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Weight Percentages or LR Bite?
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Post by otiscampbell on Apr 5, 2013 6:28:41 GMT -6
For those of you who didn't read my J-Bar thread, this is an old asphalt LM running a 3-link with a J-Bar. I used to frequent this forum when I set up our modified and this car doesn't really fit with any of the other classes on the forum so I decided to ask my modified friends.  The car: Total Weight is approx. 3100# (track min. is 3000#) running on Hoosier D35s on 10" wheels. I don't have the wheel-base or track-width yet...I'll post that on another thread or later in this one if someone replies. When we bought this car, I was told to run 54% LS and 52% Rear weight percentages with 125# of LR Bite. After scaling the car and moving the lead around as much as I could before the last race, we were at 54.2% LS, 51% Rear weight and 125# of LR Bite. The track is a 1/2 mile clay egg-shaped oval...one turn is noticeably tighter than the other. (For some reason, in all of our cars we've raced, we always were much faster in the tighter turn than in the sweeper...not sure why.) When I got to the track, I was forced to change tires and I did my best given the circumstances to get the correct size tires on the correct corner. As it turns out, though, the new tires were not the same sizes as the old ones so now all of my corner heights were off by a lot. To remedy the problem we had to take drastic measures and start adjusting coil-overs to get the heights so the car was able to race. (We did practice like that and it wasn't pretty.) During the race, the car started out really snug and as the race went on (it was 30 laps, I think) the car started getting better and better--I presume because of fuel burn-off. Knowing that adjusting the coil-overs (i.e., weight jacking) doesn't change percentages at all...the only thing that really could have changed is my LR Bite. My question is--which played a bigger factor in the car being too snug early in the race...the rear percentage (i.e., heavy fuel) or the amount of LR Bite in the car? Or, did the fuel burn-off just help the fact that we had too much wedge in the car to begin with? As I've said before, I'm looking for a good neutral baseline that we can start with. My plan is to post everything about the setup we are running in a future post--shocks, springs, corner weights, caster/camber, etc. Thanks for your help! Otis |
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Post by confused on Apr 5, 2013 15:57:27 GMT -6
Also post where on the track the car is "snug." Entry, middle, exit, on or off throttle. They have different adjustments
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Post by otiscampbell on Apr 6, 2013 7:32:55 GMT -6
I just asked my driver and here is what he said:
The car was tight on entry. He would drive the car in, it would be sluggish to turn and he would have to wait until it was pointed in the right direction to get back on the throttle. He also told me that when the car is tight, he has a tendency to run the top of the track. His theory is that he doesn't have to turn as sharp so it doesn't make the push worse and everyone knows how miserable it is to drive in on the bottom and have the car wash up the track. One benefit to running the top and not turning the wheels as sharp is that the car carries more momentum through the turns...at least that sounded good when I typed it. One other comment he made was that the car was not tight on the throttle--it was a little loose on exit as well. I don't think the looseness was being caused by the tight entry, though...
Back to the setup issue: Since I'm trying to learn this stuff, tight in on entry could be a few things... If the caster/camber weren't right, then that would cause it not to "cut" properly. I set caster/camber/toe before the last race so I don't think that was it. (I'll post that, too, a little later this morning.) From the chassis seminars I've attended, I've heard that running a lot of LR Bite also has the added benefit of "dragging" the LR tire on entry under deceleration which will loosen the car on entry. So, if we didn't have a lot of LR Bite in the car after I made the blind adjustments to get the ride heights to a "raceable" state at the track, it could have added to the tight on entry and loose off condition.
Are my presumptions above even in the ballpark? Please feel free to correct me if my logic is wrong--I'm trying to learn here. Also, what role does fuel burn-off play in this? Standing in the pit area watching the race, it looked like it helped the tightness a little, but the car was still snug on entry and getting looser on exit.
One last little thing to add--I suspect that we were also running with a blown power valve in the carb. I talked to my carb. guy and told him the goofy things the car was doing--hard to start hot or cold, running really rich at idle, it cut out at high rpm, lots of fuel in the oil, etc.--and he suspected that the power valve is blown. We went from last to 5th in this race (I think there were 16 cars or so), but the car did appear to be down on power. What contributions would that have made to the handling issues?
I'll post the complete setup in a little bit...
Thanks, again!
Otis
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Post by otiscampbell on Apr 6, 2013 8:11:34 GMT -6
Below is more information about the car before the last race of 2012. Keep in mind that I had to adjust all 4 coil-overs to get the ride heights right at the track without scales, so this may not be how it was actually raced. (I wasn't able to scale it after the race so, other than the fuel level being about 1/2, I don't know the exact scale numbers.)
CASTER
LF: +1.5 RF: +3.5
CAMBER
LF: +1.5 RF: -3.5
TOE: 3/8" OUT
RIDE HEIGHTS
LF: 5" RF: 5-1/2"
LR: 6" RR: 5-3/4"
SPRINGS (THIS IS WHAT I'M TOLD IS ON THE CAR)
LF: 600 RF: 600
LR: 225 RR: 200
SCALE NUMBERS (WITH DRIVER)
LF: 805 RF: 700
LR: 842 RR: 711
LS%: 53.86%
REAR%: 50.78%
CROSS%: 50.43%
LR BITE: 131#
TOTAL: 3058
(I WILL ADD LEAD THE NEXT TIME I SCALE IT TO GET THE TOTAL WEIGHT, WITH DRIVER, TO AROUND 3100# WITH 17 GALLONS OF FUEL--IT IS A 22 GALLON CELL.)
WHEELS
3" OFFSET ON ALL 4 CORNERS
TRACK WIDTH
FRONT: 81" (2 SPACERS ON EACH SIDE--REQUIRED ON RF TO PREVENT TIRE RUB WITH UPPER CONTROL ARM.)
REAR: 78-7/8"
J-BAR
(I'M NOT SURE THE EXACT RAKE OR ANGLE. THERE DOESN'T APPEAR TO BE A LOT OF RAKE IN IT, THOUGH.)
Please feel free to offer any advice on establishing a good base-line setup. If anything about the information I posted above sticks out like a sore thumb, let me know that, too.
Thanks,
Otis
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Post by confused on Apr 6, 2013 10:01:08 GMT -6
I think, on your other post, you said the j-bar was at the bottom of the pinion? If that is true, try raising it one hole on the pinion plate and an equal aomunt on the frame. That will help to loosen the car overall. Rake does play into the actions more than height does, but they work togethor.
The lack of rear precent is the only thing that really sticks out to me. Can you get it up to at least 54%? This will help with the drive off as well as freeing up entry some.
You/the driver is correct about moving to the top if the car is to tight. The higher momentum helps the weight transfer to make the car turn.
Is the RR leading or trailing the LR? If it is leading, the car will be tight.
Turn entry is everything. If it is not correct, the rest of the turn will be affected.
The right link can be raised to help free up entry as well.
This is just a few thoughts for now.
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Post by otiscampbell on Apr 9, 2013 18:03:50 GMT -6
Another question...
When working on the car, I noticed that the RR tire seems to be rubbing one of the bars that come off the main cage going back to the rear when the car "rocks over" (assuming it does). The rub doesn't look severe enough to flatten the tire but I'd like to prevent this rub if possible. Someone told me that the front track should be 1-1/2" wider than the rear and right now, my front track is 2-1/8" wider than the rear. If I add a 1/4" spacer to each rear wheel, then that would make the front track 1-5/8" wider than the rear which is a lot closer to the 1-1/2" goal. As an added benefit, it should free the car up some, too...at least that is what I envision it doing.
I'm replacing shocks and springs and re-scaling the car and if you guys think it is a good idea to do this, too...I will.
Let me know your thoughts...
Thanks,
Otis
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Post by flipflopoo1oo on Mar 3, 2014 17:34:48 GMT -6
J-bar angle if too much can hold it tight going in and be loose off! If rr tire under drives on lifting a angle change needs more positive and will tighten coming off more but also steers more rocked over! While the numbers are good, consider a shock change to help and less left and more cross! A 50-100 split in front will allow for swap if needed to get you there! Compressed heights on the springs will come into play in transfer as to what you want to move first to make it steer as well as shocks! Measure and decide on soft or stronger rates are needed!
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Post by flipflopoo1oo on Mar 3, 2014 17:57:58 GMT -6
J-bar angle if too much can hold it tight going in and be loose off! If rr tire under drives on lifting a angle change needs more positive and will tighten coming off more but also steers more rocked over! While the numbers are good, consider a shock change to help and less left and more cross! A 50-100 split in front will allow for swap if needed to get you there! Compressed heights on the springs will come into play in transfer as to what you want to move first to make it steer as well as shocks! Measure and decide on soft or stronger rates are needed! Also pull-bar springs or excess movements and 90 10 shocks time changes on and off!
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