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Old 09-23-2019, 01:14 PM   #1
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T14 Thill...2X within one foot of the other

Here's a nice little mystery (which I think we figured out):

Rounds 3 & 4 I put the bike on the ground during the AFM weekend in turns 14 and T11 at Thill. This the first time on the ground in several years and probably over 50 individual races. These were weird ones:

Crash 1: Bike felt quick and had plenty of grip. Bike felt confident in the front end and didn't have any ill handing issues on entry, mid corner, or exit. Practice and F1 qualifying were reasonable. I had 1 race on Saturday that went well and two more on Sunday which both went very well. 600 Superstock was my third race and later in the day. I was able to make more and more time into T14 at Thill during the race to make up time on the leader. After 3 laps of making up time, I turned into T14 on the same line I'd used the whole weekend, gave away lever as I added lean angle and was a bit more than 5% or so at the apex. @ the apex, the front end collapsed with no warning. No heavy tire feel, no wagging bars...just bang: on the ground. I chalked this up to holding too much lever to the apex.

Crash 2: Bike felt decent in practice (saturday). Front was a bit vague feeling when I'd transition to the side of the tire and start giving away brake lever, but not a deal breaker. During qualifying, I ran my first hot lap for an initial lap time. The second lap, I was using the rider (my prime competition) who was about 3 seconds ahead as a marker to catch, for a faster lap time. The lap felt pretty darn fast given the time made up on him. Enter T14, I did everything the same; braked in the same spot, turned in at the same spot, gave lever away all throughout the turn at the same spot. @ the apex of T14, with maybe 5% brake lever and almost immediately before transitioning to throttle, the front end collapsed with no warning. No bobble, no slide, no heavy bar then collapse, no light bar wagging; just, Collapsed. Oddly enough, the bike hit the ground within a one foot of the my previous AFM weekend at Thill's 600 Superstock crash. I've never crashed in almost the same spot in the same exact manner before. We got exactly 1 lap in for qualifying before putting the bike on the ground. It was very hot this weekend (110 degrees or so...probably about 105 at the time of losing the front). At this point, I didn't know what I'd done wrong, but was starting to suspect something different from rider error given the mirrored event to my previous crash in 600 super stock one month previous.

Crash 3: Same race weekend, Sunday. I had realized something was up with maybe bike balance or something, but began pussyfooting corner entries and getting off the lever much earlier than normal and not carrying as much lever and lean angle as I have several years of my racing now. It was slower and more difficult to get the bike work well, but not putting as much load on the front end. In the 600 super stock race, I got passed by Velentin Debise for the lead going into T9, raced into T10 with him and then followed him for a lap. Our next entry into T11, I tried his move of moving his head/ shoulders more to the inside a tad more. Once again, I was off the brakes and almost at the exact moment of transition to throttle, the front collapsed. I entered the corner a slight amount slower than Valentin, and did the same as always except for trying to hold the bike down the apex a bit longer with body weight. The collapse had no warning again. The front also was vague this weekend.


Changes we made to the bike from the last time we were at Thill: gearing change from 14/43 to 15/46. Ratio is the same, sprocket size is different. This necessitated a chain length change.

After scratching our heads, at dinner we came up with the following additional info: wheelbase was moved rearwards about 35-45 mm or so and not corrected on mistake. I run a very short wheelbase and that wasn't corrected. The move of the rear wheel rearward put more weight on the front end. We concluded that we did not make the correct adjustment to raise (add preload) the front forks/ change the weight bias rearward. This in turn had the forks on the bottom at the point of the smallest contact patch for the front tire. Possibly a bit too fast of front rebound may have helped the front end extend once grip was momentarily lost with no chance to regrip up.

This was our conclusion. This being Crash Analysis, discussion and comments are always welcomed. This weekend we had made the appropriate changes and had no issues.
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Old 09-24-2019, 07:46 AM   #2
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Good stuff!

The data geek in me wants to know the weight on each contact patch pre and post wheelbase change.

There has to be a formula out there already.
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Old 09-24-2019, 04:12 PM   #3
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Interesting you ask that. Several years ago, Siglin and I bought postal scales and measured the weight from moving from the front of the seat to the back of the seat: result was 12 lbs to the rear. Not as much as I'd guess.

One of my friends I race with (Robert Brittain; motogloss paint) put brake/ throttle cameras on my bike for the last two races of the day. I'm interested to see what comes out of that...how much neutral throttle I hold. His insistence came from my critique on where he was losing time all over track from his videos he posted; neutral throttle all over, in between brake/ throttle for way too long.

Fingers crossed I'm not doing that badly!
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Old 09-25-2019, 07:46 PM   #4
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Only comment I have is that it was a pretty cool, relatively, race weekend, and lots of riders were having grip issues. And, having reread the post, this makes no difference and is not applicable/
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Old 09-25-2019, 08:44 PM   #5
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I am far from an expert, but I had an identical apex crash at laguna turn 5 that I'm still trying to figure out the cause. Switching from q3's to slicks seemed to make my bike handle much more confidently, even though im pretty slow and am not even close to pushing the tire. I noticed that the front end stopped (what felt like) skating across the bumps at the apex of 11 at thill after swapping tires. I wonder if the tire diameters were different and gave me different weight distribution.

I'm interested to see what people think about your crashes, hopefully you figure it out.
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Old 09-28-2019, 10:41 AM   #6
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It’s unlikely u were near bottom at the apex with 0-5% brake pressure. Turning doesn’t create nearly as much load as braking. The corner is also flat so there’s no g-out. Maybe we can find someone with suspension pots and a Thill lap to see a possible suspension position for that corner.

Typo in your wheelbase changes? 4 total teeth ~= 16mm, 2 links ~= 16mm. And they are opposite changes. I don’t see how u got 35-45mm.

3.5-4.5mm is more likely. Such a wheelbase change won’t adjust the weight balance enough to put the forks on bottom at apex. Maybe you’d want 0-2mm fork preload to compensate, which is small.

I’d bet you wouldn’t have crashed if you held more brake pressure. My experience is that overloaded tires provide feedback and underloaded ones don’t. But that’s on Dunlops. More preload might help because it could give u confidence to hold the lever harder and deeper. But I really doubt you overloaded the front tire.
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Old 10-09-2019, 04:16 PM   #7
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I think there are a few things that either got misinterpreted;

1. At max lean, I want off the lever when it's time to go to the throttle. T14 is a flat(ish) corner and I make most of time on the side of the tire for my entries. Holding more lever in T14 would be the same problem only earlier in the corner, IMO.

2. 2 links = 35mm. One is about 16mm. using only the axle position as the measurement, the wheel was moved rearward about 35 mm or so from the previous measure we had on it. 43-46 too is 3 teeth, for whatever that is worth.

I still have nothing more. Rode the last race weekend with the original WB and bike was good, but twitchy as hell, which I don't remember from previous time we had it setup that way. Put it more on it's nose and I started to get front end feel back. I still think I might go back to a longer wheelbase for mid corner stability for the last AFM round and see how that works. Or do one link added and run with that.

Thanks for the thoughts Rob.
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Old 10-21-2019, 04:50 PM   #8
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Update: measured shock and discovered the bike sits high in the back as is. We settled on the geometry being affected as the issue. Didn't get a chance to walk T14 apex, but another of my close competitors went down in the same spot over the weekend. Suspect the combination of geometry, more weight on the front end and excessive lean angle as the prime contributors...least that's what I'm telling myself.
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Old 10-21-2019, 09:05 PM   #9
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Same place I crashed tuesday, right before the apex, carrying too much lean angle and off the breaks. Stupid.
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Old 10-22-2019, 11:13 AM   #10
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You too Ernie? Just let go w/ no warning?
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Old 10-23-2019, 08:07 PM   #11
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You too Ernie? Just let go w/ no warning?
There and gone no warning, no push. Second crash in the same place. Like the first one, everything was fine until I was on the ground.

I did get warning. About a half a lap prior my head said:" Dude you need to back out of this pace and call it a day, you're straining." I should have listened.

For me the transition between brake and throttle there, with lots of lean angle, is a bit treacherous.
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Old 10-23-2019, 11:18 PM   #12
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Same exact thing Ernie....exactly the same.
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Old 10-24-2019, 02:42 PM   #13
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I haven't been there in years: Are there bumps, seams or patches in that area? With any amount of brake still applied, there will also be some residual bar pressure. If there are surface irregularities, the front will have a harder time following contours with pressure on the bar and high lean angle.

With these incidents all seeming to happen in the same place, there has got to be something about the surface that is contributing.
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Old 10-24-2019, 04:07 PM   #14
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Good analysis Andy. I'm thinking something with the surface for sure. I meant to walk it this last weekend and didn't get the chance. If I go up for Z2 in a few weeks, I'm gonna walk it for sure. I'll check back then.
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Old 10-24-2019, 04:39 PM   #15
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It's an interesting place. We want to get as much speed as possible for T15, which is SO IMPORTANT, but at the same time don't want to overwhelm the front with lean angle. We all want as much drive as possible on the exit of T15, on the little bikes we need all the roll speed as well. On the bigger bikes I am comfortable trail braking right to the apex and then rolling on, my crashes both were on the SV, where I was desperately (the correct word) looking for lean angle and roll speed. I gave up the brake a bit early and that's what got me. Of all the corners at Thill, this is the only one that I just don't like on the SV. The front feels wooden and vague and I have no idea what's going on.

I'd agree about the surface but even at my perceived apex and Berto's, there's probably five feet of track both ways between us.
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