Correct Angles/Ride Heights or Correct Bite?

Helping a buddy with B-mod. Set up sheet states certain upper control arm angles for front suspension and measurements from top of rear housing tube to overslung tube in rear. Problem is that with these settings on the money, the bite is near zero. We’ve had good results with about 50 lbs of bite on dry tracks. Option 1: we can get the 50 by cranking in and out evenly on all 4 corners, but the front angles/geometry is not to spec (too much angle LF and too little RF). Or Option 2: we can crank into the LR spring until we get our bite while maintaining front angles. This, however, leaves the LR ride height about 1 ¼” higher than spec. Which method would be the lesser of two evils? Thanks.

I'm no expert but I'll share my opinion. I start by getting it all to the proper ride heights. I think that those front ride heights are priority. After you've got that all set maybe play with adding a little lead to the LR and see how much bite you get and if those heights change. I'd rather be 100# heavy and setup right the 100# lighter with everything wrong. If the lead doesn't do it I'd crank a bit into the LR (don't think the LR height/angles is as crucial) but then share maybe just a 1/4" on the other 3 wheels. Add all four small height changes together and maybe that gets you there? Or maybe go eat a hot dog? lol

Yah, I hear ya. Many things in race car set up seem to be a compromise. I'm leaning towards keeping the desired front end geometry, even if I have to crank into the LR quite a bit. My thinking is that the builder has figured out and accounted for roll center placement with those angles. Besides, the LR spring is dangling free with a crazy dynamic ride height when the car is on the LR bar for most of the race. Anybody see a problem with this?

#2. A lot of setup sheets will state LR R/H varies w/ wedge. Some sheets may not even list a r/h.

But I have a hard time believing it didn't change the other corners when you jacked turns in the LR enough to get the wedge you wanted adjusting the LR only.

I'd add lead to the high left side and reset the RH's.

If you're not on a super soft lr try this: I rd in rf, 1 out lf, 2 in lr, 2 out rr. That should get you very close without changing ride heights. Its not uncommon to have the lr 1/2-3/4 inch higher.

Ran into the same problem as you had with our bmod. Found out the rearend was bent. Chassis builder has what is your baseline is were you need to start with. Tweak on that baseline setup and give it what the driver and car whats. we started with 15 degrees of angle in rf and 50 lbs of bite with base line. in 3 weeks of racing ended up with 19 degrees and 100 lbs of bite and the car is a rocket on dry slick. Best advise i from an old racer was your car is like your engine find what the best timing to make it work the best. IE is the car rolling over in the middle of the corner or before the corner. it all in the timing of the chassis.

When putting turns into Lr we don't get much change (couple of pounds literally with 3-5 turns. But with one in rf and one out lf we change about 1% of cross for every turn.

Quote:

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Originally Posted by Dirt_Buster

When putting turns into Lr we don't get much change (couple of pounds literally with 3-5 turns. But with one in rf and one out lf we change about 1% of cross for every turn.

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stiff spring on the RF will gain weight quicker than the soft springs in the rear.
crank the LR in it till you get wedge in it were you want it, don't worry about lr ride height.
front is more important.

We've won a lot of races and that Lr with the springs we run needs a lot of turns. We get a lot more out of our 400s up front. Than the 75 Lr.

What are your rear bar angles and panhard bar angles? With bmods everything is a compromise. I always have liked to run higher than recommended lr bite but that fits my driving style (somewhere in the 150 range). On a black slick track we run 23 degrees lr and 0 rr angle with 3 inches of panhard bar rake at ride height.

Thank you for all the input guys. We kept the front angles and started the night with 50 lbs of bite. On back, we ran L bar up at 23 degrees and R was dropped to 6 degrees to minimize roll steer. Ran good in the heat but it looked like the feature was going to be dry, dusty and slick, so we dropped panhard at pinion to bottom hole, so like 8" split. We also put two rounds in the RF to pick up the bite quickly and keep the RF frame from scraping. The car looked pretty good. Passed some cars and kept up with the fast guys, but the RR tire came in scorching hot. The LR definitely had heat too, but the RR was noticeably hotter. I don't think the car was sliding a lot, so do ya think the hot RR is due to spinning when LR is digging?

Let more Lr chain out.

The chain limit is set to the set up sheets recommendation but still has more adjustment left. As it is right now, the LR bar goes to about 44 degrees at full droop. This brings up another question. Lately, as we've been tightening the car for dry tracks, we've been seeing the LF tire more in the air. I'm not sure we like this. Even in the slick, it's a good 5 or 6 inches off the ground. If the drack is good, the LF tires is almost a foot off the ground from the center out to about a third down the straights. I'm not sure when it got to this point. We run a 225 spring on RR and 200 on LR. How can we keep the LF down?

Has your lf been picking up the more jbar split you put in it?

No. We just tried lowering the panhard at the pinion this past weekend to try to keep the car from skating up the slick track right after entry and past the center. It's been three wheelin for several weeks. I think the LF wheely started around the time we made changes for the slick tracks. The changes included going higher on the bite than the set up sheets recommend and moving the pull bar mount to the left about an inch. Also, it looked like the car's rear end was loose and trying to drive around the car, keeping driver from getting on gas on exit, so we reduced roll steer by going way down on R bar and resquared the rear end.

Just keep the lf dangling. Barely touching. Sounds like you need maybe more side bite which equates to momentum and or a shock guy to let those shocks work more. Guessing you're tied down on rf

One of my go-to adj for a slick track b-mod is shortening the right trailing arm 3/8". That will tighten you up on entry and exit.