Modifying your suspension - part 2
10-14-2010, 02:01 PM
#1
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Modifying your suspension - part 2
This is a follow up article that wrote about our performance clinic. It covers some basic reasons why a modified car get harder to drive. All input is welcome.
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Performance Clinic – Part 2
By Larry Herman
In my last article I talked about how stiffening up your car and adding wider, stickier tires will increase its performance, at the cost of a narrower, harder to find operating window. Now, I would like to explain as straightforwardly as possible why that happens.
First let’s take a look at the tires. Street tires, especially ones that are not super-low profile, are designed to provide a very stable level of grip. The taller, softer sidewalls allow the tread to be very compliant, and compensate well for less than optimal camber. This is because the force (weight of the car) pushing the tire onto the road will help to keep the tread flat because the sidewalls do not have enough stiffness to overcome this. The tread compound is harder, and more thermally stable, so the tire can provide its maximum grip even at lower operating temperatures. R compounds, and even more so racing slicks, are just the opposite. They typically have much lower profile and have very stiff sidewalls. This requires the suspension to keep them as flat to the road as possible during cornering. Otherwise, if the tire (and wheel) leans somewhat, the sidewalls can lift up the inside of the tread, reducing contact patch, and grip will drop off rapidly. This places greater importance on suspension setup, and makes setting the right camber values for these tires more critical.
R compounds and racing tires also need heat in them for their softer compounds to provide maximum stick. There can be a dramatic difference between the grip of a cold slick vs one that is up to its typical operating temp of ~180 degrees F. As a driver, you need to make sure that you are driving hard enough to heat them up and have the correct pressures for these tires to work properly. Also, having your tire pressures off 2 or 3 lbs of pressure which may have minimal affect on a street tire will have dramatic impact on the grip level of a racing slick. All this tends to narrow the operating window of R compounds & slicks, and make that window harder to find.
Suspensions affect the operating window and drivability of your car to an even greater extent. Once again, the stock suspension, if well dampened, is a very compliant, very driver friendly system. With a little bit of additional camber dialed into it for the track, it will do a good job of maintaining predictable, constant grip and provide a very large operating window at the limit. Modified cars and full out racecars with stiffer springs and shocks, more radical camber, solid bushings and limited suspension movement will keep their outside tires flatter on the track under hard cornering. This will provide for greater grip when it happens, and this is a big reason that the operating window gets so narrow. Let me illustrate as to why this is so.
For argument’s sake, we will take the average 911 that weighs 3,000 lbs and has 600 lbs of that on each front tire, and 900 lbs on each back tire. Also, let’s assume that the stock 911 has front springs rated at 200 lbs/in. Simply put, it takes 200 lbs to compress that spring 1 inch, so at rest, the front springs are compressed 3 inches. Our modified car has 600 lbs/in, so its springs are compressed 1”, and the racecar has 1200 lbs/in so the race springs are compressed only ½ of an inch!! The difference in how these 3 cars will respond to bumps and dips in the track is staggeringly different.
If we take our 911 example, and under hard cornering shift 50% of its weight from the inside wheel to the outside wheel, we will have 900 lbs of weight on the outside spring. Our stock spring will be now be compressed 4.5”, our mod spring 1.5” and our race spring ¾ of an inch. Let’s see what happens when our car encounters a 1” dip right in the middle of our turn. Understand that as the tire becomes unloaded over this dip, the spring will extend and keep the tire down onto the track, and will maintain this until gravity pulls the “slower to respond” body down to its new level, re-exerting its original amount of pressure on the tire.
So what will happen? The stock car will instantly lose 200 lbs of pressure on the tire, but with 700 lbs or about 80% of the weight still there, the stock car will track smoothly over this dip. The modified car will lose 600 lbs of pressure (or about 66% of its grip) and will require some steering correction until grip is restored. The racecar has only ¾ of an inch compression on its outside spring, so it loses ALL grip momentarily. Imagine going from generating 1.2~1.5 G’s of side loading to zero for an instant? It will take a fast and large correction of the steering wheel to keep the car balanced and on course. This is what makes it so much harder to drive. It really requires an innate sense of exactly what the car is going to do, along with the experience to make the right corrections to be able to drive this car at its limit.
The other part of the equation is the setup of the car itself. With 3” of spring compression at rest, the stock shocks have plenty of time and movement to work with, and proper dampening of the suspension is fairly easy. Most quality shocks designed for performance applications would be suitable. With stiffer springs, time and movement decreases, and in the racecar example having spring movements of ¼” that occur in tenths of a second require adjustable racing shocks that are capable of controlling such short, quick movements. Obviously, the setup of these multi-adjustable shocks is a lot more critical. Now the track becomes a variable too, as the settings for a fairly smooth track can be stiffer and provide more stability than those for a rough track that will sacrifice overall grip for better control over the bumps and rough patches. All of this makes it harder to get your car dialed into its “sweet spot” where you can get the maximum performance out of it.
I hope that this article sheds more light on your understanding of what happens when you start to modify your car, and allow you to make the right choices in a progressive fashion as your skill and knowledge increase.
--------------------------------------------------------------------------
Performance Clinic – Part 2
By Larry Herman
In my last article I talked about how stiffening up your car and adding wider, stickier tires will increase its performance, at the cost of a narrower, harder to find operating window. Now, I would like to explain as straightforwardly as possible why that happens.
First let’s take a look at the tires. Street tires, especially ones that are not super-low profile, are designed to provide a very stable level of grip. The taller, softer sidewalls allow the tread to be very compliant, and compensate well for less than optimal camber. This is because the force (weight of the car) pushing the tire onto the road will help to keep the tread flat because the sidewalls do not have enough stiffness to overcome this. The tread compound is harder, and more thermally stable, so the tire can provide its maximum grip even at lower operating temperatures. R compounds, and even more so racing slicks, are just the opposite. They typically have much lower profile and have very stiff sidewalls. This requires the suspension to keep them as flat to the road as possible during cornering. Otherwise, if the tire (and wheel) leans somewhat, the sidewalls can lift up the inside of the tread, reducing contact patch, and grip will drop off rapidly. This places greater importance on suspension setup, and makes setting the right camber values for these tires more critical.
R compounds and racing tires also need heat in them for their softer compounds to provide maximum stick. There can be a dramatic difference between the grip of a cold slick vs one that is up to its typical operating temp of ~180 degrees F. As a driver, you need to make sure that you are driving hard enough to heat them up and have the correct pressures for these tires to work properly. Also, having your tire pressures off 2 or 3 lbs of pressure which may have minimal affect on a street tire will have dramatic impact on the grip level of a racing slick. All this tends to narrow the operating window of R compounds & slicks, and make that window harder to find.
Suspensions affect the operating window and drivability of your car to an even greater extent. Once again, the stock suspension, if well dampened, is a very compliant, very driver friendly system. With a little bit of additional camber dialed into it for the track, it will do a good job of maintaining predictable, constant grip and provide a very large operating window at the limit. Modified cars and full out racecars with stiffer springs and shocks, more radical camber, solid bushings and limited suspension movement will keep their outside tires flatter on the track under hard cornering. This will provide for greater grip when it happens, and this is a big reason that the operating window gets so narrow. Let me illustrate as to why this is so.
For argument’s sake, we will take the average 911 that weighs 3,000 lbs and has 600 lbs of that on each front tire, and 900 lbs on each back tire. Also, let’s assume that the stock 911 has front springs rated at 200 lbs/in. Simply put, it takes 200 lbs to compress that spring 1 inch, so at rest, the front springs are compressed 3 inches. Our modified car has 600 lbs/in, so its springs are compressed 1”, and the racecar has 1200 lbs/in so the race springs are compressed only ½ of an inch!! The difference in how these 3 cars will respond to bumps and dips in the track is staggeringly different.
If we take our 911 example, and under hard cornering shift 50% of its weight from the inside wheel to the outside wheel, we will have 900 lbs of weight on the outside spring. Our stock spring will be now be compressed 4.5”, our mod spring 1.5” and our race spring ¾ of an inch. Let’s see what happens when our car encounters a 1” dip right in the middle of our turn. Understand that as the tire becomes unloaded over this dip, the spring will extend and keep the tire down onto the track, and will maintain this until gravity pulls the “slower to respond” body down to its new level, re-exerting its original amount of pressure on the tire.
So what will happen? The stock car will instantly lose 200 lbs of pressure on the tire, but with 700 lbs or about 80% of the weight still there, the stock car will track smoothly over this dip. The modified car will lose 600 lbs of pressure (or about 66% of its grip) and will require some steering correction until grip is restored. The racecar has only ¾ of an inch compression on its outside spring, so it loses ALL grip momentarily. Imagine going from generating 1.2~1.5 G’s of side loading to zero for an instant? It will take a fast and large correction of the steering wheel to keep the car balanced and on course. This is what makes it so much harder to drive. It really requires an innate sense of exactly what the car is going to do, along with the experience to make the right corrections to be able to drive this car at its limit.
The other part of the equation is the setup of the car itself. With 3” of spring compression at rest, the stock shocks have plenty of time and movement to work with, and proper dampening of the suspension is fairly easy. Most quality shocks designed for performance applications would be suitable. With stiffer springs, time and movement decreases, and in the racecar example having spring movements of ¼” that occur in tenths of a second require adjustable racing shocks that are capable of controlling such short, quick movements. Obviously, the setup of these multi-adjustable shocks is a lot more critical. Now the track becomes a variable too, as the settings for a fairly smooth track can be stiffer and provide more stability than those for a rough track that will sacrifice overall grip for better control over the bumps and rough patches. All of this makes it harder to get your car dialed into its “sweet spot” where you can get the maximum performance out of it.
I hope that this article sheds more light on your understanding of what happens when you start to modify your car, and allow you to make the right choices in a progressive fashion as your skill and knowledge increase.
__________________
Larry Herman
2016 Ford Transit Connect Titanium LWB
2018 Tesla Model 3 - Electricity can be fun!
Retired Club Racer & National PCA Instructor
Past Flames:
1994 RS America Club Racer
2004 GT3 Track Car
1984 911 Carrera Club Racer
1974 914/4 2.0 Track Car
CLICK HERE to see some of my ancient racing videos.
Larry Herman
2016 Ford Transit Connect Titanium LWB
2018 Tesla Model 3 - Electricity can be fun!
Retired Club Racer & National PCA Instructor
Past Flames:
1994 RS America Club Racer
2004 GT3 Track Car
1984 911 Carrera Club Racer
1974 914/4 2.0 Track Car
CLICK HERE to see some of my ancient racing videos.
10-14-2010, 04:22 PM
#3
Rennlist Member
IMHO.
and to complicate situation even more - plus to this main 700lbs spring there is also a 150lbs tender spring.
10-14-2010, 04:34 PM
#4
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seems like you do not take to the account that tire itself also acts like a spring/damper system, so over a dip/bump tire itself will extend first and only after that spring will react and with 700+ lbs spring rates one essentially 'rides' mostly tires stiffness actually. so to loose all grip takes a bit more than just a linear amount of a spring compression.
IMHO.
and to complicate situation even more - plus to this main 700lbs spring there is also a 150lbs tender spring.
IMHO.
and to complicate situation even more - plus to this main 700lbs spring there is also a 150lbs tender spring.
10-14-2010, 05:05 PM
#5
Racer
I've been waiting on part #2 and it was well worth the wait. Thanks Larry. It allowed me a good visual of what's happening with my car and why my car behaves the way it does. 
I'm very much convinced that the better the nut behind the wheel becomes, the better the car handling characteristics and ultimately the better a driver I become.
I'm very much convinced that the better the nut behind the wheel becomes, the better the car handling characteristics and ultimately the better a driver I become.
10-14-2010, 05:06 PM
#6
Rennlist Member
Thanks Larry, that helps to understand why I had such a hard time transitioning from a slightly modified 993 to a track prepped 911 in one jump. Rough sections of the track being particularly challenging. Accuracy rewarded and mistakes punished mercilessly.
10-14-2010, 05:36 PM
#7
Good explanations.
Personnally, I prefer softer springs than most race cars drivers will use because you can have a better grip for a longer time this way. I often see very fast cars in qualification that can't keep their pace for more than 30 minutes into a race because the tires are doing all the work with stiffer set-up... and that's when you can do your move to pass them !
Stiffer set-up is probably better for sprint races... and if you have the budget for new tires for every race !
Personnally, I prefer softer springs than most race cars drivers will use because you can have a better grip for a longer time this way. I often see very fast cars in qualification that can't keep their pace for more than 30 minutes into a race because the tires are doing all the work with stiffer set-up... and that's when you can do your move to pass them !
Stiffer set-up is probably better for sprint races... and if you have the budget for new tires for every race !
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10-14-2010, 09:46 PM
#9
Three Wheelin'
I have only skimmed this stuff-- will review in detail later. Here are some thoughts from 50k feet:
1. Great points and perspective.
2. Whether it makes sense to modify a particular car pretty early in your experience with performance driving involves a lot of considerations as I am sure all will agree. I started driving on the track in a stock 1984 944 in '87 at LRP. Street tires. No Ax, no karting. During that first experience I realized a few things about stock 944s, and as my instructor took me for a ride in her 911 race car... about 911s. One thing I still remember was how much easier it was for her to rotate the car at Big Bend. When I started to AX in '88 I pretty quickly decided that a small number of suspension changes were necessary to make the 944 behave more admirably. Slightly stiffer front springs (Weltmeister), larger swaybars (from a 951), and reduction in rear ride height, some Konis and a decent alignment transformed the car pretty quickly. Now as I made those changes, I was acquiring skills all along, but with what I felt was an appropriately more responsive car. The changes were a kind of recipe that I picked up on from the writings of Bruce Anderson, Jim Pasha and from local advice in the PCA. Days before internet. These changes made a huge difference in my learning process very early. I also moved to sticky DOT tires (remember RE71 and Comp T/A R1) pretty early on so I would not destroy my street tires in AX. As I instructed others with 944 and 951 cars, many took similar approaches to make these kinds of changes early. So to me, whether to make changes early is extremely situation-dependent.
3. Upon cursory review, I think this information tends to assume slicks are radials. Bias ply slicks are a different animal and folks need to take that into account. I have run bias ply slicks for nearly 10 years, and in my area a number of fast GT cars run them-- we probably consume at least 40-50 sets a year with just our little group. In fact the preponderence of our cars run bias plys. They behave differently from radial slicks.
4. I like Matt's admonishment about diffs. We had a short discussion on these at the track last weekend. I run torque biasing diff, and Matt and I discussed whether an LSD would make a difference... we kind of reached an agreement about braking being one of the key areas of potential benefit. But then my car weighs only 1950 pounds and braking distances are pretty short already... so the amount of incremental benefit is perhaps negligible, depending on particular corners, tracks and situations. I may still try an LSD if/when budget allows. Now if I had one of these newer Porsches that was heavier (including a factory race car), I would have the spiffiest diff I could buy from GT.
Anyway, I am sure we will all agree that there are a lot of variables to consider, and I appreciate the perspectives here. Too often folks don't consider these kinds of things as they attempt to go faster.
1. Great points and perspective.
2. Whether it makes sense to modify a particular car pretty early in your experience with performance driving involves a lot of considerations as I am sure all will agree. I started driving on the track in a stock 1984 944 in '87 at LRP. Street tires. No Ax, no karting. During that first experience I realized a few things about stock 944s, and as my instructor took me for a ride in her 911 race car... about 911s. One thing I still remember was how much easier it was for her to rotate the car at Big Bend. When I started to AX in '88 I pretty quickly decided that a small number of suspension changes were necessary to make the 944 behave more admirably. Slightly stiffer front springs (Weltmeister), larger swaybars (from a 951), and reduction in rear ride height, some Konis and a decent alignment transformed the car pretty quickly. Now as I made those changes, I was acquiring skills all along, but with what I felt was an appropriately more responsive car. The changes were a kind of recipe that I picked up on from the writings of Bruce Anderson, Jim Pasha and from local advice in the PCA. Days before internet. These changes made a huge difference in my learning process very early. I also moved to sticky DOT tires (remember RE71 and Comp T/A R1) pretty early on so I would not destroy my street tires in AX. As I instructed others with 944 and 951 cars, many took similar approaches to make these kinds of changes early. So to me, whether to make changes early is extremely situation-dependent.
3. Upon cursory review, I think this information tends to assume slicks are radials. Bias ply slicks are a different animal and folks need to take that into account. I have run bias ply slicks for nearly 10 years, and in my area a number of fast GT cars run them-- we probably consume at least 40-50 sets a year with just our little group. In fact the preponderence of our cars run bias plys. They behave differently from radial slicks.
4. I like Matt's admonishment about diffs. We had a short discussion on these at the track last weekend. I run torque biasing diff, and Matt and I discussed whether an LSD would make a difference... we kind of reached an agreement about braking being one of the key areas of potential benefit. But then my car weighs only 1950 pounds and braking distances are pretty short already... so the amount of incremental benefit is perhaps negligible, depending on particular corners, tracks and situations. I may still try an LSD if/when budget allows. Now if I had one of these newer Porsches that was heavier (including a factory race car), I would have the spiffiest diff I could buy from GT.
Anyway, I am sure we will all agree that there are a lot of variables to consider, and I appreciate the perspectives here. Too often folks don't consider these kinds of things as they attempt to go faster.
10-14-2010, 11:09 PM
#11
Rennlist Member
Larry, just seeing this now. Nice work.
Professional Racing and Driving Coach
Professional Racing and Driving Coach
10-15-2010, 12:09 AM
#12
Rennlist Member
Just a passing thought to toss out though it doesn't address the complex ideas that may exist for more modern cars.
If I was to increase my torsion bar spring rate from 31mm rear/23mm front to 34mm rear/24mm front (I think that is the latest available) should I send my shocks back to Bilstein (or whoever) to have the valving changed?
I'm just a slow guy that is wondering so no secrets have to be protected...
If I was to increase my torsion bar spring rate from 31mm rear/23mm front to 34mm rear/24mm front (I think that is the latest available) should I send my shocks back to Bilstein (or whoever) to have the valving changed?
I'm just a slow guy that is wondering so no secrets have to be protected...
10-15-2010, 09:48 AM
#14
Rennlist Member
Professional Racing and Driving Coach