Bottom Out

[zeroracing]

I am having issues bottom out the rack and front bumper. I have raised the bumper, no avail. Here is what seems to be happening:

I go into the corner, roll off gas and onto brake. As I enter mid corner the front end digs into the ground and I push terribly. The rack is packed in mud, nose folded under, mud stuffed all over. I have tried the following:

1. Not let off gas: Problem solved, just cant do that all the time.
2. Shut RF brake off: Slightly better but still bottoms out.
3. Let off brake by 1/3 corner: Fixes problem but slow
4. Raised nose: Still hits, feels like rack hits and then digs in like a plow.
5. Increase RF Dynamic Load: up to 2800, still bottoms out.
6. Increased RF tire pressure and put 92" on, like 15-17 PSI, not as bad.

It feels like the tire flex is allowing dig in. I have tried trail braking, but with this one car it is impossible. If i am on the brake 20% or more at mid corner, it is toast. In traffic if you tag the brake mid corner, done for the night...

I tend to blame driver, but I drove a mod the other night same way, very fast...

This car has a raised RF rail but not a raised cross member. I ran Airshok part time, also different bump/stack/standard spring combos.

Is all of this from running aggressive set up causing cross member to hit ground? Would raising cross member make it all go away? Or am I driving this thing completely wrong?

[grt74]

I am having issues bottom out the rack and front bumper. I have raised the bumper, no avail. Here is what seems to be happening:

I go into the corner, roll off gas and onto brake. As I enter mid corner the front end digs into the ground and I push terribly. The rack is packed in mud, nose folded under, mud stuffed all over. I have tried the following:

1. Not let off gas: Problem solved, just cant do that all the time.
2. Shut RF brake off: Slightly better but still bottoms out.
3. Let off brake by 1/3 corner: Fixes problem but slow
4. Raised nose: Still hits, feels like rack hits and then digs in like a plow.
5. Increase RF Dynamic Load: up to 2800, still bottoms out.
6. Increased RF tire pressure and put 92" on, like 15-17 PSI, not as bad.

It feels like the tire flex is allowing dig in. I have tried trail braking, but with this one car it is impossible. If i am on the brake 20% or more at mid corner, it is toast. In traffic if you tag the brake mid corner, done for the night...

I tend to blame driver, but I drove a mod the other night same way, very fast...

This car has a raised RF rail but not a raised cross member. I ran Airshok part time, also different bump/stack/standard spring combos.

Is all of this from running aggressive set up causing cross member to hit ground? Would raising cross member make it all go away? Or am I driving this thing completely wrong?

put some anti dive in the rf, BUT JUST A LITTLE, IT DOESNT TAKE MUCH, don't know which car your talking about either

[billetbirdcage]

Like GRT said, least check to see if it has some anti-dive in the RF or actually has it set to dive instead of neutral or anti-dive.


However: Your load number are likely really off from what your supposed to run for that brand of car (Swartz I think you have). What inch is that 2800# at? What is the 4, 4.25, and 4.5" load numbers?


Basically we know what amount of travel on RF (with LR travel the same all the time - IE chain length) when the car will start to drag. So say this is around 4.5" on average, we have the load numbers ramp up at before that point so the RF basically can't ever travel farther then that 4.5". Like I said if I change the Chain length on LR then that will change that 4.5" on RF measurement and have to adjust accordingly.

[zeroracing]

I will check the dive/anti-dive good thought.

On the load numbers I started out running the standard for the chassis, yes a Swartz. I started out with the recommended load for 4.5" travel. When I bottomed the car out, I started to back that down to same load at 4.25" then 4.00", then got mad one night and cranked up load on the spring stack (300/700) to 2800 at 4.5" figuring no dang way I can bottom this out. I also tried a straight 300 with a tree bump, and a 200 on RSW bump earlier in the year. I don't have the load graph in front of me, but the entire time I kept the same static load number (car at ride height).

I started out the LR within builder spec, 6" of travel, then I backed that down 5.5" in hopes of limiting the ground plowing, this did help a good bit, but did not fully eliminate it.

[grt74]

just remember, not all tracks need anti dive, its an adjustment just like any other adjustment, sometimes it works and some places it will hurt ya, but if your having to get after the brakes really hard and its slamming down, anti dive will help
may have to run a bump to keep it from traveling to far

[MachineMasters]

I wouldn't be afraid to start with a higher initial ride height.

I'd back the RF tire psi down to 10-12psi at the same time. And re-set your RF loads to be around 2300-2500lbs at 4" - 4.25".

[Basecircle]

More a question than a solution, but if you are on a bump setup, couldn't you shim the bump up so you're on it sooner? Set it so you're on the bump before you bottom out? Maybe a band-aid over a bigger issue, but just a thought.

[TheJet-09]

^^^ I've been trying that on my RR for the past few races. I have no idea what any load numbers would be, and I'm not sure I'm even looking at it the right way, but with it "shimmed up" as you say (less travel before engagement), it's allowing me to lean on it harder (the RR) when the track is fast/heavy, and I remove shims when it slows down (to assist with roll in my case). I've felt like the car was falling over on the RR when fast/heavy, and wouldn't recover. This has helped that.

As for the RF, my question for the OP is, when you "cranked up the load" on your stack, were you just adding to the static preload (turning down on the spring nut)? In just now trying to understand load numbers, etc., I'm thinking that also affected the initial load? So for those with experience in this area, would you try to come up with a compromise that would keep initial load numbers close, or the same as the original, yet "ramp up" so to speak as you near that bottoming out/travel distance? And I'm further guessing you could achieve that by either decreasing the gap for the lock-out nut, or going to a stiffer primary spring, or adding a bump?

[zeroracing]

When I cranked it up, I cranked up the secondary Load numbers. Example was on a spring stack I engaged the second stage earlier so at 4.5" travel I was at a much higher number. Theory being my actual load number would now hit and Limit travel to say 4.00". I did not adjust static load numbers. I also worked on a bump some in an effort to fix the issue, adding shims, similar deal.

I think it was caused by being too soft in my "ramp" and way to linear/non raised cross member. My stack and my bump both were giving a very linear graph and so if I hit a bump I would hit some number over my original target, that would equal enough travel to catch the dirt. Where I needed to be more along the / part of the graph so a few hundred pounds of load was very little travel. I appreciate the responses and billet really helped me get it lined out I think. I was seriously about to give up on the late model over this deal, just not been our season. Thanks again guys.

[billetbirdcage]

I appreciate the responses and billet really helped me get it lined out I think. I was seriously about to give up on the late model over this deal, just not been our season. Thanks again guys.

I figured your curve had to be off as it seem to be the only logical way you where still bottoming out with the changes you said you have tried. Hopefully what I sent you will get you in the ball park, you just need to find that travel at which you need that curve I sent you. Just adjust that with packer shims to raise and lower that curve to the desired travel you need.

[TheJet-09]

Excellent info from everyone! Thanks for sharing!