Since most underbone users are fond of lowering their rides either for better look or for better performance, here's an article that will be of great help. It will give you a thing or two about things you need to know and remeber when lowering your motorcycles.
Though the article generally refer to cruisers, the concept is the same anyway, so enjoy reading.
Lowering a bike doesn't just give it that long, low custom look, it also allows shorter riders to put their feet flat on the pavement at stoplights. And if you've ever found yourself on a too-tall bike at a slippery intersection, you might be willing to trade in all the shiny chrome in the world for secure footing. (Though we know very short riders who have adopted to tall motorcycles.) "Probably more guys are after the look," says Progressive Suspension's Larry Langley, who estimates the ratio is approximately 60:40 in favor of lowering for style. "But more and more vertically challenged riders are doing it these days, too."
Regardless of your reason for lowering your bike, it's not a task to be taken lightly, and if you're unsure about the dynamics of this type of modification, consult a suspension specialist to avoid safety problems. Over the years, countless bikes have been lowered by backyard mechanics who took a hacksaw to the fork springs and bolted on a set of cheap lowering blocks to the rear shocks. Most of the perpetrators of such hatchet-jobs were happy with their work—until they rode it for the first time, and discovered that, in addition to cool-looking, the bike had become ill-handling, uncomfortable and unsafe. Langley gave us some guidelines for doing the job right, along with a few
Lowering the Front End
"First, as a general rule, never lower the front without lowering the rear," Langley says. "You can lower the rear without lowering the front, and what it does is give the bike more of a chopper effect. But if you just lower the front, you unbalance the bike the wrong way." Many bikes can be lowered by approximately an inch in the front fairly easily by modifying or removing the stock preload spacer. Some bikes come with preload spacers that compress the fork springs an inch or more when the fork is unloaded. Shortening the spacer drops the front end of a bike an amount roughly equal to what you removed from the spacer. But be careful not to go beyond the point where there is minimal pressure on the spring when the suspension is fully extended. If you go beyond this point, your bike will be effectively springless when the front extends completely, as when the front wheel drops into a dip in the road at speed. Not a pretty scenario.
If you want to lower your front end more than an inch, says Langley, probably you'll have to do it mechanically. "What we do is put a spacer, which is really a short spring, under the damper rod. That fools the fork into thinking it's shorter, and doesn't let it come back up to full extension." If the fork has a preload spacer on top of the spring, you also might have to remove or shorten it, or the spring will be too compressed when the fork is at rest. Depending on the bike, you may need shorter main fork springs because you've taken up so much travel that the springs will not let the fork compress fully before the coils contact each other, preventing the fork from compressing.
The preceding methods work for any bike that has damper-rod suspension. "If it has cartridge forks," Langley says, "it's a much bigger problem. For example, we don't even make a lowering kit for the [Honda] Valkyrie, which has a cartridge in one leg and a dummy cartridge in the other." A cartridge is like a rear shock internally. Consequently, there's not an easy way to shorten it. If the design allows, you can slide the forks up in the triple trees; make sure, however, the fender doesn't hit the triple tree when you compress the fork completely, as when hitting a large, sharp bump.
Lowering the Rear End
If you only intend to lower one end of a bike, the rear end is the better choice. And it's all some riders need, since lowering the rear end also lowers the seat substantially, making it easier to flat-foot a bike at stops. The backyard crowd has a quick fix for this—lowering blocks, which are machined spacers that relocate the rear shock's bottom mounting point several inches to the rear. They're cheap, easy to install and their net effect is to lower the back of the bike. But there's another consequence of using lowering blocks which is not so obvious—they drastically change the rear shocks' lever ratios.
A shock's lever ratio is determined by the angle at which it's mounted. To better understand this concept, picture a bike's rear suspension, including the swingarm pivot, the rear shock's lower mount and the rear axle. Next, imagine the rear wheel moving though its travel, which describes an arc, and draw that arc. The distance the rear axle travels typically will be farther than the distance traveled by the shock's lower mount. At the extreme, the shock might be moving two inches, and the rear wheel four inches.
This disparity can have unintended consequences if you decide that in order to lower your bike an inch, you only need to fit your shocks an inch shorter than stock. "On a bike like a Valkyrie, which has a 1.5:1 lever ratio, a one inch shorter shock will lower the bike an inch and a half," Langley says. Most bikes have a lever ratio greater than 1:1, and on a single-shock bike such as Yamaha's Road Star, the lever ratio may be as high as 3:1. "The only bikes that have close to 1:1 lever ratios are Harley FLHs," Langley points out.
The drastic change in lever ratio that results from using lowering blocks essentially makes the shock stiffer, reducing ride comfort. So why not just go to a salvage yard and yank some shorter shocks off a wreck? Langley warns, "Shocks are engineered for a particular model. For example, a [Harley-Davidson] Dyna Glide shock will not work on a Sportster because the lever ratio is different. The shocks on a Dyna are moved way up, and they have heavy damping and 300 pound springs. The spring on the Sportster shock is a 100-pound spring and the damping is lighter. Switch them and they'll be either too soft or too hard on the wrong bike. The spring has to be right, and the damping has to match the spring. You have to buy by application, not length."
Effects on Handling
Even if you lower your bike by the book, handling can be affected to some degree. "When you lower a bike, you also lower its center of gravity, so it'll handle a bit better in certain circumstances," says Langley. "The negative is that your initial ground clearance is decreased. Things you used to clear, like curbs or speed bumps, might now be a problem."
And that's not the only thing you'll notice during your first ride on your just-lowered bike. You've given up travel, so your comfort will suffer. As Langley puts it, "The more you lower it, the more ride quality and comfort suffer. Two inches of travel won't do the same job as four inches of travel." Why? The springs must be stiffer to keep you from bottoming out, and the shocks usually need heavier damping to match the heavier springs, which leads to compromises that might force you to re- consider lowering in the first place.
Cornering and Braking
If ground clearance is affected, how about cornering clearance? Common sense tells you a bike's lean angle should be reduced, too. While Langley (and most manufacturers of these kits) says lowering a bike seldom reduced enough to make a difference, practical experience has shown the Motorcycle Cruiser staff that cornering clearance is noticeably altered. If you drag pegs occasionally at the stock ride height, you will do so more frequently if the bike is lowered. Also, if your bike tends to drag solid mounted parts, such as its pipe or sidestand, lowering is not for you.
Braking is a performance category where few riders will notice a difference. Theoretically, lowering a bike should result in less forward weight transfer under braking. But cruisers' long wheelbases should make the difference negligible. However, if you find the fork bottoming out under braking, consider a set of progressive rate springs to stiffen up the front end in the bottom of its travel. The shorter travel may also make the bike chatter more over bumps under braking.
So far, we've seen that when you lower a bike you give away some ride quality. Langley says you also should be prepared to give up some load capacity: "You can't make a bike low and have the same load capacity. That's because you lower the bike at the expense of suspension travel." The reduced travel means the bike can bottom out easier. Those planning on extended two-up riding should forego lowering. If you want to make your cruiser a show bike and troll Sturgis, go for it. But if you want to pack some gear and a passenger and ride across the country on a lowered bike, you're not going to be happy
There's one more way to lower a bike, which is to fit smaller wheels, lower-profile tires, or both. (Of course, you can simple cut the seat down by removing foam or replace it with a thinner saddle.) The wheel change approach is an option that seems appealing, especially with the advent of a wide variety of aftermarket wheels currently available for metric cruisers. Today, you don't just see Harley customs sporting enormous rear tires and low, wide wheels at bike shows. While that setup might lower the bike, Langley suggests you bear in mind that most of those customs aren't ridden much, if at all. "Now you're into an area that drastically changes geometry and how the bike handles," he says. "You'd better really know what you're doing." We recommend changes of this type be made carefully—with the guidance of builders who have performed this type of modification before.
Langley offers some final thoughts on lowering: "The more you lower it, the more ride quality suffers. That's the first thing I tell anyone considering lowering a bike. What I generally recommend is going an inch lower front and rear, so you'll still have enough travel to give [yourself] a good ride. That's a pretty good compromise, but anything over that and you're giving up a good ride."
Regardless of whether you want to lower your bike for good looks or peace of mind, resist the quick-and-dirty fix, and remember that lowering unavoidably involves compromises, no matter what some backyard customizers say. You can live more easily with those compromises if you do the job right and don't take it too far. Just don't forget the idea is to get down...not hit rock bottom.