Handling Corrections

I’m a long way off from tuning my suspension, just trying to calculate my best guess while collecting parts. Over on Clark’s Garage under “Handling Corrections”, it says to remove front weight or add rear weight to correct understeer. That seems exactly backwards to me. Is that right or wrong?

http://www.clarks-garage.com/shop-manual/susp-15.htm

I fixed my under steer by adding front camber. I am not sure about the weight.

It looks like he basically copied a few small sub-sets of the information contained in the book "Tune to Win" by Carroll Smith for that website.  I know that chart looked familiar, but seemed to be missing a few things.  The book was out of print for a long time, and used copies were getting hard to find, but it looks like it's back in print again.  It's a very good starting reference for vehicle dynamics, with lots of usable information.

Here's a link to the book: http://www.amazon.com/Tune-Win-science-development-tuning/dp/0879380713

The answer is .. it depends.  First, is the understeer caused by a vehicle set-up limitation, set-up error or by a driving technique problem?  Is the problem a corner entry issue or a corner exit issue?  How are the dampers, bushings, and tire pressures (contact patches) & temperatures?

To really get the best result when trying to correct an understeer issue, you need to figure out exactly what kind of understeer problem you are having, and why you are having that problem.  While "rule-of-thumb" solutions may help, if you don't solve the root issue, you may fix one problem and introduce a new one all at the same time.

Buy and read the book.  You will probably save more than it costs in reduced tire wear and money spent on parts that don't solve the actual problem.  Also get the book "Drive to Win".  It's a really good primer on how to hustle a car around the track the right way.  When fixing vehicle dynamics problems, it's always better to improve the driver's performance and knowledge before throwing money at parts that may not be needed.

I'm not saying there isn't a real problem.  What I am saying is that you will likely get much more benefit from money spent by first optimizing driver skills, technique, and car set-up knowledge when approaching the problem.

erioshi wrote:The answer is .. it depends.  

I disagree, adding or removing weight from one end of a car is always adding or removing weight. I can think of no variables.

erioshi wrote:To really get the best result when trying to correct an understeer issue, you need to figure out exactly what kind of understeer problem you are having, and why you are having that problem.  While "rule-of-thumb" solutions may help, if you don't solve the root issue, you may fix one problem and introduce a new one all at the same time.

This is exactly why I asked if removing front weight will improve understeer, which I am 99% sure is wrong as understeer is caused by a lack of traction at the front tires. I was asking for confirmation on this one issue to make sure I wasn’t smoking too much dope in the ‘70s.

Based on how I want my car, I suspect it will weigh in at a front to rear ratio of around 45/55 rather than a preferred 50/50, which I am reasonably sure will give it an inherent tendency to push. Adding weight to the front is not an option I'm willing to do. My goal is to set the car up as accurately as possible in the initial parts purchases based on educated predictions rather than WAGs and limit the number of parts I buy twice.

Although I appreciate the link to the book, I doubt I’ll  purchase a booked identified as the source of my indecision.  Unless it or someone explains how removing front weight corrects understeer.

Here is the link to a good Rennlist article, take a look.

http://rennlist.com/forums/944-turbo-and-turbo-s-forum/838719-oversteer-and-tire-pressure.html

The link posted above by pormgb also recommends reducing weight in the front end as one possible way to correct understeer.  That will help if the outside front tire is being overloaded via weight transfer.  Other possibilities are stiffening the back end (which can reduce rear traction .. again, depending), softening the front springs and/or sway bar (if the issue is too much roll resistance in the front end), and adjusting tire pressures.  Not every option will result in the same level of reduction in understeer, because the different options each tend to be tailored towards solving specific handling problems.

And yes, the book will explain how removing weight from the front of a car can reduce understeer.  In fact, there is an entire 8.5" x 11" page covered in a chart filled in with typical, book-sized print that covers specific solutions for oversteer, understeer and other handling issues .. and suggests the most effective options based on the root cause.  The book is absolutely essential reading if you are truly serious about going around corners as fast as you possibly can.

Here's another example of just how finely grained some of this suspension tuning stuff can be.  It's a long story, but I think it illustrates the benefits understanding what to change and why.  And how even the right change can create unexpected problems.

I was asked by a friend to help him with the suspension set-up of his 2nd gen MR2 time attack car.  The car was basically alright, was using the tires correctly, didn't have any really bad habits (we had already done quite a bit of testing and development with the suspension components, alignment, damper and ride height settings, and tire sizing and compounds), but he always seemed to be just about .2 to .3 behind a well set-up Miata in his class on this track.

I drove his car, and to me it felt like there was just a very tiny bit of undrsteer from mid-corner through corner exit.  While it felt predictable (safe?), I felt that it reduced my ability to apply as much throttle as should be possible with the car's current suspension and tires.

I decreased the front tire pressures 1/2 lb each before the next session, and then took the car back out.  On a roughly three mile track I had made the car faster by about about .7 seconds a lap.  He was overjoyed.

On the next session he went out with my new tire pressures.  His time lap times actually dropped from where he started by about .2 seconds per lap.  To him the car felt too twitchy, and he wasn't as comfortable near the limits of the tires.  I had raised the car's corner speeds, but in doing so pushed the rear tires closer to their limit of traction.

When we reviewed the in-car video, it was obvious that I was making way more tiny (like 1" of steering wheel movement), mid-corner adjustments than he did to maximize my corner exit speeds and keep the car from going into an oversteer condition.  To me, that was comfortable, but for him, it was not.  For the next session we bumped the front tires up 1/4 lb each.  I was slower than with the 1/2 lb adjustment, but he was about where he started.  To him the car felt less stable, but he was able to use more throttle.

The next day, with the 1/4 lb adjustment to the front tires he finally managed to out lap his nemesis of the weekend, that Miata.  But to do it required three things.  The afore mentioned 1/4 lb reduction in front tire pressures, a change in his driving style, and him to working hard to drive outside of his normal comfort zone while he learned the car's new behavior and became comfortable with it.

And that is just tuning tires for one track on one weekend.  Imagine what we we went through setting the car up from scratch when starting with a worn out stock suspension.  It was a lot of work, a lot of parts, and required careful experimentation to sort out exactly what was happening every time he ran into a problem with the car's handling.  For what it's worth, I believe that car still holds a few class based track records .. including the one I set on the weekend described above.

Let me clarify my question. I’m not asking how to correct understeer, I’ve got that covered. I want to confirm that a car that is permanently light in the front end will have a tendency to understeer.

The first chart in the link pormgb posted is the same chart I linked too and the source of my skepticism.  Thanks for the link BTW! Another chart further in the thread suggests lowering the front end which will add a slight amount of weight to the front, that is correct and an adjustment in the opposite direction.

erioshi suggests removing weight to correct an overloaded tire, an oversteer problem and also an adjustment in the opposite direction. See what I’m getting at?

After a lot of thought on the subject I’ve decided that line in the chart is plain wrong and will continue in my original direction, after missing out on the sway bar I was going to buy. I basically allowed some one promoting bad information to inject doubt in my mind.