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Rear axle hop during track braking

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racer47

Still winning after 30+ years
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Your brake pads seem fine to me. Going to bigger tires generates higher forces, which exaggerates chassis issues. Brake dive is a front geometry issue. Don't mix that in with wheel hop and forward bite.

The BMR upper mount at the top hole is higher than stock. It will provide less anti squat and a longer instant center length. Both will help your situation. 90% squat with a long instant center length is better for both power and brakes as compared to 90% with a short I.C.

I also had a spherical upper arm but went back to bushing at both ends using the BMR adj upper and their axle bushing. The spherical was too unforgiving, imho. That rattle noise when it brake hopped is the spherical banging around. Not that the noise matters but I like the bushing better.

I hate to just give out setups because 1. I worked hard to figure it out and 2. It won't work for everyone anyway. But top hole BMR upper, top hole rear lca (1 down from stock), lowered such that the lca's are angled up slightly, less up than you are now (so lower rear ride ht). This is ideal for me. If I melt the front brakes, I will just start to notice the rear feeling unhappy. So I'm at the limit. Having the spherical made the good / not good line very fine. I think that extra give from the bushing helps widen the transition zone. This setup also plus power down with excellent forward bite.

From where you are now, you can't lower the rear. The upper arm is short and going down will make more squat. Going up in the back will be directionally better for squat but you're starting to get a lot of lower arm angle = more roll steer which is fine at low speed but uneasy feeling at high speeds. I was in this exact same spot and also had brake hop. As a temporary fix, I would delay downshifting as much as possible and sometimes even pushed in the clutch to take engine drag off the rear axle, so it wouldn't hop. I looked at all the options and bought the BMR upper because the upper arm has too much angle. Plus its too short but you can't fix that with bolt ons. It made enough difference to get what I needed.
 

racer47

Still winning after 30+ years
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The driveshaft is not going to fix anything for brake hop. The upper arm is not just a drag race thing. Its an important part of the rear end geometry for road racing as well.
 
The driveshaft is not going to fix anything for brake hop. The upper arm is not just a drag race thing. Its an important part of the rear end geometry for road racing as well.
Hey Racer47,

I was maybe a little unclear - I've changed multiple things in stages. What I know is swapping back to the OEM DS from a Dynotech single piece aluminum unit, all else being equal on the MM recommended setup made the hop issue go away based on ~80mph hard braking tests done by my local shop. I wasn't in the car so I'm trusting what they told me. Perhaps the aluminum DS was damaged from previous violent wheel hop situations which was contributing to the problem - there was some driveline vibration after the last big incident on track, which went away after returning to OEM DS.

The previous setup was done before I bought the car, so I don't know all the thinking that went into it. Here's the complete breakdown of stages I've gone through in my diagnosis process:

1. Original setup (wheel hop on track)
-Poly bushed, OEM length non-adjustable LCA's (not sure what vendor)
-Ford Racing relocation brackets, w/LCA mounted in upper hole, level to ground
-Roush wheel hop reduction UCA, PN13115K671LRK. I believe they are stiffer bushings and OEM geometry but not 100% sure
-Dynotech Aluminum DS
-Koni yellow / MM springs
-Nitto 275's square
-Porterfield ST47F / 43R

2. Updated to fresh MM setup, keeping aluminum DS (still wheel hop based on street test)
-MM spherical LCA's
-Ford Racing relocation brackets, w/LCA mounted in upper hole, level to ground
-New OEM UCA
-Aluminum DS
-Nitto 275's square
-Porterfield ST47F / 43R
-Dynotech Aluminum DS
-Koni yellow / MM springs
-Nitto 275's square
-Porterfield ST47F / 43R

3. MM setup, back to OEM DS (wheel hop eliminated based on street test)
-Same as above, just change out DS

I'll know for sure if I'm good once I track it again - braking zone for T7 at Sonoma will be the real test.

Sounds like my setup is pretty mild compared to what you're doing. I don't have the knowledge on solid axle geometry you do, so my approach has been guess and check with a little guidance from others. My goal is to have a stable, easy to drive platform on track which I can also drive on the street - focusing on driver development, not really chasing lap times and setup optimization. Now that everything matches the complete MM recommended package, I'm hopeful the issue will go away and I can move on. Their philosophy is to remove all the compliance from the LCA's, keep UCA flexible. Would love to learn more about serious tuning of 3-links for road racing but that's past what I can keep up with right now. My comment on drag racing is more that I've seen a lot of rear suspension parts on the aftermarket which are tuned for straight line launches, and to be careful not to use the wrong tools for the job. UCA bushing bind and too much anti-squat seem to be the potential problems. The thing that's still unclear to me is if the pinion angle itself really matters all that much for road racing performance (as long as it's within acceptable u-joint angles). Optimizing for driveshaft angle at max torque seems to be drag-racing consideration - maybe i'm wrong on that?
 

racer47

Still winning after 30+ years
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There are basic mechanical issues that apply everywhere. Pinion angle is important, drag or road race, the concept is the same. It doesn't have to be perfect just within a reasonable range. If you had a driveshaft problem, then you had a driveshaft problem. Maybe it was driveshaft and pinion angle. I don't know. But that is a different issue than squat induced brake hop.

There is no one size fits all magic solution or setup. Its all about correctly analyzing what is going on and then fixing it correctly. Too often, different issues are mixed together or improperly diagnosed resulting in endless trial and error attempts at fixing it.

The core problem with s197 wheel hop is the upper control arm. Its too short for one. Plus the chassis mount is too low to start with and lowering the car makes that even worse resulting in a very short instant center and too much anti squat. But everyone messes with the lca because its a lot easier.

Purpose built race cars do all of the squat adjustments on the upper arm. The lowers are set parallel to the ground (or slightly angled depending on the chassis and engr doing the setup) then the upper arm sets the desired squat, usually with slotted mounts on both the chassis and axle mount.

As far as compliance in the arms, thats more debatable for street/race cars. Full race is all heim everywhere with some exceptions to address particular issues. For street, full heim gets real old real fast. I like the BMR lowers with the heim on the axle and the bushing on the chassis.

As far as learning road race 3 links, just read and understand what I wrote here and above. Longer instant centers are better than short. Generally you want as much anti squat as you can tolerate before brake hop. Too much lower control arm angle will induce too much roll steer. There's not much more to it than that. All things in moderation. Nothing way out line.
 

Grant 302

basic and well known psychic
Using the Whiteline LCAs (adjustable length) and the Whiteline LCA relocation brackets. The brackets only have one hole. At static ride height, the LCA is at a 7* angle - higher on the chassis end.
Pretty simple matter of being too steep despite other opinions. Try a bracket with a 3” adjustment point. The Whiteline 4” (single) adjustment point is just too low unless you can lower the rear more. Good luck.
 
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I really don't think your pads have anything to do with this, the only time I've experienced axle hop was in my drag racing days, leaf springs were "wrapping" up and then unloading (a bit of old school here, they way around that was to reverse the springs so the "long" measurement from the spring eye, ran forward, do that to all the springs and then clamp them, they provided spring action but resisted wrap up) the other time was in my 84 camaro with a torque arm and it would hop under braking, solved by a stiffer set of trailing arms and a urethane bushing in the torque arm, the only other time was in my kart under hard braking the tires would cause axle hop because of distortion, I've never seen an S197 axle hop under braking.
In each of the above cases it was a combination of spring (a kart is just a big torsion bar) or bushing softness that would translate to the tire causing load/unload (no doubt abetted by low tire pressures) One aspect of tires that no one ever discusses is the overall diameter of the tire, everyone considers the wider tire is the best because of the contact patch, but no one ever talks about the contact patch front to rear. This is one reason that the dragster guys continued to go to a bigger diameter tire,( until the rules limited it).What a taller tire does is promote a bigger contact, but depending on the sidewall it can contribute to tire hop, the dragster guys and the sprint car guys use a real soft side wall to promote this contact patch and it also acts as part of the suspension schemes in these cars.
The point is, whenever you talk about axle hop, don't forget the part that tires play in all of that.


 

Norm Peterson

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I'm sure it's possible to have too much rear pad and/or too much rear tire for some excessive amount of anti-squat/anti-rise (or a too-short SVSA) even with a 3-link suspension. But the real problem there would still be too much 'anti' or not enough SVSA length for the rest of the setup rather than the other way around.

Certain years of 4th gen F-bodies - these being torque arm cars - were susceptible to brake hop and it was reputed that an unpleasant coincidence between some hop frequency and the ABS may have been involved.


Norm
 
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I've never seen an S197 axle hop under braking.

Well, if you watched my video posted earlier in this thread, you have now! Considering the S197s that you’ve been hanging around, I’m not too surprised - a lot more setup talent than I currently have on my team (me)! Great point on the contact patch of the larger diameter tires - that’s one of the main changes that I made before this issue began - 25.6” to 26.3” change in height, and 275/35 to 305/30 aspect ratio.

I'm sure it's possible to have too much rear pad and/or too much rear tire for some excessive amount of anti-squat/anti-rise

Perhaps with the smaller contact patch, the tires weren’t gripping enough to expose the anti-squat issue until the larger tires pushed it past the threshold. Ordered a new set of J&M adjustable LCA brackets, as well as a set of their poly/rod end combo LCAs. This will make all 3 axle connections spherical bearings and all 3 chassis connections will be poly to reduce noise. This should get the LCAs closer to level and back within the limits.
78BE4AC0-38C0-48B3-8689-8F384D55D5A4.jpeg
 
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just for fun, have you checked all of your anti lock brake connections and sensors?
Yes, I typically inspect the wheel speed sensors each time I have the wheels off after a track day. I'll be looking a bit closer and tracing each back this weekend when I get the car up in the air. No ABS light, codes or any other abnormal behavior other than the axle hop.
 
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did you really feel the axle tramp or could it have been an intermittent rear brake lock up, the axle tramp that I have observed is usually pretty violent, it really wasn't apparent in your video (probably was very apparent to you, I would imagine).
 
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did you really feel the axle tramp or could it have been an intermittent rear brake lock up, the axle tramp that I have observed is usually pretty violent, it really wasn't apparent in your video (probably was very apparent to you, I would imagine).
I would describe it as violent - could definitely feel the up and down motion. It damn near sounded and felt like the whole axle was going to drop out of the car. I think if it was intermittent brake lock, I'm not sure it would have swapped ends like it did.
 

racer47

Still winning after 30+ years
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That was definitely axle hop in the video. My car did the exact same thing even with the exact same noise. And I know for sure mine was axle hop.

I'm not a J&M fan. I think BMR is made better and typically from heavier gauge steel. I originally had Ford racing lca brackets. But then another local guy bought BMR lca brackets. I measured them and the top hole on the BMR was 1/4" or 3/8" higher than the fords. So I bought BMRs. I don't know where J&M falls but you really don't want to move the rear mount too low. I looked up the whiteline brackets and their single hole is in about the same place as BMRs second hole. Thats for sure too low.
 
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I was very impressed with the build quality of the J&M UCA mount, and I think both BMR and J&M relocation brackets are made of 3/16” steel. Great thing is they’re both made in the US, and really comes down to preference. I decided to go with the J&M based on a bit finer adjustment between holes - they’re 2” - 2 3/4” - 3 1/2” vs 2” - 3” - 4” for the BMR. They’re also the only ones I’ve seen with a reinforced upper mounting point on the side that usually just has a more pronounced bend on the FRPP, BMR, etc.

Thanks again for the validation of the issue and for real world feedback on how to correct. I know that sharing setups goes against every grain of your racer DNA - thank you for sharing.

3DD0502F-6CAC-4864-A6E4-4B206AFD5BE3.jpeg
 

Grant 302

basic and well known psychic
I was very impressed with the build quality of the J&M UCA mount, and I think both BMR and J&M relocation brackets are made of 3/16” steel. Great thing is they’re both made in the US, and really comes down to preference. I decided to go with the J&M based on a bit finer adjustment between holes - they’re 2” - 2 3/4” - 3 1/2” vs 2” - 3” - 4” for the BMR. They’re also the only ones I’ve seen with a reinforced upper mounting point on the side that usually just has a more pronounced bend on the FRPP, BMR, etc.

Thanks again for the validation of the issue and for real world feedback on how to correct. I know that sharing setups goes against every grain of your racer DNA - thank you for sharing.

View attachment 17718
Sounds like you have the right dimensions on the adjustments, and would agree that it’s a better choice. Thanks for posting.

@s9669s - might be the bracket to try based on previous discussion with me and Norm.
 
Thanks for tagging me on this. I just ordered UMI brackets (instead of BMR) for a couple reasons. The holes being at 2", 2.75", 3.5" instead of 2"-3"-4" (similar to the J&M pieces) was one of them.

The J&M piecess look really good from a design perspective, but I wasn't sure about clearance for my Strano adjustable bar with them.
 
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racer47

Still winning after 30+ years
392
497
Exp. Type
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SE WI
I was very impressed with the build quality of the J&M ...

Thanks again for the validation of the issue and for real world feedback on how to correct. I know that sharing setups goes against every grain of your racer DNA - thank you for sharing.

That must be a newer version. Their old one did not look like that. I'm sure it will be good and you won't need the lower holes, keep it at the top one.

You're welcome for the validation. Everything I post is from my real world testing and racing, not regurgitating stuff I've read or armchair racing. My setups work. Finally got to race this weekend. 1st race of the year = 1st win of the year.
trk.jpgwinner.jpg
 
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JDee

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That must be a newer version. Their old one did not look like that. I'm sure it will be good and you won't need the lower holes, keep it at the top one.

You're welcome for the validation. Everything I post is from my real world testing and racing, not regurgitating stuff I've read or armchair racing. My setups work. Finally got to race this weekend. 1st race = 1st win.
View attachment 17785View attachment 17786
Winning race car with a trailer hitch? That's gotta be a first!
Must be a story there, do tell?
 
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Winning race car with a trailer hitch? That's gotta be a first!
Must be a story there, do tell?
That’s awesome! I’ll be curious to hear what @racer47 was towing. I have the Blowfish receiver hitch sitting on the garage floor ready for install - the trailer just arrived this week. The Corvette pic is from their website:

https://www.leroyengineering.com/paddock.php

D5B9EB8B-BFD5-4458-869A-2AFB7A12B269.jpegD6DF047A-5694-4000-92BB-6C738AA4C564.jpeg
 

racer47

Still winning after 30+ years
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I have a tire trailer, Blowfish hitch and drive the car. I raced for real for 25+ years, super lates, GT1 and GTO. I don't want to deal with the truck and trailer anymore. So a tire trailer is enough. I'd pull the receiver hitch except that I bolt it in tight so it doesn't rattle. When I'm not racing the whole bracket comes off and the mounting tube stays on hidden behind the bumper cover.

67GTA is obviously a baller with your Leroy trailer (kidding). They are super nice. I just built mine based of a northern tool trailer kit but it does the job

trailer.jpgt2.jpg
 
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