2-Speed vs. 3-Speed Savage Tranny

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Doesgo

Doesgo

Well-Known Member
Messages
290
Location
Minnesota
I'm trying to calculate through the driveline gears of the Savage 2-speed and 3-speed in order to approximate road speed given no variables (power, drag, tire growth, etc.). The mathematics aren't the problem, the Savage's transmissions are the problem, or the gears therein.

From what I see in my Savage 21 manual, when in second gear, the top shaft is using the 23-tooth gear to spin the 39-tooth gear on the idler shaft, then the gear at the end of the idler shaft (32-tooth) drives the 29-tooth gear on the bottom (output) shaft. So input to output I get 23/39*32/29 = 0.65075.

Applying that to the 3-speed, the largest gear on the top shaft is 24-tooth, it mates to a 38-tooth gear on the idler, the gear at the end of the idler driving the output shaft gear is a 31-tooth, and the output shaft gear itself is a 30-tooth. Thus 24/38*31/30 = 0.65263.

There's a difference, but it's EVER so slight, and given the same rpm, bell, spur, and tire diameter, the difference in top speed is less than a tenth of a mile per hour. HPI claims 10% higher top speed potential due to the different gearing in the 3-speed, so obviously I'm doing something wrong....or is HPI's advertising more than a little misleading?
 
that's pretty deep doesgo. i hope someone in here has some productive information to keep this going enough to decide if we're being misled. i have seen some info on other websites who have used radar and i believe telemetry and who claim something like 5-10 mph difference. i bought my savage x from you and my 3spd tranny and never ran the 2spd that came in the truck, so i don't know from experience w/ the 2spd if there is a difference. i bought a 3spd for my bro-in-law and he swears his truck is faster. i had a 3.3 revo and he was beating me, but i was new to tuning and having some other issues so that's probably not a good example.
 
thats really interesting to read as i've also read that the 3 speed supposedly adds alot of top end speed now u have me thinking maybe it was exaggerated!!
 
Maybe this will help. I am not sure where I found these (probably on this forum but I can't find where so I uped them again)

the second page has the ratio's for both gear box's

13.jpg


22.jpg
 
Thanks Wister! I'll run through the gears again and try to come up with the numbers they did in the magazine so I make sure I'm calculating correctly. HPI already got back to me, which was nice, but the guy didn't have any good information for me.
 
Thanks for posting the magazine pages! Unfortunately they don't tell the whole story, instead a very misleading portion of the story.

I got no help from HPI, but I've figured out a couple things. First, a little background for those don't understand how it works and want to know. If there's anyone fitting that description, that is... ;)

If you look at a transmission you see the input shaft, the case, and the output shafts. If there were no speed-changing gears inside the tranny (like in a buggy or truggy), the output shafts would spin at the same rate as the input shaft.

With a two-speed Savage, however, there are gears inside. There are three shafts with gears on them. The top shaft (input) has two gears (18-tooth and 23-tooth) and they drive two gears on the middle shaft (44-tooth and 39-tooth). The 23T top-shaft gear drives the 39T middle-shaft gear, and naturally the 18T top-shaft gear drives the 44T middle-shaft gear. Next to those two gears on the middle shaft is another gear (32-tooth) that drives a gear on the bottom (output) shaft (29-tooth). First gear in the Savage uses the 18T/44T gearset, second gear uses the 23T/39T set.

Note: the top shaft spins at the same rate as the spur gear, the bottom shaft is what drives the center driveshafts.

A gear ratio is calculated simply by dividing the number of teeth on one gear by the number of teeth on the gear it drives. Then divide one by that number. For example, the two-speed's first gear uses the 18T top gear and the 44T middle gear. To calculate the ratio: 18/44 = 0.40909. 1/0.40909 = 2.44. Therefore, the gear ratio is 2.44:1. For every 2.44 rotations of the spur gear, the middle tranny shaft will spin one time when in first gear. The gear ratio for second gear is 23/39 = 0.58974. 1/0.58974 = 1.70, so that ratio is 1.70:1. So in second gear, 1.70 rotations of the spur gear will spin the middle tranny shaft one time.

Now we apply that thinking to the 3-speed transmission.

The top shaft has three gears instead of two, but the math is the same. The gears are 18T, 21T, and 24T. The gears on the middle shaft are 44T, 41T, and 38T. First gear uses the 18T/44T combo, just like in the 2-speed, for a ratio of 1:2.44. Third gear uses the 24T/38T combo for a 1.58:1 ratio. Just to complete the picture, the middle gear on the three-speed has a 21T/41T gear set which is a 1.95:1 ratio.

To sum up:
2-Speed
1st Gear: 2.44:1
2nd Gear: 1.70:1

3-Speed
1st Gear: 2.44:1
2nd Gear: 1.95:1
3rd Gear: 1.58:1

Remember, all else being equal, the "steeper" the ratio (the higher the number on the left side of the colon), the faster the acceleration rate, and the lower the maximum speed. That's why 1st gear has a steeper ratio than 2nd gear, etc.

-----------------------

Okay, that wasn't so bad, was it? Well, this is where I have problems with HPI's claims of 10% better acceleration and 10% higher top speed just from slapping in the 3-speed transmission:

Remember that gear on the end of the middle shaft I mentioned above, along with the gear it drives on the bottom (output) shaft? If they were the same on the 3-speed as the 2-speed, the story could end here. BUT THEY'RE NOT! The 2-speed uses a 32T/29T combo in this location, while the 3-speed uses a 31T/30T combo. Why do I care? Because that difference plays a part in the overall gearing situation of the transmissions and thus, the acceleration rate and top speed. The top-shaft-to-middle-shaft ratios don't mean much on their own if there are more gear differences farther down the line.

Let's quickly run through the numbers and just get an input shaft speed ratio to output shaft speed ratio for low and high gears in each transmission. That's what we really care about, after all. The gearing inside gets us there, but the final numbers are what are important.

Overall Gear Ratios
===================
2-Speed
1st Gear: 18/44*32/29 = .45141 = 2.22:1
2nd Gear: 23/39*32/29 = .65075 = 1.54:1

3-Speed
1st Gear: 18/44*31/30 = .42272 = 2.37:1 (6.7% increase)
2nd Gear: 21/41*31/30 = .52927 = 1.89:1
3rd Gear: 24/38*31/30 = .65263 = 1.53:1 (0.7% increase)


What's it all mean? Well, if my calculations are correct and all else is equal, switching to the 3-speed transmission will get you 6.7% faster acceleration in first gear and a 0.7% higher top speed. The acceleration improvement is nice, but not 10% as claimed. And the 0.7% increase in top speed? You'd never even notice it. At 40mph, that's a difference of only 0.28 mph. Oh, and there's additional rotating mass inside the tranny due to the addition of two metal gears, which will have a slightly negative effect on acceleration. The middle gear of the three-speed helps bridge the gap between low and high, however, which will also aid acceleration.

I believe HPI's claims of 10% better acceleration and top speed are inaccurate and very misleading. I have trouble even figuring out where the 10% number came from. Sure, when you show a vehicle's top speed in an advertisement we let it slide because so many variables are involved and in a perfect world with a perfect tune and a perfect tailwind, perhaps those ideal numbers could be reached. But this is a direct-swap transmission. All it does is transfer power from the input shaft to the output shaft and change the rotational speed along the way. There are no variables, it's pure mathematics! If you change the overall gear ratio by 10%, you can potentially get 10% more speed. But they didn't change the overall ratio very much at all, so how could someone POSSIBLY get a 10% improvement?

If I'm wrong somewhere, please tell me! My purpose isn't to blast HPI; most of you know I've been a huge HPI supporter for over five years! My point is that if you're considering HPI's 3-speed transmission to improve your Savage's acceleration and speed, use these numbers to put the expected outcome into perspective before you drop $50 or more on a new tranny.
 
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kinda interesting ive been thinking about a 3 speed. One thing is I really don't need any more bottom end and wasnt sure if the little bit of top end was worth all the cash. Thats why I have never invested in a 3 speed.
 
Doing the 3-speed could give you more top end if you just regear your bell/spur combo appropriately. Then you'd maintain your current rate of acceleration in first gear, enjoy the benefits of having that middle gear (which will also likely improve your acceleration), and then gain 7% on the top end, all else being equal. 7% for $50 (plus re-gearing costs) though, might be a bit hard to swallow. 7% more top end on a 35mph truck will only bump it to 37.5mph.
 
UPDATE

HPI is releasing their high-speed "X" 3-speed transmission as a complete unit and also will be offering a the high-speed 3rd gear gearset separately as well. These gears are 26T/36T and will replace the current 3rd gear's 24T/38T set, which will increase the mathematical top speed by about 14%, or nearly 15% over the 2-speed.

In my opinion they should offer this gearset for free to anyone who has an older 3-speed simply because it didn't (and couldn't) live up to their 10% claims. But I'm not holding my breath...
 
DOESGO,

Hi here is the gear set, I back ordered a set to see how they do, for 10 bucks I figure it is worth it.

http://www.horizonhobby.com/Products/Default.aspx?ProdID=HPI77065

I am lost on yor thread with the calculations, I guess it is hard for me to follow because I have no idea how the transmission works, I do understand the ratio's between 2 gears, I will have to read your thread again, I would like to understand this better, It seems you have it down, but the only thing that makes me question your math is, well I would think some competetor would be all over this unless they just never bothered calculateing it, the other problem I have with it is that it would be pretty foolish fr hpi tooutright lie or mislead people, it could turn around and bite them in the ***. I am by no means disputing your math, I cannot because I don't quite understand it, but I would really like to.

this brings up a question I have, since you seem to have a good grip on this I would be be gratful if you would check my math and see if I did this correctly, what I did was to calculate the numbers to enter into the nomadio sensor for the speed sensor, the numbers for the tach I just guessed at by spinning the cb and countingf the turns till the other side of the transmission spun once I couldn't begin to calculate them.
here's the link to what I am talking about https://www.hpisavageforum.com/forums/showthread.php?t=3534 I am pretty sure I got the speed settings correct, but well like I said I just guessed the tach settings. Sorry for getting off subject, but if you have the time I would really be thankful.
 
I completely agree with you, it makes no sense for HPI to claim something they can't back up! That's why I keep saying my math might be wrong and I encourage anyone to tell me if it is, and where! I've even contacted HPI about it, but the guy who's been talking to me doesn't seem to understand gearing too well.

This is really bugging me because I like HPI a lot and I don't think they'd intentionally mislead their customers, yet I just don't see how the gears included with the transmission can do what they claim.

What's funny to me is even the magazine article is wrong. They seem to have missed the fact that HPI changed the bottom gear and the end gear on the middle shaft, which screws up the so-called gearing advantage.

I'll check on the speed sensor thing...
 
hi guys...pretty impressive findings.

I'll tell you though I had the 2 speed on my savage xss and just installed the 3 speed about a couple weeks ago. I've been in this hobby for over 20 years no and I immediately experienced a marked difference with the 3 speed. Definitely the acceleration was much better and so is the top speed. I do not have actual figures but I can definitely feel a marked difference and I haven't even tuned the shift points perfectly yet.

Just my 2 cents...
 
someone w/ a telemetry kit needs to review this w/ the 2spd in and the 3spd. anyone? i have the spektrum, but i can't shell out the money for the telemtry set-up. any racers?
 
hey doesgo, do u have the spektrum or namadio w/ telemetry? if u do u could probably get a good answer from running each transmission.
 
No, I had a Nomadio for a while, but it's gone. We've got a radar gun available, though. Problem is my 3-speed is fragged right now (just found out a couple hours ago), so I'm not sure when I'd be able to get speed runs in, especially back-to-back on the same day.

I'm hoping to throw those high-speed gears in it when I rebuild it. I'm not too sure I want to put the stock gears in, do a run, swap to a two-speed, do a run, and then take it apart yet again and put in the high-speed gears! I'm a team player and all, but... ;)
 
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Plus like you said, in a perfect world the radar gun thing would help, but in our orld, I think it would be very hard to get anything usefull just by going full speed with both transmissions, everything would have to be the same for both runs and we know ho that goes....

I called HPI once and got a guy that had no idea what he was talking about, I mean he was really talking nonsense, must have been very new a the job, I really needed to know the answer to the question and was aggervated at the nonsense this guy was telling me anyway I asked to speak with someone more knoweledgable with the product on the phone, I did get to talk to someone that was very good, In fact I have even had that guy call me back without having to, because he didn't completely answer my question when I spoke with him the first time, I guess they got my number from CID. So you can get to the people that know whats going on, just ask for someone else, it does work.

I hope you are wrong about the gearing too, but I did think it was strange when this new high speed gear set came out and they don't make any claims on how much faster, just that is is better top end, did you notice that?
 
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Yeah, there's something messed up with their three-speed stuff, but few people are talking about it.

Now they show the old 3-speeds (87218 and 87220) and also the new X 3-speed (87257), but they have essentially the same description and performance claims. What makes it an "X" tranny, though? It's got a beefed-up case, but it's not accessible through the top like the stock X 2-speed. Maybe that design just isn't beefy enough? I was thinking of switching my transmissions to the X case, but perhaps I won't...
 
i swear i have been reading dis 4 da last hour and its only just making sense
BUT i have been having so much trouble getting it 2 hit seccond and wen it does it is stupidly quick so y bother unless ur drag racing and den y do u have a Savage y odnt u get a onroad car
but doesgo i take ma hat off 2 you with all dat maths
 
Why bother? Because even in off-road, speed is fun! I got the 3-speed because I wanted better acceleration and higher top-speed. I'm just disappointed that the transmission isn't what HPI claims it is.

And I do have an on-road car. :)
 
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