TremorX

iTrader: (1)

I've been looking all over and I haven't had much luck searching, so I thought I'd post this here.

My LHS runs a VTA class, allowing 17.5 brushless or 27T brushed. I have an HPI Sprint 2 Sport and a Novak 17.5 and I've been having a real bear of a time trying to get this thing geared to the target 3.5 FDR.

Right now, I have a 69T Spur (AE) and a 48T pinion (Robinson) in it, which calculates out toEXACTLY 3.5, but I'm having a problem with both belts rubbing against the motor, with the rear belt wearing through after about 5 minutes. If there's a way to adjust the belts, I haven't found it.

I was wondering - what gears are people using with this car for VTA? If what I read is correct, the 25.5 motor's target FDR is also 3.5, so I'm thinking that either way I'd be running into the same problem.

I'm kind of new to RC stuff, so figuring out what spurs to start with is kind of over my head, but I know enough to find a pinion once I get the right spur. The problem I'm having is that I can't find any pinions that match what the spurs call for (ie, >50t in 48 pitch). I'm totally stumped.

Thanks!

kwkride

iTrader: (6)

I assume that you went to Gearchart.com to figure your gear ratios.
It's a great site but unfortunately they have the transmission ratio wrong for the Sprint 2. It should actually be 2.1305
That means with your 69t spur and 48t pinion, you are running a 3.06 fdr.
Try switching your spur to a 79t and you will be at 3.51

Also, keep an eye on those motor temps. When I was running my 25.5 motor with a 4.0 fdr it was getting pretty warm.

TremorX

iTrader: (1)

I was using comeseethis.com and the Excel spreadsheet on the VTA site, and, yeah, they all list it as 2.4375. Wow, this makes life a lot easier!

locked

iTrader: (1)

kwkride is right about the internal ratio. A bunch of sites have it wrong.

Hopefully once you adjust the gearing to the correct 2.13 IR, you won't have quite as much rubbing on the motor. I know what you are talking about, but I don't have a solution to it. It's the poorly designed motor/spur/layshaft pulley area on the HPI Sprint 2. Only certain gear combos work without the belts rubbing on the motor. You can adjust the layshaft slightly, but it only helps if just the rear belt was only slightly touching, you can adjust it a little so that the rear doesn't touch, then if the front one is rubbing the motor you can adjust the belt tensioner to hopefully free it up.

I'll refrain from commenting further about this car before I start getting too negative. There are just too many things I dislike about its design.

TremorX

iTrader: (1)

Just from the little I've played with it, I can definitely understand. I mostly just wanted something to jump into the Vintage class, and learn the ins and outs of driving a TC chassis. For that, it seems decent enough, but even being a total novice to RC I can kind of tell it's not the best design ever. I'm nowhere near experience enough to run up against the limitations of the car yet, though, so it's more than enough for me right now.

Thanks for the help!

redron123

iTrader: (87)

So what is the whole purpose of the 3.5 final drive ratio?

TremorX

iTrader: (1)

We run the older VTA rules, allowing more than just the one motor so no one has to buy new stuff to run.

metalflakeking

iTrader: (2)

I have been running the same chassis and motor combo for the last year at my local track. Here is what you need.... Associated spur 75t pt#3923 and robinson 39t pinion pt#1339. This will give you a final drive ratio of 4.09. This ratio is to be used with no dynamic timing ( spedo timing ), and a lipo battery. With the spec VTA tires and bodies this is where you want to be. This chassis is tricky with gearing but put a new rear belt on with this combo and you should have no issues. By the way check the instructions and you will see that the top layshaft that the spur rides on has ecentric bearing holders and can be "clocked" to adjust rear belt tension. Practice and learn to tune this chassis. It is inexpensive and durable = well suited for VTA.

redron123

iTrader: (87)

I did find an iphone app thats free that figures all fdr calculations for on road and offroad. It even has the sprint 2 sport internal ratio right at 2.13.

redron123

iTrader: (87)

I also figured out that you have to take tire height into consideration. The app i have does all this you just put all three inputs in and it gives you your fdr and a chart to adjust by.

Scottrik

iTrader: (261)

Pulling this thread out of the vault...

I am in the process of helping set up a new Sprint 2 for VTA and dug through the manual to find the gear sizes. Came up with 32:15 for the same 2.1333 ratio mentioned above.

Went to GearChart.com and found, again as described above, that they show 2.4375 "internal" ratio.

So I did a search to try and find "who's right" and found this thread, so I'm feeling much more confident about what I calculated.

IF YOU WANT TO PRINT YOUR OWN GEAR CHART ON GEARCHART.COM with the correct info just enter the info for an HPI RS4. Turns out they have the same 2.13 ratio.

Now to find out if my 88t spur and 47t pinion (3.99 FDR) will fit without belt problems, etc. If I HAVE to I'll use one of my 77t spurs with a 41t pinion (makes both gears smaller...less chance of belt rub) but I'd REALLY rather use the 88's as they're pretty much "obsolete" for my racing now since 1/12 pretty much requires spurs in the 70's now.

Not sure how you're getting 2.1305 (not that it matters a WHOLE bunch) but the 32:15 gearing divides out to 2.133333333333333333-repeating to infinity.

I'll have to be sure and watch temps. I know with our cars we've found 4.0 to be pretty much THE setup for our typical layout so we don't even think about it much. Any lower FDR only raised motor temps without a corresponding drop in lap times.

When you mix tire diameter into the mix you're finding "roll-out". Roll-out is only really important if you're running foam tires where the tire OD changes. For rubber tire classes the tire diameter is moot (it really doesn't change) so we use FDR in those classes. FDR is only taking gear ratios into account and ignoring tire diameter. This COULD lead to a challenge if you're looking at two different classes that run a significantly different tire diameter, but the VTA tires are pretty much the same diameter as TC rubber tires so it's a non-issue.

Ffejdat

iTrader: (5)

Scottrik- I run a Sprint2 in VTA myself, the problem with the gearing is, if you run the motor too high in the mount it will rub on the belts, to get the ratio you want and keep the motor away from the belts I would recommend a larger spur gear, like a 104, presuming 64 pitch, then pick the appropriate pinion to get the ratio you want. We run our VTA cars in Minnesota on 1s and a 17.5, , for those motor batt combo we shoot for a 3.0 fdr, I do that with an 88 spur and 62 pinion... Basically you want the motor as far away from the spur as possible, without hanging out the bottom of the car...

Ffejdat

iTrader: (5)

The other problem with a Sprint2 chassis is, the motor clearance is pretty tight for the wires of some brushless motors on the left metal chassis plate. I clearanced the plate little where it hits, but again running the physically biggest gear combo you can helps, plus it puts the weight of the motor lower in the chassis...
The plastic bearing holders that press into the upright metal side plates, that hold the spur gear layshaft, tend to fit very loose in the plate, and get looser over time, leading to sloppy gear mesh and worn out gears and belts. I take them out of the plate, and put a thin layer of tape, such as electrical tape, around the mounting area, then put back together. You can also put some CA around the outside of the plastic, LET IT DRY, then put back together, basically just trying to make the plastic bearing holder fit a little tighter in the metal part...

Scottrik

iTrader: (261)

When they were looking at changing the USVTA motor/battery spec we tested several combos and provided our input. For naught. You guys are SPOT on with your 1s/17.5 combo, though the gear ratios needed (we had limited time but had gotten down below 3.5 and were still showing improvement) would likely have obsoleted some cars.

Good info on going with the "largest" gear combo. We're running everything pretty much at 4.0-ish on our typical track. I have a bunch of "obsolete" (for me) spur gears which I can combine with larger pinions out of my 1/12 kit. I can do the aforementioned 88:47 (3.99FDR), 96:51 (4.01FDR), 100:53 (4.02FDR) or 108:57(4.04FDR). That last combo seems too crazy to even fit...the motor and layshaft would be WAAAAYYY apart with that one.

Great input on the layshaft bearing holders too. Maybe I'll try a dab of Shoo-Goo (nee Amazing Goop) on each of the holders. The neat thing with these products is that they shrink when they cure so they tend to hold non-porous things like this together very nicely yet peels off reasonably easy.

Any other Sprint 2 tricks or tips would be very much appreciated...I'd like to make getting started with this chassis as painless a process as possible for some of our newbie racers here.

What has created this interest in the Sprint 2 is that the folks at the LHS and I spent some time trying to find the lowest cost new setup that an interested newbie could jump into VTA with and we hit upon the Sprint2 vintage combo since it already has legal body, wheels and tires with it and has a 2.4 radio for $240. Throw in a Novak VTA combo for another $160 and a LiPo battery (and some appropriate gears) and we SHOULD be at the races.

The LHS brought a couple of these in and a supply of the typical "crash" parts. Now, thanks to HPI's "Demo Program" they have decided to make one of the cars available as a Demo with the complete VTA "set-up" as best we can determine it. I'm headed over there in a little bit to install the motor/speedo and gearing so we can let folks try it out at the races tomorrow.

ADDED BONUS is that we even have a "home" for the cast-off motor and esc from the Sprint 2 RTR...summers we run a "spec" class specifically for RTR Blitz which use these items. The motors have a very finite life and even the esc's have been known to get cranky.

Ffejdat

iTrader: (5)

Sounds cool... i have no other problems to report on this chassis as I haven't gotten to run it much, but i bought this chassis on purpose over a more expensive car to show it can be done inexpensively, plus I think the gear diffs and shocks work very well for this class, and the chassis is just flexi enough for rubber tires... I was gonna put the stock ESC and Motor in my F103 pan car, and I could not find the Sprint 2 with the legal camaro body and tires anywhere in town, i ended up with a drift version....