Hey everybody, so I am here with Shaikh of Fat Cat Motorsports and you know when I was pulling apart the suspension on my M2, Shaikh also came over and took some measurements as well for a custom M2 suspension he's working on for another customer. And this customer is going to be a, sorry what?

And many more.

Yes, and many more. And this customer's going to be using that car for competition, right? Yep, street cars all across. Great. And so one of the things that we both did is we both measured the motion ratios. And this is something I learned, is that you really have to measure the actual stroke of the parts, and not necessarily just measure the points between geometries and calculating it. Like for example, when I was looking at the front, you know, I thought, okay, maybe you can just use some trig because the strut's at an angle, right? And then, you know, you can calculate, you know, if your wheel moves so much, then what is this leg of the triangle going to be? And doing that, the motion ratio was a little bit high to me. It was about .99, and then when you actually measured it, Shaikh, I believe you had lower, yeah, it was .95. So, you know, not a huge difference, but still a noticeable difference. And then also in the rear, for the motion ratio of the components back there, I was just measuring the camber arm and then where along the camber arm the spring and the damper sat. And like, for example, the damper, I measured a motion ratio of like 0.7. And then, Shaikh, when you actually did it, you know, we started with the wheel carrier at normal position and then raised it up in order to get that motion ratio, and yours came out to 0.76. So once again, you know, not a massive difference, but still a significant one.

About 8% or so.

Yeah.

8%, so that's noticeable.

Yeah, so what this has taught me really is, you know, doing just some rough measurements will get you ballpark. We're all within 10%, but if you're really going for the accuracy of it, then you really do need to measure the motion ratio by doing the stroke and measuring how much clearance you've gained or lost. Going from those motion ratios, well that leads us into the bump stops and how much can these wheels travel before you engage them. And looking at this wonderful data package that Sheikh provided me from all of his findings taking measurements here, In the front, we have 18 millimeters of wheel travel before your bump stops are engaged, so a hair under three quarters of an inch. And in the rear, there's actually about 33 millimeters of wheel travel, so more than an inch. Do you have any thoughts about how significant those numbers are?

Yeah, what's interesting is that you had mentioned when you looked at the ride height because the static heights are the same front and rear, right?

Yeah, on one side they were.

Okay, at least on the one side that we were measuring was the right side. But that being the case, you have... less wheel travel in the front before the bump stop starts to engage and that's common on a lot of high performance cars and most cars in general because they want to make sure that the car engages the front and starts to understeer rather than the rear engaging earlier and starting the car to oversteer. So it's the safe approach to suspension

tuning. And I think they also do that to give the front a supported feel. A lot of people, what is the common thing that people say about their new aftermarket suspension? Oh, it rides on rails. Why? Because it doesn't lean. And I think the bump stop tuning is a big part of that. Absolutely.

The bump stops are designed to be a secondary spring, and they do that job very well, especially the modern microcell bump stops, which BMW uses the foam, people say, but it's really microcellular polyurethane. But that does give you that supplemental spring rate, and then... other effects can come into play.

And one of the other really cool things that Shaikh did is, you know, I gave him the dampers, the springs, the bump stops, so he could bring it back to his shop here and do some more measurements on it. He put it on his spring tester to actually see what the spring rate of the bump stops are. And, you know, looking at your notes, once again, the front bump stop, initially, because these are progressive bump stops, they're conical shaped with different partitions or segments, The initial rate was about 129 pounds per inch. Yeah, in the front, for the first one inch of wheel travel, and then for the next little bit more than three quarters inch of wheel travel, that rate ramps up to 329 pounds per inch. So that's actually quite a difference, and then, you know, when you're talking about a front spring rate that's, you know, right around 200 pounds per inch, you know, you're well more than doubling it.

Yeah, and that's making a big difference in terms of how the front end will initially feel in just like light maneuvers, and then if you really lean and you go into a hard corner, you get all of that added spring rate from the bump stop in the front.

And then looking at the rear, we have a somewhat similar initial rate of about 138 pounds per inch for about the first inch of travel, and then you have once again for about the next three quarters of an inch travel, that goes up to 239 pounds per inch. So not nearly as large of a ramp up. Is there a reason why you're familiar with why they might have a softer ramp or rate in the rear?

Well, it's like you said when you were commenting on the front and the support of the bump stop, that's one of the reasons why they use more front. And also to make sure the car doesn't oversteer, that's why they're using a softer rear bump stop. And also keeping in mind the motion ratio difference, the front is 0.95, the rear 0.76. The rear bump stop at the wheel is effectively contributing even less because it's even more. So it's a 230 pound per inch secondary rate. But when you multiply by 0.76, you're getting a lot lower contribution than the 330, 340 front secondary at 0.95. So the bump stops are definitely going to make the car understeer more noticeably if you're driving it hard.

So, going from, you know, the spring rate of the bump stops, let's talk about the actual spring rate of the springs themselves. And, you know, you have this handy dandy long-acre spring tester to actually measure the rates as, you know, it goes through the stroke. And at least for the front on this car, you measured about 216 pounds per inch, which, you know, seems... It seems about right from the other things that I've seen where other people have done measurements. And in the rear, you measured the initial rate. So once again, these springs are dual rate. So that means I have this Bilstein spring here as an example. This is a progressive dual rate spring. And progressive doesn't necessarily mean that the rate is changing all the time. With these dual rate springs, you can see we have a segment with coil spaced closely together and we have another segment with them spaced further apart and this closer spaced coil pack, this is the low or the soft rate and then only once these are all compressed does this main larger rate area begin to work. So, you know, on the M3, M4 competition package, as well as the M2 competition, this rear spring is a dual rate spring. And, you know, looking at the notes you gave me, you said, or you measured rather, there's less than an inch of travel on the spring before the low rate is fully bound and we get into the main rate. And for that low rate, you measured just about 500 pounds per inch. And the main rate was about 605 pounds per inch. So with those spring rates, you calculated a front ride frequency of 1.4 hertz and a rear one of 1.6. And that gives you a flat ride ratio of about 15%, right? When you did that, was that including a nominal driver weight or...? Was that just chassis?

No, I had that with about a 180 pound driver.

Okay, so nominal weight. And you know, looking here, you noted that when the bump stops are engaged, those frequencies do change quite drastically. The front going up to 2.3 hertz and the rear going to 1.9. So from flat ride to now, pitch. So you know, you've given me these wonderful damping curves. And we can see that BMW is using a digressive damper in the front and the rear. And what that means is instead of these lines just going straight up straight or straight diagonally up down, they do, let's say, saturate or taper off. And do you have any comment on what you thought about the forces or the shape of these damping profiles?

Yeah, something that I noted immediately is that they have a very smooth transition between compression and rebound, and that's what's called an S-curve, so it's very, it allows the tire to kind of be compressed and extended fairly smoothly at low speed, so that's good. You don't necessarily want a really abrupt jerky curve. So that's good. The forces are definitely rebound biased, which anyone who's driven the car, you can feel it. And it's very typical, you know, Germanic tuning. I mean, it's not just German cars that do that. The force levels on compression are fairly low, in my opinion. Again, they're using the bump stop to provide sort of a secondary compression support because the car is designed to use the bump stop.

One of the things that I thought was really interesting from your damper dynographs was how much hysteresis there was in the rear dampers. What the hysteresis is, is that's the variability in the output based upon the previous stroke of the damper. As the damper is working, because it is working and not starting from a static position, that is changing its output on the next cycle, the next stroke. Really what it is is variability and well, we want this to be as consistent as possible because now you're changing the force of your damper so how do you know exactly what damping force you're getting going over whatever road features you're driving on. So another thing that you took a close look at were the sway bars, and in particular, how much the sway bars affect your roll stiffness or counter the roll. And there's a huge disparity in the front and the rear. So in the front, when your bump stops are not engaged, you measure just about 60% of your roll stiffness due to the sway bar. And when the bump stops are engaged, that's still not that much lower. It's about 46%, so still a very sizable contribution. But the rear, you have only 8% roll stiffness or roll stiffness contribution when the bump stops aren't engaged. And that drops even further to only 6% when the bump stops aren't engaged. So that really tells me, right, the way BMW designed this is, you know, the front sway bar is doing quite a bit and your rear sway bar is really not doing anything at all.

It's sort of a trim tab. But the benefit of having a rear bar is often, as was actually explained to me by another racer, that having a bar present at any end of the car helps to reduce your droop. And it actually affects your droop, your roll center movement. So if you took off the rear bar, as some people do, you can get better power application. But the downside is that because the rear has more droop and the front has less, the rear roll center will actually move more than the front. So when you're cornering, the rear is going to actually, it's going to feel like there's more understeer dynamically because your rear front center stays better controlled and the rear moves more.

Interesting.

So having a small but present rear bar actually keeps the car more neutral in terms of that roll center movement, migration.

Okay. So it's not really for typical sway bar applications. It's really to stabilize the rear. And maybe that is why if you look in the BMW parts catalog, BMW calls it a stabilizer bar. They don't call it a roll bar or a sway bar. Looking at the hard numbers and the contributions, this really shows me why in your video where you talk about your different principles of ride harmony or race harmony that you really do want to tune the front bar, but you really don't want to do anything with the rear.

At least for a rear-wheel drive car, I would say that's the wrong approach. Because you already start with a car that needs at least 40-50% of contribution from the front bar because of the strut geometry, the lack of camera control. So you don't want to drop that, and just going with a lot of spring gets you into pitch territory, which has its own problems. From both ride and handling, and the car's consistency on a bumpy road.

Yeah, and how it doesn't necessarily want to rotate anymore going into a corner. Well, hey, thank you so much, Shaikh, for spending some time with me and sharing with us the insights into how these things are set up and the stock suspension here. It might be a little while before, but this is definitely going to get one of Shaikh's suspensions, so stay tuned for that one, guys.