naynay12

On my hoemmade gas rc car 26cc weed eater engine in am on my second tank,the first tank i had a glitch ,it went into a tree and then tranmission gear gave way and stripped/gave way..In the second tank i was driving no more then 3/4 throttle maybe 40km max with short full throttle bursts.As i was driving the plastic tranmission gear gave way again when i was driving maybe 1/4 throttle.
It is an HPI 2 speed tranmission found in a HPI savage.THe gears are all metal apart from the two plastic first and second gear.
No what i am wondering is that thir are atleast 6 other people thatr has made a gas car with the same tranny?What would i be doing differently,and im not even driving it that hard?

Fabe

Hmm, the plastic gear gave way...I've got 4 questions. Did the people who used the HPI tranny have the same size engines? Do you think their weights were around the same or less? Did they have a slipper clutch of some sort? Were your short bursts of throttle similar to peeling from 0 to 40, or more delicate?

In my setup, I take the more gradual acceleration approach to keep the diffs alive. I'd say that the plastic wouldn't hold up terribly well if any of the above went wrong. Got any pics of the gears/tranny opened up?

Sorry to hear about your experience though. Down time is bad, especially when it's home-built.

naynay12

The people that were using the same tranny had engines ranging from 18cc to 18 cc as far as i know,none had problems.I normally dont gradually pull the trigger since i like the torque,and dust flying everywhere etc.A few vids i saw they were doing the same thing anyway,and my savage with the same tranny seems to make just as much torque.
My weight is similar,maybe a pound or two more.
One of my theories is that the homemade dogbone that connects the rear diff to the tranny,since it is not precisely engineered,when i turn the rear wheels it should move smoothly,though it is a bit more bumpy,if you understand me.So that could put pressure on the gear.The plastic gear is the weakest part in the drive train.

bsrman14

AS I have told you before," it's all about the gearing and weight"! Tooo much weight for the gear ratio you selected will cause damage. I suggested you lower the gearing or use bigger gears. Plastic will only last so long. I have never had a gearing problem ever!.


Origionator of the $250 1/4 scale bugget buggy.

mooman007uk

Hpi make all steel gears as an upgrade, I used to have em in my savage...but even with steel your asking a 1/8 trannie to handle 1/5 scale power and weight which might be asking a bit too much

naynay12

The all metal gears would be nice.I can't seem to find them anywhere you have a link?The gear that keeps stripping is the 44 tooth.

mooman007uk

here's the one i had[link]http://www.hobbyetc.com/cgi-bin/item.cgi?part_id=46402[/link]
this site has both the steel one and a titanium one
[link]http://www.thefind.com/query.php?query=savage+3+speed+metal+1st+gear[/link]
2nd gear steel is HPI77073 or they are on these sites as well

sam123

My Feedback: (1)

I think the main problem is the weight. IMO you should just remove the 2-speed gearbox assembly entirely, and just replace it with a chain drive going directly to the rear diff. This will save weight and make the whole car more reliable.

naynay12

Thanks alot mooman ,did you use to have it in your hpi 3 speed ,not 2 speed right?Since that gear is ment for the hpi 3 speed,so i would have to upgrade to a 3 speed,to use that gear.A bit pricey though probably my best option.What would be better,steel or titanium?
Sam123 if i have a problem after this, i will have to try a different setup.Now that i think about it, chain setup will work out cheaper,though there will be no real brakes??Any ideas?

sam123

My Feedback: (1)

No, you can still have brakes. You would have to mount the disk on the clutch output shaft. If you keep having problems with the transmission, you may want to think about a smaller engine also.

Fabe

As Sam mentioned and I've tried as well, chain can be more rugegd than these types of transmissions. The other question I had was does your engine have a clutch or is it the HPI trans that engages internally?

mooman007uk

oops sorry naynay I though you had a 3 speed, those links should also have the 2 speed metal gears...yeah i had a 3 speed . Steel is stronger than titanium but I'm still dubious that it'll hold up under the weight and power you have...no harm in trying though.

PEERPSI

[quote]ORIGINAL: mooman007uk


????????????????????????????????????

What side of the tooth is the wear occurring on? Could the damage be from acceleration or deceleration?

Could the damage be from friction heat softening the gear? If so metal ones may solve this problem.

Is there some sort of shock dampening in the drive train? The spur gears on my 1/5 scale Baja have rubber dampening to absorb shock loading.

naynay12

Yes the nengine has a clutch,when the plastic gear stripped,im not to sure if it was accelerating ior decerlarating.Theior is no shock absorbing on the drivetrain,apart from the actual shocks.Mooman are you sure they make 44t steel or titanium upgrade for the hpi 2 speed,since i can't seem to find one...

mooman007uk

[quote]ORIGINAL: PEERPSI


I'm sorry but since when has an aluminum gear been stronger than a steel gear??????? I believe your posted graph is in reference to mild steel. the strongest high carbon steel is stronger than the strongest titanium.

scarletboa

My Feedback: (1)

[quote]ORIGINAL: mooman007uk



+1........if i can ever figure out how the whole +1 thing works lol. i know why but not how

mooman007uk

i know I've seen steel gears in the past and i'll remember where when my brain comes back on line..busy last few days and i'm fried, link here is for all aluminum set[link]http://www.hot-racing.com/?partnumber=SAV1000H&c=129[/link]

SAVAGEJIM

As everyone has already said, it is because the Savage tranny is designed and made for 8th scale RCs and the miniscule torque of nitro engines, not the massive torque of our gas engines.

Gearing to weight is best used to refer to if an engine has the torque to accelerate the heavy vehicle. There is no doubt in this case that the engine he chose has more than enough power to run his RC.

Instead of gearing to weight, we need to look at the parts failure in his Savage tranny in a more engineering standpoint: materials science. What we need to look at is the load bearing capability of the gears in this case. Simply gearing up or down will not prevent the gears from stripping, though it will vary the load bearing somewhat. (That is really moot because gas engine easily make 4times more torque than nitro engines, so that massive torque would still overcome what little load bearing relief gearing higher at the pinion and spur would have given.)

In this case, Moonman gave the best advice: go with all steel gears inside the tranny case.

mooman007uk

the 4.6 savage ss has all metal gears so hpi sell them...still don't think they'll hold up though, but you can try.

SAVAGEJIM

I agree too. Something I noticed about HPI's nitro gears: They are not machined high carbon steel. They are cast, and strongly leaning to the iron side of teh iron-steel grades of metals. Thus, the Taiwan HPI metal gears are really best to only take the very small torques of nitro engines (and sometimes, nitro engines will even tear teeth off of HPI's taiwan metal gears).

bsrman14

Chain drive is way cheaper than a steel gear for a hpi savage. Bigger cars require bigger gears or drive line.




Orginator of the $250 1/4 scale bugget buggy.

SAVAGEJIM

He was trying to take advantage of a 2 speed tranny he had. Otherwise, I can see him mounting the engine with the shaft transverse to the CL and using like a chain saw sprocket and gears to drive a chain to teh wheels.

naynay12

I ended up finding only one stronger gear for the tranny,which is a alluminium 7075 gear,made by gpm.I payed $27 usd shipped.I couldn't find anything better,was hesitatinmg to buy it,since it is alloy ,not the titanium one that is made for the 3 speed tranny.A bit expensive ,but is i converted to sprockets and such,it would cost about that,with a bit of mucking around,and i would not have 4wd.Also i think the gears could strip on decacceraleration,since the vehicle is heavy(18 lb) and since i made the rear dogbone the drivetrain does not exactly run as smooth as something machined.
Also for be to get the titanium gear,which should definately last,i would have to by a 3 speed,or atleast the conversion kit,which i would be looking atleast another 60 usd shipped.
The hpi 4.6 2 speed transmission is what i have,all gears are metal apart from the 2 plastics ones for the first and second gear.
The reat of the drivetrain is pretty rugged,ill just have to hope this alloy gear holds up,otherwise i can see myself going chain drive...

SAVAGEJIM

GL on getting it all going. As for decelleration,do not worry abut the gears because the engine should disengage the clutch at idle and of course, the Savage tranny's brake is on the rear output shaft of the case.

As for brakes, I would look into adding another brake set in front of the tranny case. The single brake set is barely enough to stop a Savage, let alone a larger and heavier gasser RC. On 5th scales, for the ones that have only one brake set, the brakes are easily twice as large as teh Savage's brake set.

naynay12

Ok,so thats what the two little brakcets that are at the front end of the tranny are for.Will any savage break set work?