This is a re-post from the Brushless Oval Racing Thread - I wanted to make sure the guys who don't hop forums were able to see the info...

I appologize for posting the same info in multi locations, but I want the results to be seen for those interested...


...The saga of the 17.5 continued this past weekend...

Saturday June 9th - Metro R/C Speedway - JOSH SMITH running the 17.5 w/ the 3200 Carbon Lipo, once again a very strong showing against some heavy hitters in the 4300/4 cell class. I believe Josh finished 2nd in the main, but more impressive was still the fact that this motor competes are virtually the EXACT same level as the 10.5 on 4 cell....

I'm sure if Josh or Don reads this they will chime on on setup and/or other info.

Sunday June 10th - Racers Haven R/C Raceway - I had some spare time on my hands just running one class, I decided I'd try running the same motor Josh ran on Saturday...on Sunday with just 4 cells.

That race had a small 'sportsman stock' class, and I've been promising to give this motor a try as a STOCK type B/L replacement because a lot of guys have found the 13.5 motor a bit faster than their liking.

The plan was to run one qualifier and the main, since I didn't so any practice or any other running...the heat should let me know how close my calculations were going to be on the gear. Well dummy me, got the main order screwed up and missed the main so I was only able to get one run in...

The cool part was the days TQ ran a 62/5:02.50 in the previous round - so that was the target I was aiming at. I ended up running a 61/5:05.29, and some how..and for some reason (uh, cause the battery I ran had just run the 4300 class 10 minutes before and didn't have a full charge) was dumping at the 5 minute mark.

In 2 days of racing, I ran 2 packs of 4200's at least 10 - 12 times each. By the time I ran this 17.5 motor this battery pack had run 5 times that day in about a 4 hour period...in 95+ degree weather.

The TQ guys times in this heat were

5.2
4.7
4.7
4.7
4.5
4.5
4.7
4.7
4.6
4.7

for the first 10 laps. He had the hole shot, and checked out.

I on the other hand, not having any idea how the car would handle running these slower speeds (This was my 10.5 car) grided the car in the rear, and let the rest of the guys go on the tone before I pulled the trigger...here's my first 10 laps.

5.9
4.7
4.7
5.8
4.8
4.8
4.8
4.9
4.7
5.1

Most of the next 3+ minutes were pretty much the same 4.8 to 5.0 range, but then with less than 30 seconds to go...it really fell flat..and times dropped to

5.0
5.2
5.4
5.4
5.6

for the last 5 laps.

I will also note, my batteries are NOT batteries I would race in the STOCK Class cause the voltages suck in part to my abuse, but because I'm a mod guy I buy run time packs, where the other guys in the class had really good stock packs...and they are babied....with all that info, I was VERY pleased to be so close with no other runs....

I had planned to drop 2 teeth in the main, just to see how that felt and if it improved times at all. (The ratio I ran was a 100/58 w/ 2.17 - 2.18 tires)

With NO further testing..and I was to give this motor a grade for 4 cell SPEC or SPORTSMAN type racing, I'd give it a A++ rating. I also let NOVAK know this..and the reasons I felt as WHY...so IF you and/or your group is looking for a possible SPEC/SPORTSMAN type STOCK based B/L motor to be used w/ 4 cell - THIS COULD BE IT. (I hope to be able to get a few more runs in with in the month...and I really think it's going to go faster yet)

Something I didn't really mention on the original posting was...if you notice I ran about a .1th slower than the TQ(Leader) in those early laps. But, I must also say since I started at the rear of the field..and let them get a good jump so I could be in clean traffic, I caught the group on lap 2 so I was in traffic the entire race (the cars were so evenly matched) and because we were battling, the leader was able to pull away a bit w/ no challenge from us.

I'm not sure why your pack fell flat at the end, but I would suggest actually gearing it up, a lot. It looks like you had about a 3.95 rollout, judging by the lap times that sounds about right for a 13.5 motor on that track. It seems that the 13.5 likes to be geared about 60 points higher then a 10.5. If you carry that logic forward I would guess a rollout in the neighborhood of 4.50+ for that track and a 17.5 motor. That would just be my guess.

PM,

I agree, I felt before hitting the track that the 17.5 motor would probably require closer to a 100/60 and thought I was starting conservative.

I think I mentioned...I was running a really abused 4200 pack that had been run at least 5 times that morning already, including the 4300 heat just about 20 minutes earlier (IT ONLY Got 20 minutes of charge into it...I was busy changing motors I didn't have time to change the pack...I just immediatedly put it on the charger when I got off the track..and the race director was kind enough to give me a pass on corner marshalling to get the motor swapped in time.) I didn't think battery would be an issue...so I didn't worry about topping it off any harder, and making time w/ the 4300 motor wasn't an issue and it had plenty of RIP w/ that motor.

I did have a fresh pack ready for the main, but they ran the STOCK "A" before the 4300 "B" and I put the wrong motor in first, and didn't realize it until I was carrying it to the line...

I am very excited about doing some more testing with this..and will focus on running JUST it instead of double duty next time...

I will also have another driver testing it as well - (since I have 2 of them now)

Where the 13.5 motor is approx 3300KV or RPM per volt, I believe I calculated the 17.5 motor to be closer to 2600-2700KV or RPM per volt...

The 10.5 motor is rated at 4200 (use to be 4300KV), so the difference between a 13.5 and 10.5 is larger than the difference between the 13.5 and the 17.5

Thats some good info

The fact that it was so close to stock with a questionable battery, maybe not the right gearing and probably not the most ideal run is a bit surprising. I think one time I mentioned wondering if adding turns to the BL motors would result in diminishing returns. Just hang even more gear on it and still go the same speed after a certain point. I wonder if that is ending up to be true?

PM, that's the same way I've felt about the B/L deal too. KNOW the differences in the motors...RUN for POWER - GEAR for RPM

if a 10.5 runs 4200 rpm per volt UNLOADED, and you (I'll use 5.0 volts for simple numbers) use 21,000 unloaded RPM - I figure @ 30-35 amps you'd lose what...20% or so, plus what ever you lose in the corners with the speed drop (and I don't care HOW fast you are in the corner, I've proved..the cars ARE slower watching them w/ the radar gun) but that drop can be another 8 - 20% depending on the track and/or how hot of a motor class you are running (for instance, a 3.5 will drop a LOT more than a 13.5)

So for yucks, I'll use a RPM drop of .25% so if I take the 21,000 unloaded RPM and x's by .75, a 4300 motor under load will be close to 16,000 RPMs

Take the 17.5 motor @ 2700 RPM per volt unloaded...it will have less RPM drop in the corners, and I figure unloaded w/ 5 volts it should be about 13,500, and with the drop around 80% or a 20% drop. So my guess is the 17.5 will run about 10,500 LOADED on 4 cell.

If you were to be able to run a 200 ft. track in 4.00 seconds per lap, that would be an average of 50 ft. per second. With a 2.15" tire (.56287 ft. per rev. of the tire) that would be 88.83 Tire Rotations per second, or an axle speed of 5330 RPM. 10,500 RPM motor divided by 5330 AXLE RPM would be a starting ratio of 1.97:1 or a 100/51 (or really close) for a rollout of 3.44 (THIS Is for the 17.5 motor)

Adjust accordingly for YOUR situation...

Or at least that's the way I start...

You're too scientific with it for me! I just go by gut feel. :-)

...I like going by feel too, but I've always liked crunching numbers on this stuff...and verifying the numbers on the track can be fun too...

A lot of times though, I can crunch the numbers..but need someone else driving the rig to get the times...(I get lazy behind the wheel..and with the wrench)

I've got about 8 tracks where I know what rollout I ran, fast lap times, track length and corner diameter (approximately). I could put all of that into a spreadsheet and then attempt to derive a relationship between the variables. Have you tried doing anything like that?

I've got about 8 tracks where I know what rollout I ran, fast lap times, track length and corner diameter (approximately). I could put all of that into a spreadsheet and then attempt to derive a relationship between the variables. Have you tried doing anything like that?


I did stuff like that back when we had 'tracks' that didn't change..and we were running stock.

Since I switched to Mod, it's been tougher trying to keep up w/ what motors to run, what the power/rpm numbers were...

The first couple years running the 'limited' mod class we had, we had to use 1400 packs so we went with a mod motor that was efficient at the lower amp output of those batteries. (We calculated we had approx 23 amps continuous for 4 minutes) That was if we used a TORQUE LIMITING ESC, set to 30-35 amps...so as not to SPIKE the amp load off the starts. (Which we could otherwise see as much as 100+ amps off the line causing the batteries to run 8 - 15 seconds less time.)

I recently found some old notebooks w/ rollouts, gear ratios, battery voltage tables...from 1992 (The last year I cared enough to keep a full notebook)

It was very cool reading some of it...sadly the ONLY facility still running that we used then and now is the ENCINO VELODROME

We DO have a couple tracks now, that the #'s could be tracked, but I don't race at them...or don't race at them enough to work up a great set of notes. Hopefully, we'll get good notes again on the METRO(formerlly SHOWTIME) track....since we've got quite a few dates there now...