WE RIDE HT RACING’S PROTOTYPE KTM 350SXF MOTOCROSS BIKE:

July 4, 2010
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Bill Thomas from HT Racing is an aerospace engineer by trade. He also has plenty of experience in the motocross industry; working with Team Honda and running a Honda support team are just a few of his credentials. HT Racing isn’t about the money for Bill?it’s about fun. Luckily for MXA, Bill can’t ignore his dirt bike fascination. To keep himself entertained between Boeing projects, Bill specializes in doing big-bore KTMs (and if you were a hardcore KTM aficionado, you’d know this already).

Three and a half years ago, Bill got a call from KTM asking him how close he could get to a 350 big-bore using a 250SX as a test mule. Bill’s answer at the time was, “Not very close.” But, he agreed to work on developing a prototype engine for KTM to reach their goal. What was KTM’s goal? 350cc of displacement, 50 horsepower and 30 foot-pounds of torque. What did Bill and HT Racing want for their work? No money?just the parts needed for the project and the permission to publicly claim their role in the development. KTM agreed, and for the MXA wrecking crew it was a rare opportunity to talk to Bill about the development of the prototype KTM 350.

Bill and HT started building, testing and tinkering with all kinds of big-bore KTM modifications in order to select the correct path to follow. He first built a 309cc, went to 312cc, boosted that to 330cc and eventually crept closer to the 350-50-30 goal. HT Racing built nine different engines over a span of about 30 months. The math finally topped out at 340cc. It turns out that going bigger than an 85mm bore would encroach on the water cooling passages. The final 340cc version put out 49 ponies and 29 foot-pounds of torque. KTM was more than satisfied with those numbers. Nevertheless, HT Racing continued to work. He got another head and figured out how to make a proper combustion chamber by rewelding it and recutting the water passages. Through this process, Bill and KTM learned a ton about all the other engine components that needed to be updated to suit the jumbo engine. For example, 20 ounces of tungsten were added to the crankshaft, while a 41mm Keihin carb was used.


 
This 350cc beast couldn’t breathe though a stir-straw sized 250 pipe, so FMF custom built this header with a larger diameter for this project.

It is true that HT Racing never handed KTM the 350cc engine they were looking for, but by this time the Austrians had taken over the development of the production engine. But the last prototype that HT Racing built is still running and is surprisingly owned by Superfeet Insoles owner Tim Norton. Norton is one of those Vet guys with meticulously prepped bikes that is in love with mechanical trickness. As a Vet racer at Glen Helen, Tim prefers to race big-bore 250Fs?and that is where HT Racing comes into the picture. Norton convinced Bill Thomas to build him a special one-off version of HT Racing’s final prototype…and as a kicker, Thomas promised to give Norton’s bike the extra displacement that KTM never got.

Thomas built the prototype for Norton, and after he raced it once, he handed it over to the MXA wrecking crew to test. This was a rare opportunity to test a piece of the 350SXF development puzzle (and get a preview of the highly anticipated 2011 KTM 350SXF).

SHOP TALK: HOUDINI WITH A WRENCH

Our test bike’s powerplant was slightly different from the final HT Racing KTM prototype. The final KTM prototype had an 85mm by 60mm bore and stroke (340cc), 13:1 compression ratio, 250 works camshaft, factory ignition, 41mm Keihin carb and rev limiter set at 14,000 rpm. Our test bike had an 86mm by 60mm bore and stroke (348cc), 11:1 compression ratio, HT Racing cam, Vortex ignition with similar programming to the KTM unit and a 39mm Keihin carb. Finally, complementing all the motor work was an FMF Factory 4.1 muffler with a custom-made, larger-diameter MegaBomb header.

The history of how the bike was built is quite interesting. Tim Norton bought the frame and swingarm on eBay for $500. He hand-selected the accessories and aftermarket parts to suit his personal tastes. The rims were Notako by Excel, mounted with a Dunlop MX51 rear and 756 front. The KTM 348SXF had Fastway axle blocks, a BRP chainguide, PWR oversize radiators, CV4 radiator hoses, R&D PowerBowl and Flexjet, anti-slosh gas tank foam, billet KTM gas cap, SDG gripper seat, AME grips, Vortex ignition with handlebar switch, Motion Pro throttle housing with optional-sized throttle reels, Hinson slipper clutch, Cycra vented front fender, and Renthal 7/8-inch bars and sprockets. LightSpeed provided the carbon fiber frame guards, gas tank cover and fork tube wraps. In a unique little touch, Tim stashes an Allen wrench under the crossbar pad for suspension adjustment.

For the suspension chores, Tim went to Ohlins. The bike has an Ohlins steering stabilizer, Ohlins fork internals and a complete Ohlins TTX shock. Like the production 350, Tim’s bike has an electric starter, but instead of a stock battery it is powered by a lightweight and compact Turn Tech 5.0 Ah battery.

The most notable ergonomic modification was at the rider’s feet with Fastway footpegs. Since lowering them on the axis of the pivot also moves them rearward, Tim decided to have the shifter cut and rewelded shorter. Finally, the KTM 348SXF had a standard Brembo front brake caliper and an upgraded rear.

TEST RIDE: HUNTING FOR 450s

You never know how efficiently a project can be done until it’s done privately. This 350SXF has a full-race engine, aftermarket suspension and all the bells and whisles. Best of all, it didn’t cost as much as a new OEM from the showroom.

The displacement may split the difference between a 250 and a 450, but the bike’s performance is a whole lot closer to a 450 than a 250. The HT Racing 348 laid down smooth power in the bottom end that was extremely usable when carrying momentum. The delivery became stronger in the middle and started stretching riders’ arms. The powerband definitely had a sweet spot in the middle. Power revved out freely without dropping off, and we could wait to shift without major penalty, but it was better to click up.

A traditional big-bore 250 invariably runs like an overpowered 250. The HT Racing 348 is more like a slightly underpowered 450. MXA test riders went looking for 450s to drag race on uphill straights. In the sweet spot, the 348 didn’t lose any ground. Immediately popping out of ruts and turns, the lightweight 348 had the advantage, but when the straightaway demanded a long pull, the 450s were stronger.


Gold means Ohlins parts. The fork internals are a quality mod made cheaper by utilizing the standard WP forks. The steering damper would be better for offroad.

One complaint on Tim Norton’s bike was with his tall gearing. While it is true that more power necessitates taller gearing, Norton’s 14/45 versus the stock 250SXF 13/48 gearing was close to 6.5 teeth off the rear. Test riders thought a 46 or 47 on the rear would make the bottom end more potent and prevent some clutch use. As for failures during our test period, we broke the shortened shifter on its welded spot. Who would have guessed?

In the handling department, the combination of 350 power in a lightweight 250 chassis made the bike a dream to ride. Less weight is a big plus when a rider is trying to change directions or come out of corners (which is 100 percent of the time on a track if you’re doing it right).

Normally we would make a point to look for any issues under engine braking situations because big-bores can create an unwanted dragging feel. The Hinson slipper clutch let the bike freely coast into turns and was an excellent match for the 348cc engine. And KTM’s potent Brembo brakes did the rest. Very nice.


 
Once we pulled the gas tank foam out of the fuel petcock, twisting the R&D Flex Jet for fuel screw adjustments facilitated perfectly tuned jetting.

In the handling department, the combination of 350 power in a lightweight 250 chassis made the bike a dream to ride. Less weight is a big plus when a rider is trying to change directions or come out of corners (which is 100 percent of the time on a track if you’re doing it right). The way the power was delivered eliminated a scary power rush and instead produced a hooked-up feel that let the test riders go where they wanted.

As for the suspension, we have tested the last couple generations of Ohlins KTM shocks. We weren’t surprised that the suspension worked well through every type of bump we hit. The lowered Fastway pegs give a rider more bike to clamp with his legs, and the sharp cleats hold onto the boots. The trade-off is that the pegs and rider’s feet hit the ground more.

WHAT DO WE REALLY THINK?

KTMs are amazing engines, and when you get enthusiast engineers like Bill Thomas in the mix, then they become phenomenal.

This is not the engine that KTM will sell later this year. Absent from the list of exciting new KTM features was, of course, the linkage-equipped rear suspension, new chromoly frame and electronic fuel injection. Nevertheless, if HT Racing’s prototype is any indication of the performance of a purpose-built 350cc engine, the world is in for a treat.

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