Advice on curing understeer

[kosingcurry]

Hi all,

I have just joined this forum, seeking advice for my new car. It's a bike engined single seater, originally built as a class 8 autograss car but converted latterly to a car for use on tarmac sprints.

It runs an 1100cc blackbird engine (rear engined).

These have been used in the past for such events as tarmac sprints, relatively succesfully, but they always tend to suffer with understeer.

This I would like to cure. I always raced 911s succesfully, so in my head this should be a similar beast to drive.

Well.......this Sunday I had my second event in the car (on the Isle of Wight), and although we did pretty well, it was struggling massively with front end grip.

So far, since getting the car I have changed the pedal box and steering box setup as this was all very crued, which now allows for better throttle use (very difficult becuase it's a bike enge so very revvy).

I have just put 500lb springs on the rear to attempt to cure it 'squating'. The car is approx 350kg with 180bhp rwd. Running Med compound tyres on the rear and soft on the front.

This video is from the event, which shows a very clear view of what's going on with the front end.

https://www.youtube.com/watch?v=s4Y2r5pAUbg

I would like some advice from you guys in the know as to how best cure the understeer. Currently I'm considering ballasting the front end, but not sure if is the correct route to go.

I'm running -3 degree front camber and 1.5 degree tow out

Any help would be much appreciated.

Will

[Richard]

That looks like fun!

Is this your car at 6:08 in the video below?

Direct link to the right bit - https://youtu.be/WW1S6NCAqeU?t=6m8s

[Jolle]

I don't know how to eliminate your understear, but bare in mind (I've driven different types of 911 as well in anger). a 911 is something special... it's prob the only road car that will always turn in, even if your speed is 20km/u too fast, only the backend will go on straight. I think thats one of the most fun expects of a 911, you have to really know your entering speed, a few kp/h too fast and you are facing the wrong direction instead of (with a M3 or AMG for instance), just schrobbing to the outside of the corner.

I presume you've loosen up the rear roll bars already?

[kosingcurry]

Richard that's the car yes.

Jolle, I've race a 911 for years succesfully now. When running a very light 911, you will encounter understeer occasionally upon turn in, particularly on hillclimbs and sprints when the tyres aren't hot but this can be balanced pretty successfully by having different levels of fuel in the tank up front.

I don't have this option with my car obviously.

In terms of rear ARB, why would you want to slacken this off? This would make the rear roll more and therefore surely lift the inside front wheel.

My whole issue is around weight transfer being over the rear when accelerating.

Thoughts?

[Jolle]

from memory (but it has been years and years ago that I did stuff like that), by loosen the rear roll bar, you put extra weight on the outside front wheel at turn in.

But I could be completely wrong...

I miss the tricky 911's though...

[Jersey Tom]

Looks fun!

The typical basic big hitters I'd say would be (a) stiffen rear / soften front, (b) move the CG rearward, (c) work with the diff.

I'm curious what kind of diff the thing has in it. If it's a spool that's a pretty big one!

[Advino116]

I have a few suggestions, but I'm not sure if any of them will work, having no real experience tuning suspensions, just a bit of theory from some past university modules (which TBH are a bit hazy now)

(i) Stiffen up the rear. could do by fitting stiffer springs or fitting an anti-roll bar.
(ii) Softening the front, but I dare say the front is soft enough already
(iii) Lowering front tyre pressure, but not too much.
(iv) install wings/air dam in front, but they might do more harm than good...

[AndrisV]

I'd try a little bit more caster + narrower back or wider front. Spacer fitting time And yes, as mentioned before, is that a spool dif (no diff) or something is there at the back?

[George-Jung]

Do you know your current weight distribution?
When on throttle the front lifts up pretty much, therefor I think you loose some 'wheel pressure' on the asphalt.. and therefor having the understeer..

Would it help if you stiffen up the front damper out movement, so the front doesn't lift up that quickly?

[Tim.Wright]

Yea I think we need to understand a few things first:
Mass and mass distribution Tyre sizes front/rear
Differential type + setting
Static toe + camber

Also if you have it:
Ackermann
Caster
KPI

One aspect could be that if you have a very rearward weight bias, your front tyres are simply not coming up to temperature.

[garygph]

Lots of good advice above from the likes of Tim etc. Just thought I would add some thoughts and it would be interesting to hear the views of our more esteemed members here.
Firstly is "normal" tuning changes like stiffening or softening one end of a car makes no discernable difference at all then there is normally a massive overiding problem like a seized shock, fouling suspension linkage etc. I am guessing that in this case you have a wide track to short wheelbase ratio vehicle with a locked diff(guessing). This is a massive understeer recipe and the only way I know how to help this one is by setting it up like a kart and not a car. That is getiing the load trasfer to happen predominantely on the rear and not the front (typical rear wheel drive scenario). This will tend to lift a REAR wheel and not the front. It is the only way to get a kart to not understeer. On the driving side I drove a Shelby Can Am sports car where you had to finish braking and maybe just touch the throttle a little before heavy turn in otherwise..understeer. However I do not think that would be the solution here.

So either attend to the diff or setup as a kart. Besides that I think your front has a very big camber change going on as well as general suspension geometry problems, but still think they are not the absolute cause of your issues.

Just my 2 pennies worth and maybe it is not even worth that

[Phil]

Didn't read all the replies so far, but if you want more 'oversteer', you need more front-end grip. Front-end grip could be either achieved by going wider tyres or, as I have done, more camber. This has made my formerly rather understeer/neutral car to a more neutral/oversteer behavior.

[Facts Only]

For sprints a touch of Toe Out on the front will help on corner turn in, you'll trade some high speed stability though.

[olefud]

For sprints a touch of Toe Out on the front will help on corner turn in, you'll trade some high speed stability though.

Pro-Ackerman may get you the turn in without the hunting during braking.

[Silent Storm]

Try lowering front tyre pressure and increasing at rear.
Increasing rear width might help.
I don't know if this will work but try to setup it for oversteer. A bit of toe out at rear or negative camber at rear.
Lighter front wheels can also give you better turn in but might not be noticeable.

It would really help of you give more information