RCU Forums - OFFICIAL HPI Sprint 2 Thread (Sport and Flux)

<span style="font-size: medium">www.drccentral.com/old/hpi-sprint-rc-drift.html</span>

CLICK the top link to reach the archived web page - It shows the pictures that won't show in what I pasted below -

Also, this article is dated so some of the links incuded on the web page are dead.

<div style="margin: 0in 0in 0pt"></div><div style="text-align: center; margin: 0in 0in 0pt; background: #e5eaf5" align="center"><font size="4"><span style="font-family: 'Arial','sans-serif'; color: #333333">R/C Drifting FAQ</span></font></div><div style="text-align: center; margin: 0in 0in 0pt; background: #e5eaf5" align="center"><span style="font-family: 'Arial','sans-serif'; color: #de007e; font-size: 10pt">HPI</span></div><div style="text-align: center; margin: 0in 0in 0pt; background: #e5eaf5" align="center"><span style="font-family: 'Arial','sans-serif'; color: #de007e; font-size: 10pt">The Complete Sprint Hop Up Guide</span></div><div style="text-align: center; margin: 0in 0in 12pt; background: #e5eaf5" align="center"><span style="font-family: 'Arial','sans-serif'; color: #333333; font-size: 10pt">Sprint Hop Ups</span><span style="font-family: 'Arial','sans-serif'; color: #333333; font-size: 10pt">
The following parts are listed by manufacturer and are followed by the part numbers for easy ordering... If a part takes modification to fit, I'll note it in parenthesis beside the part number. The list will be constantly updated...

<span style="font-family: 'Arial','sans-serif'">**Drivetrain**</span>
-HPI Aluminum Center Pulley 15T Purple #72172 (2)
-HPI Pro Spur Gear Hub LW RS4 Pro 2 #72039
-HPI Ball Differential #86040
-HPI Front One-Way Differential 32T Sprint RTR #86042
-HPI Universal Dogbones Sprint RTR (2) #86043 (2)
-HPI Thin Aluminum Hex Hub Purple RS4 Pro 4 (4) #72044
-HPI Ball Bearing 5x8x2.5mm (2) #B020 (2) (belt tensioner bushing replacement / ball-diff bushing replacement)
-HPI Ceramic Nitride Differential Ball (12) #75053

<span style="font-family: 'Arial','sans-serif'">**Chassis**</span>
-HPI Sprint Front Shock Tower Graphite # 73106
-HPI Rear Shock Tower Woven Graphite #73107
-HPI Woven Graphite Upper Deck 2.0mm for Saddle Pack #73108
-HPI Main Chassis 2.5mm Woven Graphite #73829
-HPI Graphite Bumper Brace Pro 4 #73086

<span style="font-family: 'Arial','sans-serif'">**Steering**</span>
-HPI Turnbuckle Set RTR/Nitro RS4 #72064
-HPI Titanium Turnbuckle Purple RS4 Pro 4 #93464
-HPI Ball Bearing 5x8x2.5mm (2) #B020 (2)
-DuraTrax Aluminum Servo Arm Single Purple #DTXM5012
-HPI Aluminum Servo Mount 10mm Purple RS4 Pro 4 #75167

<span style="font-family: 'Arial','sans-serif'">**Suspension**</span>
-HPI Tie-Rod Set Sprint RTR #86007
-HPI Pro Aluminum Threaded Shock Set 48-60mm (2) #75142 (2)
-HPI Competition Aluminum Threaded Shock Set (2)#75194 (2)
-HPI Composite Threaded Shock Set 50-55mm (2) #72423 (2)

(if you perform the pro 4 block swap, the following will fit)
-HPI Carbon Graphite Block/Hub Set RS4 Pro 4 #73507
-HPI Carbon Graphite C Hub Set 0 Degrees RS4 Pro 4 #73516
-HPI Carbon Graphite C Hub Set 2 Degrees RS4 Pro 4 #73517
-HPI Carbon Graphite C Hub Set 4 Degrees RS4 Pro 4 #73518
-HPI Carbon Graphite C Hub Set 6 Degrees RS4 Pro 4 #73519
-HPI Titanium Turnbuckle Purple RS4 Pro 4 #93464
-HPI Titanium Turnbuckle M3x24mm Purple RS4 Pro 4 (2) #93462

<span style="font-family: 'Arial','sans-serif'">**Aftermarket Goodies**</span>
-Square R/C HPI Pro4 Aluminum rear hub carrier (0.5 deg.) (Pro4 block swap only)
-Square R/C HPI Pro4 Aluminum rear hub carrier (stock deg.) (Pro4 block swap only)
-Square R/C Pro4 Aluminum Front Steering Knuckle (Pro4 block swap only)
-Square R/C Pro4 Aluminum 3 Deg. Front Caster Block (Pro4 block swap only)
-Tobee Craft Light Weight Aluminum Rear Body Mount for Pro4 (will work on stock Sprint)
-Tobee Craft Light Weight Aluminum Front Body Mount for Pro4 (will work on stock Sprint)
-Integy Super Lightweight Antenna Mount Purple</span></div><div style="margin: 0in 0in 0pt"><hr width="100%" size="1" align="left" noshade="" style="color: #333333" /></div><div style="margin: 0in 0in 0pt; background: #e5eaf5"><span style="font-family: 'Arial','sans-serif'; color: #333333; font-size: 10pt">Blue Print Your Sprint's Steering for $12-20</span><span style="font-family: 'Arial','sans-serif'; color: #333333; font-size: 10pt">
Your sprint's steering uses bushings as opposed to bearings, adding resistance to your cars ability to steer which limits your servo's transfer speed. After performing a mod like this your car's steering response will feel much crisper.

*Note* Before performing this mod, take a good look at your steering set up to help better understand what needs to be done. Look at how the top deck contacts and pinches the top of the steering arms where the bearings will be installed. The top of these plastic arms will need to be sanded down (slightly) and have .2mm spacers installed between the bearings and the top deck to prevent contact from occurring resulting in friction in the steering set up. If done correctly, there will be a small gap in between the arms and the top deck so that the two will not contact each other.</span></div><div style="text-align: center; margin: 0in 0in 0pt; background: #e5eaf5" align="center"><span style="font-family: 'Arial','sans-serif'; color: #333333; font-size: 10pt">
http://www2.towerhobbies.com/c.....65&P=7
More economical alternative-<span style="color: #006699">http://www2.towerhobbies.com/c.....S5&P=7</span>

2) 5x7x.2mm shims

(this is the only way that you can get shims with the right dimensions, but you can use the 10mm shims to get slop out of the front & rear differential fitment by placing these shims in between the diff housing and the 10x15mm bearing)

3) 1000-750 grit sandpaper

Step One- Begin by removing the top deck from the Sprint to gain access to the steering rack. Disconnect all the linkages from the rack (except the metal bar that connects the two arms) and remove both arms from the aluminum steering posts. After removing the arms there should be two collars left on the posts that the arms sit on, remove them as well.

Step Two- Now that the rack is out, remove the black bushings from the top and bottom of both arms (should be 4 total) and toss em in your spare parts box. Lay the 750-1000 grit sandpaper on a flat surface (like a table, desk, etc.). Begin by sanding the 2 black collars. You only want to sand the top of the collar (the part of the collar that contacts the bottom of the steering arm). Sand in a circular motion as though you would wax a car to help insure that you don't sand the surface unevenly. Also make sure that you hold the collar perfectly upright. Your only trying to sand enough to make space for a .2mm wide shim, so don't over sand. Once this process has been completed for both collars, slide them back onto the aluminum steering post and then slide a 5x7x.2mm shim on top of them.

Step Three- Time for the steering rack itself. Repeat the sanding procedure explained above on the top of the steering rack (the part of the rack that contacts the chassis top deck). Do not over sand. After sanding, clean the accumulated dust off the rack arms and install the bearings in the top and bottom of the arms. Slide the rack assembly back onto the posts (make sure that you remembered to put the 5x7x.2mm shims on top of the collars to prevent the collars from contacting the bottom of the steering arms). Now install the remaining 2 shims over the bearings in the top of both arms to prevent contact between the top of the plastic arms and the top deck. Test fit the chassis top deck to make sure that you sanded enough to prevent the top of the steering arms from rubbing (you may need to test fit the set up a couple times while sanding, I did). Amazing difference, huh? The whole setup moves with ease, smooth and completely friction free. Now go hit the pavement and enjoy your blueprinted steering setup!

*Note* If you want to take it a step further, you can remove the metal bar that connects the 2 steering arms and put a turnbuckle in its place so that you can make toe adjustments for the front wheels!!!</span></div><div style="text-align: center; margin: 0in 0in 0pt; background: #e5eaf5" align="center"><span style="font-family: 'Arial','sans-serif'; color: #333333; font-size: 10pt"><hr width="100%" size="1" align="center" noshade="" style="color: #bec9e1" /></span></div><div style="text-align: center; margin: 0in 0in 12pt; background: #e5eaf5" align="center"><span style="font-family: 'Arial','sans-serif'; color: #333333; font-size: 10pt">
<span style="font-family: 'Arial','sans-serif'">Pro 4 Suspension Swap</span></span></div><div style="margin: 0in 0in 0pt; background: #e5eaf5"><span style="font-family: 'Arial','sans-serif'; color: #333333; font-size: 10pt">What this will accomplish-
1) Higher graphite content makes the hubs stronger than stock
2) Allows for conventional turnbuckles to be used for camber adjustments
3) Bearings sit much more snuggly into the hubs, eliminating wheel slop due to the bearing sleeve being too loose or stretched
4) The car will be slightly lower with stock sprint dampeners after this swap, but it is recommended that you switch to a coil over dampener (HPI now makes a composite coil over which is said to be lighter than the aluminum coil over I run, and has updated features and improvements and runs about $35 for all 4-5) The Pro 4 hubs will actually increase the efficiency of the Sprint's belt drive (Spacers that come with the kit allow you to torque the wheel nut down without laterally pinching or straining the inner races of the 5x10mm ball bearings)
6) Your turning radius will be greatly decreased due to the fact that the new steering setup will give maximum sweep compared to stock (refer to the picture at the bottom of the previous page on this thread)
7) Through the use of spacers that secure the hubs firmly and prevent movement along the length of the suspension pins that retain the hubs, you will be able to adjust the length your wheelbase by 5mm

What you will need-
1) Pro 4 Blocks (2 sets @ $5.79 each) (part #73507)
2) Pro 4 C-Hubs (1 set @ $10.19) (part #73518)
3) HPI Aluminum Washer 3x6x.75mm (1 set @ $3.89) (part #Z814)
4) HPI Aluminum Washer 3x6x2mm (1 set @ $3.89) (part #Z817)
5) HPI Steering Block Bushings (1set @ $3.89) (part # 75122)
6) HPI Sprint Suspension Pin Set (1 set @ $5.79) (part #85021)
7) Dremel tool with cutting wheel
Drill with 1/8 size bit, preferably something that will not mangle plastic but instead leave a clean hole (I used Bosch 1/8 titanium self tapping bit for this)

(*Note*- just wanted to give a warning, parts for this project come out to be $39.23, but for the tunability and performance I believe it is well worth it)

Step 1- Begin by disassembling your suspension and stripping everything off of your suspension arms. Even though you will be replacing your blocks and hubs it is advisable to hang onto all old parts anyways. Your Phillips head suspension shafts that secure the blocks and hubs to the arms will also be useless, b/c of the threaded section near the head. Measure the length of the stock Phillips head pin, and make marks on your new plain shafts (refer to picture below), and cut the plain shafts to the length of the Phillips pin with a Dremel tool fitted with a cutting wheel (Wear goggles and be careful with your fingers!).</span></div><div style="text-align: center; margin: 0in 0in 0pt; background: #e5eaf5" align="center"><span style="font-family: 'Arial','sans-serif'; color: #333333; font-size: 10pt">

</span></div><div style="text-align: center; margin: 0in 0in 12pt; background: #e5eaf5" align="center"></div><div style="margin: 0in 0in 0pt; background: #e5eaf5"><span style="font-family: 'Arial','sans-serif'; color: #333333; font-size: 10pt">Step 2- Now we prep the new Pro 4 hubs. The suspension pins used in the Pro 4 to secure the hubs and blocks are smaller than the 3mm thick pins the Sprint uses, so it is necessary to bore out the hubs (refer to the pic below). The 1/8 size drill bit is slightly larger than 3mm, so there will be a small amount of play in the suspension shaft when installed into the hub- this will be corrected with setscrews (don't worry, the setscrew holes are already pre-drilled into the hubs and blocks, so no need to drill this too). Bore both the rear hubs and both of the front C-hubs as well.</span></div><div style="text-align: center; margin: 0in 0in 12pt; background: #e5eaf5" align="center"><span style="font-family: 'Arial','sans-serif'; color: #333333; font-size: 10pt">
</span></div><div style="margin: 0in 0in 0pt; background: #e5eaf5"><span style="font-family: 'Arial','sans-serif'; color: #333333; font-size: 10pt">*Note* You will notice in the picture that there are 2 spacers pictured, when you assemble your hubs with the bearings these spacers will be sandwiched in between the bearings inside the hubs- without anything filling the empty space between the bearings inside the hub, pressure is placed on the inner races of the bearings when the wheel nut is torqued down tight. What essentially happens is the inner races pinch the chrome balls against the inner edges of the outer races... you might have to read that a couple of times... and will result in added friction when the setup is assembled. In a stock Sprint, this is not an issue because the wheel axel is allowed to float, but what this amounts to is a large amount of lateral play within the wheels which will translate into twitchy handling and the car not wanting to track straight. The Pro 4 hubs will allow the wheels to be held much tighter and thus have less play, but at the same time not cause this pinching that will cause bearing failure, high efficiency.)

Step 3- The Pro 4 hubs are slightly narrower than the Sprint's stock hubs are, so once you do the initial mock up of the assembly the hub will have a good amount of lateral (from side to side) movement on the newly fabricated suspension shafts. To correct this on the front, you only need to use two 3x6x.75mm aluminum spacers on either side of the C-hub. The rear will require one 3x6x2mm spacer on one side of the hub (refer to picture below), and a 3x6x.75 on the other (refer to picture below). The spacers will provide a snug fit and may require a little sanding of the hub at the contact point of one of the spacers to assure a perfect fit (as it did in my case). Its important not to over sand, a snug fit will eventually loosen with regular use of the car, and will eventually be the perfect tolerance). Also notice the set screws in the middle of the lower part of the hubs. The hub on the left in the picture below had to have the pre-drilled setscrew hole sanded down, due to the fact that there is a 'ring' that protrudes out and will contact the outside of the A-arm (the area that needs to be sanded is in the red circle in the picture below)</span></div><div style="text-align: center; margin: 0in 0in 12pt; background: #e5eaf5" align="center"><span style="font-family: 'Arial','sans-serif'; color: #333333; font-size: 10pt">

</span></div><div style="margin: 0in 0in 0pt; background: #e5eaf5"><span style="font-family: 'Arial','sans-serif'; color: #333333; font-size: 10pt">Step 4- Now that the suspension shafts have been cut to size and the hubs have been bored out and fitted to accept the spacers, mock up the hubs onto the arms with the suspension pins set in place and the spacers on. There are setscrew holes pre-drilled into the hubs that will secure the suspension pins, but to make sure the setscrews do they're jobs and firmly retain the pins during cornering, we will need to prep the surface that the setscrews will contact on the suspension shaft. With everything lined up the way it should be, stick a needle through the setscrew hole in the hub and scratch the suspension shaft so you will be able to tell where you will need to sand. After this has been done on each shaft, remove the shafts and either sand or notch them where you see the needle scratches. This assures that everything will line up for final assembly with the setscrews.

Step 5- Now that everything is completed (spacer in place between the bearings inside the hubs, hubs bored out and sanded to appropriate width to accept spacers, and the suspension pins cut to the appropriate length and notched for the setscrews) you can assembled the arms. The last and final step is to insert the wheel axel, connect the dogbones, and screw ball studs into the new hubs for the turnbuckles. Congrats! Now you have a cleaner looking, completely adjustable and lighter suspension setup that will also improve your steering due to the fact that your hubs will now allow your bellcrank to turn the wheels to their limit!!</span></div><div style="text-align: center; margin: 0in 0in 0pt; background: #e5eaf5" align="center"></div><div style="text-align: center; margin: 0in 0in 0pt; background: #e5eaf5" align="center"><span style="font-family: 'Arial','sans-serif'; color: #333333; font-size: 10pt"><hr width="100%" size="1" align="center" noshade="" style="color: #bec9e1" /></span></div><div style="text-align: center; margin: 0in 0in 12pt; background: #e5eaf5" align="center"><span style="font-family: 'Arial','sans-serif'; color: #333333; font-size: 10pt">
<span style="font-family: 'Arial','sans-serif'">Pro4 Coil Over Tuning and Building</span></span></div><div style="margin: 0in 0in 0pt; background: #e5eaf5"><span style="font-family: 'Arial','sans-serif'; color: #333333; font-size: 10pt">There are a couple things to keep in mind before you build the shocks... whatever you do, protect those shock shafts. Leaky shocks are super annoying- The riskiest thing you will do during assembly is put the ball connect onto the shock shaft. Whatever you do, don't grip the unprotected shock shaft with a pair of pliers.</span></div><div style="text-align: center; margin: 0in 0in 12pt; background: #e5eaf5" align="center"><span style="font-family: 'Arial','sans-serif'; color: #333333; font-size: 10pt">

</span></div><div style="margin: 0in 0in 0pt; background: #e5eaf5"><span style="font-family: 'Arial','sans-serif'; color: #333333; font-size: 10pt">When you assemble the shocks and are getting ready to thread the ball connections onto the end of the shaft, protect the shaft by wrapping a hand cloth around the shaft (half fold the terry hand cloth around the shaft). Take needle nose pliers, and clamp the cloth wrapped shaft. USE THE FLAT PART OF THE PLIERS... the pliers have a raised serrated surface to grip the heck out of stuff. Further up the face of the pliers, the serrated raised part turns flat. Use the flat part of the pliers face to grip the shaft through the towel. If you use the serrated part, it may cut through the cloth under pressure and mar the shaft, which will cause leakage. Also make sure the shaft has been cleaned. If you try to grip an oily shock shaft, it will just spin in the cloth as you try to thread the ball connect.

Also, pre-thread the plastic ball connect with a screw before you put it on the shock shaft. This will ensure that it fully threads on the shaft, and you will not have to back the ball connect out several times.

Many people advocate using masking tape to protect the shock shaft (the HPI directions also recommend it). Don't do it! Masking tape is weak, and when you use pliers to secure the masking-tape covered shaft, they will cut straight through the tape and knick your shaft. I've done this before, and it really sucks...

Also, when you assemble the shock, you will have to thread the shock shaft (with the piston attached) into the shock body through the o-rings (before you pour the oil in). Before you thread the shock shaft through the o-rings in the shock body, lube the shock shaft with the same silicone oil you will be running. What this does is lubes the threaded part of the shaft so that it won't tear the o-rings (it may if the threaded part is dry when you thread the shaft through the dry o-rings)...

if you keep this stuff in mind, you will have a set of leak free shafts...

Also, a word about tuning the shocks-
The end goal of building should be to have 4 smooth shocks of equal length and dampening characteristics. You can ensure this by using a gap gauge and a caliper as you build-
Caliper-<span style="color: #006699">http://www.glue-it.com/model-e.....er-001.jpg</span>

gap gauge-<span style="color: #006699">http://imageserver.isnweb.com/KDT164.jpg</span>

When you put the shock cap on after filling the shock body with oil, screw the cap down but don't screw it so tight that you can no longer twist the cap. Treat it like an oil filter... you don't want to screw too tight. You just want the cap to create a seal with the shock bladder. Once you get the cap to the desired tightness, use the gap gauge to measure the gap under the cap (you may not understand it reading this, but once you build the shock you will)... replicate this same gap under the cap of every other shock. This will ensure that they have similiar characteristics.</span></div><div style="text-align: center; margin: 0in 0in 12pt; background: #e5eaf5" align="center"><span style="font-family: 'Arial','sans-serif'; color: #333333; font-size: 10pt">

</span></div><div style="margin: 0in 0in 0pt; background: #e5eaf5"><span style="font-family: 'Arial','sans-serif'; color: #333333; font-size: 10pt">After you have gapped all 4 shocks, use a caliper to measure the total length of each shock. If you get slightly different lengths for each shock, its because the plastic ball connects are threaded to different depths on each shock shaft. You fix this by using the longest length of the four shocks to tune the others.

Example: You have 4 shocks that are 42mm, 42.2mm, 41mm, & 42.5mm when they are fully extended. Its harder to thread a tight ball connect further onto a shock shaft than it is to back it out slightly. Because this is the case (and we don't want to hurt the shock shaft by putting them under too much pliers stress), we want to back all of the ball connects out slightly to attain an equal length of 42.5mm across all four shocks. Some of the ball connects may have mold-flaws that only allow them to thread onto the shaft for a minimum overall length of 42.5mm. If this is the case, you can try all day to thread the ball in further and it will get you nowhere. Therefore its better to tune all the shocks according to the longest one in the group.

</span><span style="font-family: 'Arial','sans-serif'; color: #333333; font-size: 10pt">Last edited by Sprinter on Sat Jan 13, 2007 11:26 pm; edited 7 times in total</span></div>