cork44

Hi, I just got a new to me dragster, it is super fast and I am trying to make improvevments in it. How do I calculate the potential top speed with my gearing? Its a single speed and the gears are 130t and 20t. I figure the engine is around 40,000rpm, and the tires are the standard 3.25" nitro dragster tires. What is the formula for top speed? Thanks

Nick2618

This page has some info on gear ratios and caculating top speed. Not sure if its only for electric though... Anyways check it out.

http://www.rcuniverse.com/forum/m_3255405/tm.htm

cork44

HI so if i do the math right, I have a 130t spur gear and 20t pinion gear and my tire diameter is 10.25". I come out with a top speed of 161MPH at 40,000rpm. Sounds good in theory doesn't it? I'd settle for half of that!

ahr43

Hi cork44 and welcome to the wild world of RC drag racing. Yea, 161mph would be pretty good if you can get there.

When we did the math on your set up, we came up with 58mph. Here's the formula we've been using for the R2E:

MPH = 0.0029 x Tire Size (diameter in inches) / Gear Ratio x RPM

Our rail is running belt drive at 2.00:1 transmission ratio, with a PGR of 3.11. Our O/A GR is 6.22, so pretty close to your O/A of 6.25. We are running TM Racing Tires with 3.25" diam and and RAE NEO2 motor turning 40,000rpm. We ran a best of 60.8mph during timing runs and a best of 57.7mph during eliminations last weekend, so that sort of validates the above formula.

Could be you're plugging in your tire circumference at 10.25" instead of your tire diameter at 3.25". Anyway, keep working at it and let us know what you come up with.

Hey nick2618 - thanks for the link. We were using a FASTMATH RC Calculator on another website; but it went away. This in-house link is a great substitute.

//AC//

cork44

Well I knew it wouldn't go 161MPH, but I was hoping for more than 58MPH. I did put in the 10.25" for the circumference. I'll have to bring it to the track to get a time, but it sure looks faster than that. Thanks

cork44

What is the final draive ratio? is it Transmission ratio, or pinion:spur ratio, or rollout ratio?

ahr43

Hi cork44,

Final Drive Ration = 130/20 = 6.5

Spur / Pinion = FDR (Final Drive Ratio). Your PGR (Primary Gear Ratio) and O/A Gear Ratio are one in the same. You have no transmission ratio to factor in. We are "assuming" your car is "direct drive".

Rollout ratio takes both the FDR and the tire diameter into account in achieveing a final figure, e.g., "rollout".

Assuming your rail is "direct drive", e.g. spur attaches to RR wheel hub. Pinion engages the spur and is affixed to the motor shaft. That is the RC electric drag racing standard at present. Our rail is an anomoly with belt drive. So we have to factor in a transmission ratio. Works out the same mechanically. We just suffer having additional rotating mass. Rotating mass is the bane of RC drag racing. Lose any rotating mass you can.

Hope this helps.

//AC//

cork44

Yes it is direct drive here it is:

ProRCRacer88

nice car man![8D]

cork44

Hold on! I made a mistake my gears are 120t spur and 30t pinion. According to the formula provided by nick, my rail should be able to hit 97MPH at 40,000rpm. Thats more like it! Thats what I'm shooting for!! Wish me luck, I'm gonna need it.

griz11

I saw one go 94 mph last Sat. Man that is quick. One guy from the club I go to hit 104 at the east coast nats. Watch out if the surface is bumpy. They had big problems in Cali with tires exploding on the electric rigs. Hit a bump get them in the air and whammo big chunks all over the place. I noticed some were running very thin slicks Sat. Less mass so they would tend to stay together if they got off the ground. I'll have some more videos next week from the 4th of July money race at Litespeed. Thats always a good race. Is that a custom built chassis? Doesn't look like anything I've seen before.

Griz

cork44

Yes this is a prototype dragster, check it out!

[URL=http://imageshack.us]

ahr43

Trick looking rig. Curious, what's the wheelbase?
//AC//

cork44

Thanks, the Wheelbase is 29 1/4"

Nick2618

That dragster looks awsome. Also i am glad that formula i found helped you.

2916jr

that might be a little lighter than walbern cars probably

griz11

I've seen the arrow shaft chassis before. Actually I have one. Mine is a dual shaft instead of a single. Looks good but I'd make sure the shaft is pretty stiff especially since its a single piece. Should be a killer.

Griz

2916jr

I hope the electric guys dont get a hold of this chassis we will definatly never beat them then lol

cork44

Hey Griz, I was thinking of reinforcing it with another shaft, but this one is pretty stiff. I want to keep it as light as possible. These shafts are pretty light, but every little bit helps right? How can I tell if I need it? Thanks

2916jr

how bad does it flex if fit flexes really bad then you will need to run a small shaft to the front of the car

ahr43

cork44,

Here's a thought that may help you in the thought process to double up on the forward shafts to stiffen the chassis up a little:

Having a long single spar AND a long drag link from the steering servo to the front axle MAY result in some unwanted draglink "whip" and resulting steering and/or servo issues. Maybe, maybe not. Just a thought. Draglink "whip" was an issue in full size rails back in the day. End result was teams inserting an idler halfway b/w steering box and front end. Not sure if that action transfers to 1/10 scale RC drag racing, but, again something to think about.

Anyway, please keep us appraised of your progress. Like any new car, it'll be a work in progress for a while. Good luck.

//AC//

cork44

OK, I learned many things today. 1.) This rail is FAST! 2.) AHR43 was right about the draglink "whip" 3.) I should have waited until my spektrum radio which is on its way arrived 4.) I need VHT and traction compound to keep from breaking loose. 5.) Runaway dragsters and curb don't mix well.

I ran a couple of passes today. It was running good, but I couldn't hook up in the parking lot with no VHT. After about 6 passes, the failsafe was acting up so I took it off. You guessed it, a couple of passes later, it took off to the left and smacked a curb. luckily it was only coasting and the only damage was the shaft and some scratches. I'm getting a spektrum in the mail any day now, so hopefully that part is solved. I did have too much chassis flex and I think the steering was affected. I am going to modify it for 2 shafts. What do you guys think I should do, stick to 2 carbon shafts or go with 2 aluminium shafts? Think one has an advantage over the other?

Heres the damage:

builder jim

[&:]WATCH THE WEIGHT, LIGHTER MASS'S SPIN FASTER, WEIGHT TAKES TIME TO SPIN UP.

studysession

cork44 -
Nice car. Sorry to see it crashed though.

cork44

Thanks, and its back in business and better than ever! I shortened it up about an inch and doubled up on the carbon shafts. Its very rigid now and doesn't flex under power. I have done a couple of quick passes and it stays straight as an arrow now!