Street Bike Front-End Swap onto a Dirt Bike

After learning about the Formula 450 racing class, in which motocross race bikes are fitted with sport bike front ends and raced, I got excited. Not excited enough to spend ten grand…or even a third of that…but excited. I wondered if I could build one myself, with far less loftier goals than racing. Trials and tribulations remained.

Swapped front end

Avenues

There are a few ways to go about getting the front end better suited to the street.

1. Street Bike Front-End Swap - The Formula 450 guys eventually figured out that swapping on sportbike front-ends wasn’t the way to go about building race bikes. From this May 2009 Motorcyclist article: Contrary to popular belief (and earlier efforts), you cannot successfully repurpose sportbike components. Sportbike forks are too short, which takes away too much trail (degrading front-end grip), and excessively flattens the swingarm angle (making the bike squat too much under power). A better solution comes from Paul Thede at Race Tech, who spent countless hours analyzing F450s on his in-house chassis dyno to develop a turnkey tuning package that reconfigures the stock motocross suspension for roadrace use. Technicians internally shorten the stock shock, ft it with a shorter, stiffer spring and re-valve it using their proprietary Gold Valve technology. The fork legs are also shortened, and likewise retroftted with Gold Valves and stiffer springs. New triple clamps radically reduce offset to increase trail (lessening the tendency to tuck the front), and new fork bottoms have provisions for mounting a radial brake caliper. Lastly, Race Tech machines the tapered upper fork stanchions to a constant diameter to accept clip-ons.So, basically, there’s a better way, but it’s pricey. In the end, the sportbike swap rides pretty well.
2. Street Bike Fork Swap - Another option would be to just swap forks (the XR uses 41mm in both clamps). You would also need to fit the stock wheel, brakes, and axle to the new forks, or you could try and use the sportbike wheel, brakes, and axle. This would probably require some machining and spacers, because the forks will probably not be the same distance apart on the dirt bike as they were on the sportbike.
3. Shorten Forks - Yet another option would be to shorten the dirt bike forks (with new internals), re-spoke the front wheel hub to a 17″ rim, and put on some street tires. You could also use 21″ street tires, but the selection is skinny and limited.

Parts

I went with a sportbike front end swap on my 1992 XR250L. Some front-ends will work better than others. My first plan was to use a 2000 Suzuki  GS500 front, but the triple clamps and steering stem required too much machining ($300). The top clamp would have to be drilled out, and the XR stem would have to be sleeved and pressed into the lower triple. The bang for buck wasn’t there for not-awesome GS parts.

I started with the front of a 2003 Suzuki GSXR 750. I found steering stem info here. The XR steering stem was sleeved and pressed into the GSXR lower triple. Since the XR head and stem are shorter than on the GSXR, I made sure there was enough room on the GSXR forks to move the triples closer together. You want to be sure you’re not clamping on a part of the fork that is tapering.

XR250L with top triple removed

It would have been possible to use the GSXR steering stem with a set of 30x47x15 mm bearings, but these are difficult to find and are commonly machined. You would also need to shorten and re-thread the GSXR stem (or use a spacer between the upper bearing and triple clamp–A bit iffier). It seemed easier to sleeve and swap the stem and use standard bearings–especially if I ever need to replace them in the future.

Stock GSXR front on XR250L head...The steering stem is too long.

Here is a look at the XR250L triples after the stem was removed… You can see that the upper has a bevel and the lower has a step… Should you ever choose to do this, you might find this info handy…

Dirt Bike Front-End Removal

• Handlebar clamp bolts – (4) 12mm
• Clutch lever – (2) 10mm
• Headlight switch clamp – (2) Philips
• Gauge cluster bolts – (2) 10mm
• Front fender bolts – (4) 10mm
• Steering stem nut – 30mm (socket)
• Fork clamps – (2) 12mm (each of 4 clamps)
• Brake line clip – (2) 10mm
• Speedometer clip – 10mm
• Clutch perch – 8mm

Lift the front wheel of the dirt bike so that it is off the ground. Unbolt everything around the handlebars, headlight, and forks. You will be very happy later if you label each wire you disconnect. Remove the top triple clamp nut and triple clamp. Support the front end, and remove the notched steering stem nut with a special wrench or by hitting it with a screwdriver and hammer (it isn’t on tight). Remove everything and keep track of the bearings and various handlebar bits, levers and switches.

Behind the XR250L speedometer

Labeling the mess of wires for future reassembly

Now you need to get the XR steering stem into the GSXR lower triple. Talk to a machine shop, unless you are one. I had mine done by Zoran Vujasinovic over at Twin Works Factory in Reno, Nevada. $100 and it fit beautifully.

Street Bike Front-End Installation

• Notched steering stem nut – 22 ft-lb, then 3.3 ft-lb
• Top triple clamp nut – 72 ft-lb
• Lower triple fork clamps – (2) 12mm, 24 ft-lb, 1-2 pattern (each side)
• Upper triple fork clamps – 10mm, 24 ft-lb (each side)
• Axle – 26mm, 63 Nm
• Axle pinch bolts – (2) 23Nm, 1-2 pattern (each side)
• Calipers – (2) 12mm, 28 ft-lb (each side)
• Caliper banjo bolt – 23 Nm

“1-2 pattern” means tighten each bolt back and forth until both are to spec. For these bolts, the torque of one bolt effects the torque of the other. Also note, most of these torques were pulled from the web somewhere as best I could.

Clean the steering stem bearings and races with brake/carb cleaner, and put some fresh grease on them (I used Mobil-1 all-purpose grease. It’s red, which adds horsepower (Note: This is a lie)). Install the lower bearing on the steering stem (it should still have the race pressed on it).

I found it was easier to install the forks into the lower triple before putting the setup on the bike rather than afterwords. You don’t have to be exact yet. Install the steering stem, upper bearing, and notched steering stem nut. Tighten it to 22 ft-lbs (or until you can’t move the front end back and forth easily), turn the forks from side to side and give them a good shake to seat the bearings, then loosen the nut and re-tighten it to 3.3 ft-lb. I tightened it until I could just feel the steering begin to tighten. You just want to make sure it doesn’t feel loose or clunk at all when you pull on the forks.

Slide on the clip-ons, attach the upper triple (loosely) and torque the top nut. Now, make sure the forks are perfectly aligned vertically in the triples (you may have to loosen the lower triple clamps to adjust) and torque all the clamps. Get your switches and levers on and tighten the clip-ons. Depending on which bits you’re using from which bike, you may have to get creative to attach the brakes, clutch, throttle, and electrics. I used all the XR stuff. I just attached the GSXR brake line to the XR master cylinder with no issue. Same with the rest.

GSXR front end on the XR250L frame

Results

At this point, there is a sportbike front end and a dirt bike rear. It’s a bit ridiculous. The front will have been lowered by 6-8″ including the wheel. The kickstand will also be close to useless. I ended up cutting a few inches off.

At this point, with no rider, the rake of the bike is about 21 degrees (measured by iPhone and MultiLevel) and the trail is about 2″. By all accounts, this should be a very twitchy bike prone to tank slaps (you usually want 3-4″–For all you’ve ever wanted to know about motorcycle geometry, try The Racing Motorcycle: A Technical Guide for Constructors).

For whatever reason, when I rode it, it felt completely fine. I believe it is because of how much the rear end compresses compared to the front with a (150 lb) rider. At all speeds, including highway, when I wiggled the bars, the bike righted itself right away. I bounced on it. It felt fine in turns.

Swapped front end

The dirt bike seat, at this point, is way too high, though. You feel like you’re doing an endo the whole time. That said, it felt better on the street than with the stock front.

New riding position

Next: GS500 Rear Wheel onto an XR250L

Swapped front end