FAQ: HPI Baja Top Speed and Gearing info...
 

I was wondering about 5b gearing, and ratios etc, as a full set of steel gears is not in the budget right now, so I got to calculating the ratios using only the available hop up pinions, and the stock plastic spur. Assuming a gearbox reduction of 2.4, a tire diameter of 170mm, and a top engine speed of 19,000rpm, the ratios and top speeds came out as follows.

Pinion (T) / Spur (T) / Final Ratio / Rollout (mm) / Top Speed (mph)

16 / 57 / 8.550 / 62.46 / 44.2mph
17 / 57 / 8.047 / 66.37 / 47.0mph (stock setup)
18 / 57 / 7.600 / 70.27 / 49.8mph

For comparison, here are the calcs from the popular RR steel gearsets...

16 / 58 / 8.700 / 61.39 / 43.5mph (Torque set)
18 / 56 / 7.467 / 71.53 / 50.7mph (Speed set)
18 / 55 / 7.333 / 72.83 / 51.6mph (Top Speed set)
19 / 55 / 6.947 / 76.87 / 54.5mph (Super Speed set)

Although rpm/torque loss caused by drivetrain friction and engine/pipe choice, may lower these figures a little, the swelling of the tires at speed will cause an increase in final drive, balancing out somewhat. For example, if the tire diameter expands to 180mm, for an increase in total tire diameter by 10mm, the theoretical top speed of the 'Super Speed' set increases by 3.2mph.

RE: FAQ: HPI Baja Top Speed and Gearing info...
 

Gearbox ratio is 2.4:1

We have a crew that is pulling 60mph with the 19/55 combo at about 18,500. We use these numbers to get an idea of tire size at that speed (They do expand).

My two speed has 50, 52, 54 spurs for the second gear. But, I have only been running my two-speed in the sand so I haven't tried any land speed runs with it yet. With the right engine it should put the baja up in the mid 70's low 80's.

The 20t is not a pinion, it is the layshaft gear that goes inside the gearbox.

RE: FAQ: HPI Baja Top Speed and Gearing info...
 

will a 20t pinion bolt on without a smaller spur?

RE: FAQ: HPI Baja Top Speed and Gearing info...
 

I trust ya Brice, you'd know, but I just want to point out that with 17/57, HPI puts the final ratio at 8.382 (it's in the specs of the car). The tires are 170mm, that's not in question (and for the purposes of just raw figures like this, I'm not going to apply tire 'swell' to the equation, even though it would be a factor, I know), which leaves the missing part of the equation (gearbox reduction) at 2.5. Where am I going wrong? Let me know and I'll edit the figures appropriately.

Revoqprs, based on Brice's reply, I'd say no. lol, it's not even a pinion, he's right. Like I said, I ordered it accidentally. [&:]

RE: FAQ: HPI Baja Top Speed and Gearing info...
 

Interesting...

RC411's review of the RTR says: 17/57 / 2.4 / 8.047
HPIracing.com product info says: 17/57 / ** / 8.382

WTH is going on here? The missing figure from HPI's info, is 2.5. It's the only that gear ratio can be calculated to get the correct final ratio. Misprint on HPI's site? Figures from a prototype?

Check it out... http://www.hpiracing.com/kitinfo/10600/

RE: FAQ: HPI Baja Top Speed and Gearing info...
 

The diff gear is 48t, the layshaft gear is 20t, and the idler gear is 30t. The idler does not affect the ratio. So 48/20=2.4 or 30/20=1.5, 48/30=1.6, and 1.5*1.6=2.4.

I would say that HPI site is wrong.

Perhaps the earlier prototypes had the 2.5 gearbox and they never updated their numbers

RE: FAQ: HPI Baja Top Speed and Gearing info...
 

OK, thanks Brice, I'm gonna update the figures using a gearbox ratio of 2.4. Good to know I'm not going mad, it's just HPI. :eek:

No excuse for that kind of error.

RE: FAQ: HPI Baja Top Speed and Gearing info...
 

Figures all fixed, check out the original post. Thanks again Brice.

RE: FAQ: HPI Baja Top Speed and Gearing info...
 

I have a question, how come you didn't use 18/58 into mix? Integy makes alloy spur thats a 58tooth and HPI has an alloy 18t.....Just a question....or are you just using stock ratios?

RE: FAQ: HPI Baja Top Speed and Gearing info...
 

He probably wasn't aware of the available ratio. I certainly wasn't.

Anyways here are the numbers.

18/58/7.733/69.06/48.9mph

RE: FAQ: HPI Baja Top Speed and Gearing info...
 

with the 19/55 gear combo and 18/55 combo , how come when i bought a 18t pinion it didnt match up to my 19/55 combo ? are they matched sets ? i wanted to run a 18t on my 19/55 set so all i had to change was the 19pinion to the 18t and the gear wasnt meshing up , why?

RE: FAQ: HPI Baja Top Speed and Gearing info...
 

Yes at the moment the sets are matched. Because you can't move the engine or the gearbox you have to fudge the gear numbers and tweak the pitch when you are cutting the gears to get the right mesh. For this reason they are matched.

RE: FAQ: HPI Baja Top Speed and Gearing info...
 

why is the 18/55 set more expensive than the 19/55 ?

RE: FAQ: HPI Baja Top Speed and Gearing info...
 

Just wondering i've been looking for a 19/55 super speed gear set for my Baja, but can't seem to find them????? Did RRP stop making them?

RE: FAQ: HPI Baja Top Speed and Gearing info...
 

Tyre swell or ballooning isn't as big a speed gain as you might imagine, as they ballon traction decreases meaning high speed slippage.

Nice work on the raw numbers btw, its intresting to put a figure on the potential differences between the gearing options, they really don't seem to add up to a whole lot. In reality you have to be careful with overgearing a vehicle, even with a big enough runway the engine may not have the high rpm torque to fully spool out if the gearing is too high, and a similar setup with lower gearing can actually have better top end.

RE: FAQ: HPI Baja Top Speed and Gearing info...
 

tire ballooning can be a huge difference in top end speed, how do you think top fuel dragsters have become able to push over 300mph....its the new tire design, more ballooning=more top speed....traction at the speed when the ballooning occurs usually isn't a issue since its already almost on max rpm range risk of slipage is minimal...running surface has alot to do with speed also as does the tire tread, slicks are faster than tires with tread...concrete is faster than blacktop...alot of factors in speed running involved.....ultimate speed rc car would be a turbine driven without gearing to cause friction and speed limitations....maybe i'll toss my jetcat 120 into the old traxxas mb i got here and give it hell on the salt flats this august and see what kinda speed it can push

RE: FAQ: HPI Baja Top Speed and Gearing info...
 

Oh sure, controlled ballooning could be useful with a tyre that is designed to do it, problem is its not that well or easily controlled on these RC tyres unless you start venting them with different numbers of holes to find what works best, the control issues with them expanding at different rates will enduce torque steer which then has to be manually countered which will cause friction and reduce your overall speed - that is if they don't just tear on you. I've played alot with tyre venting and using tyre balloon to try and act like an extra gear, problem is when you start hitting the kinds of speed where it would come into play wind drag becomes an ever increasing factor and you do need all the traction you can get to push it beyond that.

I've out dragged other similar setup nitro's with little tyre balloon compared to them, it really isn't as big a factor in victory as you'd think - and if you have to make a turn in your race forget it, unless you've got lots of holes for the air to escape through then you'll have some control issues.

RE: FAQ: HPI Baja Top Speed and Gearing info...
 

Thanks Foxy for the gear ratio/top speed chart I've been wondering which ones to get and you settled it!

RE: FAQ: HPI Baja Top Speed and Gearing info...
 

I was confused when Robinson made the 18/55 and Hostile made thier plastic spur a 56 to go with the Robinson or Turtle 18. I copied the below from a post elsewhere. They didnt have any speed info but the chart is nice and it was good info. i hope it helps:

The fixed gear mesh on the HPI system uses a total number of 74 teeth. To change gearing and use the perfect tooth profile, the pinion must be changed with the corresponding spur to total 74 teeth. For example: 17/57, 18/56, 19/55 and so on... For convenience, HPI chose to offer pinions of 16, 17 and 18 while still using the 57 tooth spur gear. The 17 being the proper gear to mate with the 57, meant that they would need to modify or deviate the tooth profile of the 16 and 18 tooth pinion to still create the same diameter and proper mesh. This deviation of 1 tooth either way is still within reasonable tolerances to be functional and convenient. It does however require the tooth profile deviate from the perfect shape and does have some drawbacks.

With the 16 tooth pinions made for the 57 tooth spur gears, the deviation of tooth profile and spacing to compensate for the missing tooth creates more space between the teeth. This spacing creates the situation of only one tooth being engaged at one time. This means all of the force and pressure is being applied to only one tooth. With the 17 tooth and larger pinions, the spacing is such that there are always 2 teeth engaged thereby cutting the forces applied to the teeth in half resulting in a stronger drive connection. This is the prefered set up.

In keeping with perfect tooth profiles for longest wear life, most efficient transfer of power and highest strength, Hostile Racing Products designed thier spur gears to work directly with the perfect matching pinion as well as allow the use of the deviated HPI 16 and 18 tooth pinion with the 57 tooth spur as well as the Robinson 16 tooth pinion for the 57 tooth spur.

http://www.hostilehobby.com/images/gearchart.JPG

This is from the DDM part listing:

These high quality injection molded gears offered in the stock 57 tooth size as well as 56, 55 and 54 tooth to match the 17, 18, 19 and 20 tooth pinions respectively.

Matched gear sets will also be available with pinions made by Robinson Racing.

Hostile spurs offer:

1. Harder material for longer life and less wear.
2. Higher strength material to reduce gear failure
3. Handles higher temperatures
4. Lightweight and economical alternative to steel
5. Available in several sizes for a variety of gearing options
6. Made in the USA

how would acceleration differ from 18/56 to 19/55

It will get a tad slower or more sluggish in the acceleration, but more top speed.

first is 3.111:1
Second is 2.894:1

Baja 5b
 

Hi I ready buyer for my baja 5b buggy scale 1/5 48 spurgear and 26 gear bell and when I put in the floor is not running but when I put the original gears is running good thank sorry for my English but you can't help me