Those #@*%! redlights!

[five7kid]

I know if I leave as soon as I see the last yellow, I'll bulb by -.04 or so. Therefore, I need to give it a tick before leaving. I try to get in a rythm, but if anything distracts (like issues getting the stall speed right), I'm dead. For instance, 1st TT today was a .021, not bad; 2nd took a little longer to stall up, 1st light was on when I looked up and 2nd came almost immediately, no rythmn, resulting in a -.020 - not good. 1st round of eliminations, I'm the slower car, tried to give it the proper tick, afraid I gave it too much - nope, -.007. Out.

I've tried the usual - lower front tire pressure (minimal effect), lower launch RPM's (goes dead, resulting in .1+ RT's). I don't have a 2-step rev limiter, would be nice but other priorities have prevented thus far.

In desparation, I'm going to try the taller highway tires on the front I usually use on the rear to drive to the track. A racer bud I pit with a lot also suggested softening up the rear suspension. There's a T&T Wednesday night, I hope to try the other tires on the front, and less pressure in the air shocks. Would be nice to lose the air shocks, but it sits pretty low without them.

[Trevor86TA]

What size tires are you switching to and from?

[five7kid]

205-70x15 to 225-70x15. About an inch taller. More sidewall to work with pressure variation as well.

[five7kid]

Made it out to T&T this evening. A wet cold front moved through last night, the high for the day was at 12:30 a.m., with drizzle most of the day. By mid-afternoon the drizzle had moved out, but the temp stayed below 50 degrees. Good news is turn-out was light, so got plenty of runs in, even though the 1st one wasn't until 7 p.m.

I put the taller tires on the front, dropped pressure to 25 psi (had been running 30 in the smaller tires), dropped the air shocks from 100 to 25 psi (minimum recommended for the shocks), and slicks pressure from 18 to 16 psi. 1st run, I guess I couldn't convince myself to leave on the light, cut a .068. But, the 60' was actually quicker, so I figured the roll-out must have been responsible for a good share of that. Next pass I upped front pressure to 28 psi, left absolutely as soon as I saw the last yellow, cut a .001. I was basically able to hot-lap, after a quick spray down and getting back into the lanes, the fronts were up to 29 psi, where I left them, but dropped the slicks back to 16 - leaving right on the appearance of yellow again, cut a -.002 (but ran a best-ever at altitude MPH of 99.24).

Next run I staged went back to 28 in the fronts, and when I went to stall it up, ran out of gas (oops) - they pushed me back and I trudged up to the pit to get the gas can. The next two runs were "mediocre", .022 & with some tire slip, .033 (13.522, best ET at altitude ever), but I know I wasn't dead-on the light on those. The staging lanes were closing, so I jumped back in for one more, .009 with a LOT of tire slip (lost a tenth in 60') - guess they weren't going to waste any more VHT on us.

DA was around 6400' during the runs, temp 47-48 degrees (they were lighting the VHT - when they sprayed any - to get it to evaporate), wind calm, humidity was around 80% - moisture was condensing on the face of my weather station. If the humidity had been "normal", I'm sure it would have been down into the 13.4's.

Looking forward to the races this weekend (temp will be back up into the 80's and dry). Nice to have a launch plan that doesn't involve an internal stop-watch.

[Trevor86TA]

Good to see that you were getting some good lights out of the new routine. 6400' seems like pretty low DA for you guys - what's the lowest you've ever had? And 99.24 - wasn't there something special at the end of the year for the 100mph club?

[five7kid]

And 99.24 - wasn't there something special at the end of the year for the 100mph club?Yes, they had that run-off on September 11th...

[five7kid]

6400' seems like pretty low DA for you guys - what's the lowest you've ever had? Best I can recall, about 6200' at Super Chevy in 2002. It had rained Saturday, snow at about 6500', storm moved out overnight and Sunday morning it was clear and about 38 degrees. No time trials before eliminations, nobody had a clue how to dial.

[Jaberwaki]

your cutting some great lights sounds like you got yourself a very consistent car there...
what kind of 60's are you cutting? i question 2 things.

1. why less tire pressure in front?
2. why so soft on the rear suspention?

[Jaberwaki]

well the only reason i could think of for number one would be weight?

meaning reducing the weight by reducing the ammount of air in the tire?

and given that rotating mass weights more the faster you go even a small weight must matter at the big end?

since i will not be here to see your answer, the thought intreeged me so i searched and worked some math...

i found that it takes about 66 liters of air to fill a tire. (average size) to 30 psi at room temp. at sealevel
and 30 psi is 2.0 atmouspheres

air weights 1.25 grams per liter at 1.0 atmouspheres.

so lets say we increase the tire pressure to 35 psi

35psi = 2.33 atmouspheres

so....

66 liters x 1.25grs x 2.0at = 165 grams @ 30psi

and

66 liters x 1.25grs x 2.33at = 198 grams @ 35 psi

giving a 33 gram differance for 5 ps1

or

6.6 grams for 1psi.

there are about 453.6 grams per pound

meaning 6.6 grams = about 0.095 lbs.

i cant seem to find the formula for rotating mass, but i cant see 1.1 ounces changing your weight enough to over come the added friction being given by the tire with less pressure. ???

sorry if i gave anyone a headache too early in the day :D

thoughts?

[Trevor86TA]

I think the theory on the tire pressure is that the softer tire will absorb more blow and take a touch longer to react to the touch of the gas pedal. If he was getting small redlights before, taking a hundredth out here and there while leaving with the same technique can only help.

[Jaberwaki]

I think the theory on the tire pressure is that the softer tire will absorb more blow and take a touch longer to react to the touch of the gas pedal. If he was getting small redlights before, taking a hundredth out here and there while leaving with the same technique can only help.

which i would agree with for the BACK tires, but the front tires dont absorb anything at the launch.

(FTR i was refering to the front tires.)

[Trevor86TA]

OK...the theory with the fronts is rolling resistance. Everyone pumps the fronts up when looking for slips as they create a bit less drag when inflated to the max. Airing them down will create a bit more drag and will slow the car a tad.

[Jaberwaki]

again i agree with, but only at the big end of the track, not in the grooves IMHO

[five7kid]

The theory on the fronts is roll-out. The taller the tire, the quicker it lights the staged bulb relative to the prestage bulb when you stage, and the more you "roll out" before clearing the staged beam when you launch. Therefore, the longer the time between starting to move and clearing the staged bulb.

As for front tire pressure, same theory. With more pressure, the tire sits taller, therefore presents less of a block down at the staging beams. Less pressure, more tire across the beams, etc.

So, to decrease RT's, increase front tire pressure and/or use a shorter tire. To avoid redlights, decrease front tire pressure and/or use a taller tire.

By the way, more roll-out also means quicker 60', and probably quicker ET, because you're getting more of a "running start" - the timer doesn't start until you clear the staged beam.

Since I combined front tire, rear tire, and rear suspension changes all at once, I don't know how much each affected the RT. I suspect each was worth a good hundreth plus, with the tall tire probably producing the most.

[five7kid]

Oh, about my 60' times: I've typically been in the 1.89-1.91 range this year, with 1.87-1.88 being common since putting on the RPM Air Gap intake manifold. Last night there were 4 in the 1.861 to 1.866 range, one 1.877 (2nd run), and the 1.966 for the last no-traction run.

The soft rear suspension was a new concept to me. The explanation was it absorbs some of the initial hit. The tires still planted well, I suppose partly because of the lower tire pressure - which also helped to absorb some of the initial hit.

Throwing out the 1st and redlight runs, average RT was .015. With them, .021. I think I could live with that most race days, except we typically get 2 time trials, so that third run would have been the 1st round of eliminations!

[five7kid]

Saturday Fall series:
TT's, .018 & .013. 1st round of eliminations, .006, 2nd round, -.026.

Sunday Fall series:
TT's, .019 & .063 (I knew I didn't hit the light for the 2nd one). 1st round -.012.

Good thing the dog wasn't waiting for me back in the pits. Took it out on the lug nuts changing back to the the highway tires.

4 Fall series race dates, 2 round wins, 4 redlights.

This is approaching silliness.

[Jaberwaki]

good info thanks... :thumbsup:

[five7kid]

Launch RPMs is probably a big variable that is not completely in control. I don't have a 2-step rev limiter, so my launches are probably varying at least 200 RPMs - even though I try. I'm going to try to get a better handle on that this weekend and see if that changes anything. A fellow racer might have a 2-step he can loan me - that would require some wiring changes.