Tech: rake and trail

[Captain]

Ever had the front dropped? here's something to think about. And even if you haven't, it's a good article :ayyy:

http://i3.photobucket.com/albums/y93/GBM2004/RakeandTrail.jpg

[Captain]

What happens when I turn the handlebar on a moving motorcycle? A lot happens in response to that push on the bar. A firm push on the right handlebar has the immediate effect of leaning the motorcycle over to the right for two distinct reasons. First, the center of mass of the machine has an intrinsic desire to keep going straight. But turning the front wheel to the left moves the contact patch to the left as well, and the whole motorcycle tilts to the right, pivoting around the center of mass. Also, the spinning front wheel acts as a gyroscope, and as such it is subject to gyroscopic precession. That is, if you try to steer it to the left on a vertical axis, it instead wants to tilt to the right on a horizontal axis. The two tendencies interact to lean the bike over to the right, and once it's leaned, a neat trick called camber thrust takes over.

What is camber thrust? Thought you might ask. Because a motorcycle tire is rounded, its rolling surface is smaller in circumference around its outside edges than in the middle. When the motorcycle, and thus the tire, is leaned over, it acts like a rolling cone: the outside edge of the contact patch wants to go farther than the inside edge. This sends the tire rolling in an arc over the road and results in the miracle we know as cornering. Because of camber thrust it's not necessary for the front wheel of a cornering bike to be pointed into the turn, as with a car; the rounded profile of the tire does the job instead.

Rake, Trail and Offset are explained in the previous thread.

How does rake change affect handling? A steeper rake angle, one that is closer to vertical, tends to make a bike react more quickly to steering inputs. A shallower rake angle tends to make a bike more stable but at the expense of increased steering effort. Cruiser type motorcycles, which are designed for stability above all, tend to have shallow rake angles of as much as 28 to 30 degrees from vertical, while GP racers with the emphasis on instant steering response, are now set up in the 22-degree range. With more rake, the steering head of the motorcycle tends to drop more with steering deflection, giving rise to the tendency of some raked-out cruisers to flop over into low-speed turns.

How does trail change affect handling? More trail increases the tendency of the front wheel to return to a straight-ahead position when deflected by a steering input or bump. The word trail refers to the position of the contact patch behind the steering axis and road intersection. Because the rolling resistance of the tire is located behind the axis, trail makes the contact patch hang behind the axis like a trailer (pun inevitable) hanging behind the hitch of a car. With zero trail, the front wheel would be flicked sideways by the slightest bump or bobble at the handlebar. Lots of trail would make the bike very heavy to steer. By trial and error over the years, engineers have discovered that the optimum trail for most sport bikes and racers ranges between 3.5 and 4 inches, with cruisers up close to the 5-inch mark.

How does a change in wheelbase affect handling? As with rake and trail, more wheelbase tends to give slower, more stable steering and less gives GP-quick response. Small, light roadracers generally have between 53 and 55 inches between the axles, while cruisers and tourers can run beyond the 60-inch range.

How does wheel diameter affect handling? Because the wheels of a moving motorcycle act as gyroscopes, smaller-diameter wheels tend to make a bike more nimble and quicker steering and larger wheels more stable and forgiving. A 19- inch front wheel, for example, tends to steer more heavily than a 16, but it will also want to hold its direction more tenaciously and slide more predictably. Also, a smaller-diameter wheel has a contact patch that's relatively wide and short, while a large wheel's patch is longer and thinner. The orientation of the patch tends to give a large-diameter wheel more self-aligning torque; that is, it will have a greater tendency to align itself with the bike's direction of travel.

How does tire shape affect handling? The profile of a tire, its shape when viewed from ahead, has a great deal of influence on how a motorcycle behaves as it is leaned over. A tire with a triangular profile, one with a narrow center section and relatively flat cornering patches on either side, tends to flop quickly into a turn but then reacts with excellent traction and stability once it's leaned onto its side. A rounder tire exhibits more predictable, neutral steering habits from straight up to full lean but at the expense of a bit of traction at the cornering limits. A tire that's flatter in the middle gives slower steering while straight up and down but quicker, crisper response when leaned over. Some recent tires are more mushroom-shaped; these depend on the deflection of the tire carcass to keep the patch on the ground in heavy cornering. There are various theories of tire shape and profile at work, even in very competitive production racing tires. The main message is that there is no right or wrong profile, just different trade-offs in response and steering effort.

How do rim width and tire size interrelate? With a given rim width, a wider tire is forced to curve farther around toward the rim at its sides, making it rounder than it would be on a narrower rim. Conversely, a narrow-tire, wide-rim combination gives a flatter final profile, and in extreme cases it is possible to ride right off the edge of such a rim-tire combination at extreme lean angles.

[Captain]

And if you're really into the engineering, you can read even more on this here: http://www.sportrider.com/tech/146_0310_art/index.html
Fascinating stuff.

[Loco976]

wow, interesting as always, cap.

[stetto]

nice find cap

[Johnny]

Forgive me if Im wrong Capitano, but, does your second post, totally undermime the first ??

Rake and trail, first suggests its not safer and is a myth, then why does my bike tip in better under trail braking ??? it reduces rake so is that a myth too, just like the guys on superbikes using bigger wider tyres ?.


I reckon all that falls under suspension set-up, the rest is too much food for thought :lmao:

[Captain]

No, I don't think it undermines it. Perhaps they're in the wrong order :) but both describe the principles of and the effects to making changes to these. The first article simply concludes that with the super-modern sportbikes they are (probably) already set up with a minimum of rake/trail, for optimum performance, so that it's likely that there will be very little for 'improvement'. So that any steepening on the rake angle - or reduction of trail - could actually be counter-productive. Does that make sense?

[Johnny]

Makes sence mate.

But I still reckon dropping the front and raising the rear makes a difference for the better, even the modern weapons (it did on mine, although Zeno did his magic along with that) .
Yet to have anything concrete suggest its a myth.
Looking at the Gp bikes with 16" front rims (making even lower), rear high up too, makes me wonder were it can be proven otherwise ?.

p,s, now this is a discussion !, will keep out of (from here on in) but will be reading with interest..
Nice one Captain :ayyy:

[Captain]

I am certainly curious, as Zeno raised the rear of my bike when he fitted the new rear shock. I also read that the Pitt's WSS 600RR is similarly raised in the rear. Although to be honest, I cannot say that the bike feels like it's 'tipping in' any faster than before ... but then I am probably too inexperienced to tell. So I'm going with what has been suggested by one with just a touch more experience :), for now.

http://i3.photobucket.com/albums/y93/GBM2004/PittsCBR600RR.jpg

[zenodamper]

You know all of what James Parker has done by the ways?

[Captain]

He sold out of Consolidated Media Holdings didn't he? Oh wait, that's James Packer :dohsmiley: .... never heard of James Parker ... :confused1:

[zenodamper]

Yamaha GTS1000?

[Johnny]

Yamaha GTS1000?
Forking forkless suspension ?

[*Oggy*]

Another Tony Foales copier then I take it.....

[Johnny]

ie, the ultimate cheese, even more so than the suzuki 750 wankel :lmao: