Hi, I had my first offroad race today in a 2wd b4 with a 21.5 motor.
I thought with this I'd be quite even with the field but I was at least 7 seconds a lap slower and was told it's to do with my gearing.

I've read a bit about it but still seems confusing. What set up do I need to look at for a 2wd B4 running a 21.5 Reedy Mach 2?

A 21.5? What speed control are you using? Is it in no timing 'blinky' mode, or something with boost?

That's likely going to need a huge pinion, especially if blinky, with the smallest spur they make. For reference the 17.5 blinky guys are typically using 30 to 35 tooth pinions with the smallest spur (69-70 tooth or so). A 21.5 likely needs a larger pinion than that. Some speculation on my part though, never tried a 21.5.

Even then I suspect it would be down on power, offroad typically starts with 17.5. 21.5 is only an on road thing, maybe oval, to the best of my knowledge. Depending on the brand of motor you might be able to just swap to a different wind stator, otherwise suggest getting a different motor.

Sorry I'm not sure what blinky is. My ESC is a Hobbywing EZRUN-60A.
I'm not sure what you mean by 'no timing 'blinky' mode, or something with boost?'. I'm really am an absolute beginner.

My offroad club runs a 21.5 class and the motor is approved for competing offroad so I don't have plans to change the motor.

Try to get top speed about 3/4 down the straight. You will be close to what you need speed wise.

Is timing advanced allowed in your class?

What spur and pinion are you running?

Also, post a link to the track you are running. Maybe the rules are posted and we can check.

Boost refers to ESCs that adds timing advance, boosting the speed and performance. Also known as dynamic timing or ramping. But stock got out of hand and was nearly as fast in some ways as modified.

So no timing rules were developed, which typically requires a blinking light on the ESC to confirm it is in no timing mode, thus ‘blinky’.

Important to know which rules the class is.

Gearing, if blinky try gearing up, bigger pinion / small spur. Watch motor temp, it can get too hot if overgeared, and go by track performance and lap times.

Ahh makes sense now. Then yes the class is 21.5 blinky. Will try a bigger pinion and small spur to see what results I get.

If the motor temp gets too hot will it just cease to work anymore?

Varies some by motor, but generally as the motor gets to around or over about 60C / 140F it likely will fade in power some.

Generally above about 70-75C / 160-170F it starts to demagnetize the rotor. That reduces power even when cooled back off and makes the motor run hotter. Most motors have replaceable rotors.

Hotter risks burning up the motor.

See if some of the guys will help you with an idea of the gearing that works for the class and track for a starting point.

As Dave H said, it will damage the motor if it gets too hot. The trick is to find a gearing that will give you the performance you want, but not burn the motor up.

Try different gearing. If you don't have a temperature meter, buy one, or ask one of the faster drivers at your track for help and ask if you can borrow their meter; they'll for sure have one and most people are friendly enough to help.

What gearing are you running right now?

For 21.5 blinky in a buggy, try to shoot for a final drive ratio (FDR) about 5.5. Take the number of teeth on the spur, divide it by the number of teeth on the pinion, then multiply that number by the transmission's gear ratio (your B4's is 2.4). So, say you have a 72 tooth spur and a 26 tooth pinion, the FDR is 72/26*2.4 = 6.6. To lower the FDR, increase the pinion, or decrease the spur. So, try a 32 tooth pinion with that 72th spur, and now you have 72/32*2.4 = 5.4.

Thanks Corners, seems like something I need to keep trialling until I get it right. My current gearing is 84/23 which seems quite high for the pinion.

I bought the car second hand which had a 12.5 turn motor so assuming by putting the 21.5 in with the same gearing made it a lot slower than it should be with a 215.

Yeah, that gearing is way too low (FDR is too high) for that motor and would be rather slow. Your theoretical top speed with a fresh lipo is 15.5 mph (25 kph). Try 72/36 and see how that goes (should be 28.5 mph (45.8 kph), FDR = 5.2). Watch those motor temps!

Rule of thumb, the lower the motor turns, the higher the FDR. Higher motor turns, use lower FDR. But overall, keep motor temps below 160F (71C). This isn't "exactly" the right thing to do (you should gear for the track), but basically, you keep lowering the FDR (bigger pinions, smaller spurs) until your motor temps start to get too high.

Edit, for some reason I though the B4 had a 2.4:1 transmission, but really has a 2.6:1. My bad. Changed my mind to start trying 72 and 36 and go from there. Here's a chart for you...
FDR - http://www.arroyoc.com/index.cfm?fus...=Gear&Plot=mid
MPH - http://www.arroyoc.com/index.cfm?fus...Speed&Plot=mid
KPH - http://www.arroyoc.com/index.cfm?fus...Speed&Plot=mid

Double Edit: I see you're from one of those countries that doesn't have their flag on the moon, so I included metric units.

Thanks for your help Corners. Do you think starting at 72/36 is on the side of caution? I'm just aware of overheating the motor. What happens when it over heats anyway. Fire, electrics cut out?

No problem. What DaveH wrote is right on. You will damage the motor above 160F. Not a fire. But you can render it useless.

That 5.20 FDR should be pretty close. It's not that conservative. What you settle on using will depend on the track, your motor/ESC, battery, and driving. We can't give you the final answer over the internet. You'll have to try it for yourself. If it's too hot, then go smaller on the pinion.