expense of straight line stability.

Take a look at: http://gallery.uunet.be/heremanss/c3/

There are some other good resources on the web wrt the effects of toe, camber etc - if only I could remember where...

Mike
Mike Bees

try also http://www.familycar.com/alignment.htm
that reminds me my automotive classes @ university !
Fabrice

Hi Fabrice,
that one is great :) why didn't you mention it before :)

Rob, that one covers all the missing vocabulary we didn't get two years ago :)
I'll rework the suspension crap at my site ASAP.

Related to the lively steering effects I would like to notice that it's really a huge difference between a standard weak rear suspension and a uprated one ( added PU-bushes or compliance washer)

Made my own poor experience the other year in wet conditions !! :( with parallel all around (slight Toe IN at the front)

Dieter
Dieter Koennecke

Hi.

Thanks a lot Mike, but I'm starting to get confused. What you said and what is written on the link provided is exactly the opposite to Rob's explanation:
"I actually do use toe-in at the front, and yes, it does improve stability and turn-in, as well as sharpening the steering response." Where is the error?

There's a great book about this things: "Race car vehicle dynamics", but I don't remember where I kept it...

Cheers, Valter.

Valter Fernandes

Rob's agreeing that toe-in improves stability, but I can't explain why he finds that it improves the turn-in & response.

Mike
Mike Bees

I am going to need to return to my text book on this to explain the apparant dicotomy in what we are saying and what we've found.

I think that my simple explanation holds in the context of the MGF with undriven front wheels.

Been thinking about this, and this is my first thought:
If one looks at the bump-steer data that John Englaro posted on this board last year, as the suspension compresses, the wheels toe-out. With 20 mm of suspension compression, one sees 15 minutes of toe-out. Given that the standard front static figures are toe-out (10 minutes) makes me wonder if the total toe-out angle changes beyond what is desirable when the outside front wheel becomes loaded and the suspension compressed? Bare in mind too that standard suspension systems are rather soft, so in cornering could easily result in this kind of toe-angle change. BTW, these figures don't take into account of the toe-angle changes that result from suspension bush-compliance.

I need to read that URL resource again Mike, and compare it with the books I have- but there is a surprising amount of experience that supports the empirical observations of improved turn-in with the toe-in angles on the MGF. I would not recommend these angles for an Elise or Caterham...!

Rob
Rob Bell

I look forward to hearing about your findings in "Race car vehicle dynamics" Valter.

I've just read through the URL that Mike provided- nice site BTW.

I think that there are two different states being described here: inital steering response where there has been no weight transfer to the outside front wheel, and slightly later then the car starts to roll into the corner.

The initial state relies on the grip being generated by the inside front tyre- its turning angle is greater than the outside front. The centropedal reaction of the inside wheel will outweigh that of the outer, and hence initial turn-in would be greater.

So my challange to anyone would be: based on this wouldn't the same effect be mediated by an equivalent toe-in angle?
Rob Bell

Rob,

I finally found the book. Next follows a synopsis of what I found:

Dropped Trhottle: "Toe-out with bump travel on the front end will reduce the steer angle on the outside front wheel. This will reduce the front side force and lower the amount of tuck in. In moderate turns, some toe-in on rebound at the rear will reduce the effect of dropped throttle as well."

Rough Road Cornering: "As an outside tire hits a bump the load on it increases. If the front wheel steers out as it moves up it will tend to compensate and the car will tend to hold a smooth path. On the rear, toe-in with bump has a similar effect"

General:"If the effect steers the front wheel out of the turn, it reduces the lateral force and is said to be understeer. On the rear, the opposite is true."

"Minimum static toe is desirable to reduce rolling resistance and unnecessary tire heating/wear. Alignment changes can be udes to overcome deficiences in the car. Example: Rear toe-out can be used to improve turn-in."

"In braking it is important that the tires are pointing straight ahead for best traction." - Therefor I think static toe-in will be better, as the load tends to toe-out the front wheels.

"Static toe only improves under limit cornering. In racing, if the tire is already saturated, the effect will be little."

I expected to found more stuff about this, but this is as good as it gets...

Cheers,

Valter.
Valter Fernandes

>>Therefor I think static toe-in will be better, as the load tends to toe-out the front wheels.<<

I agree, and it seems to concur with our experience with this toe-angle setting.

With your permission I'll use your quotes for the MGF suspension page?
Rob Bell

Certainly Rob!

Itīs a pleasure to help for the first time... since I learnt a lot already from you guys!

Cheers,

Valter.
Valter Fernandes

Cheers Valter

BTW I am learning stuff about the MGF (and cars in general) all the time- I doubt that I'll ever stop learning either!!! It's all part of the fun :o)
Rob Bell

This thread was discussed between 30/05/2001 and 05/06/2001

MG MGF Technical index

This thread is from the archive. The Live MG MGF Technical BBS is active now.