Motorcycle wheel alignment

[fixer]

Hi all,

This is technical question, though it is not strictly-speaking a bucket question. But I know there are a lot of smart and experienced motorcyclists on this forum, so I thought I'd throw this question out there.

First confession ... I've recently bought a 2014 Triumph Scrambler. Yeah, yeah, feel free to take a pot shot or two. Anyway, the front and back wheels are not rotating in the same plane and Triumph NZ are telling me (and apparently Triumph UK agree) that this is correct and not unusual. However, in 33 years of riding motorbikes, i don't recall coming across this before. The longitudinal vertical centrelines of the front and rear wheels on my new bike are parallel and offset by about 15mm. The front tyre is 100mm wide and the rear is 130mm wide and the right hand side of both tyres line up precisely. The left hand side of the front tyre is offset to the right of the same side of the rear tyre by 30mm. It doesn't seem to affect normal handling though the bike does pull noticeably to the right if I take my hands off the handlebars. Though it might be a factor that the entire, heavy exhaust system of the Scrambler is on the right.

So basically, my question is, does anyone know what the effect is on the handling of a motorcycle of having the wheel centrelines offset (by say 15mm) and are there motorcycle that are constructed this way intentionally?

Thanks guys.

Tim

[nodrog:Me]

Bmw are the same

[Blackbird]

It does affect the handling but by how much is open to debate. It also depends what you use your bike for and how hard you push it. I know that top race teams are very fussy but I guess that's the ultimate test. The alignment marks on a swingarm are only an approximation and even small differences of how each fork is clamped can lead to an offset. If that all sounds to hard and depressing, it's not hard to measure accurately and correct it.

Back in 2003, I built a simple laser alignment rig for my Blackbird because I was fed up with tyre fitters not realigning my wheels properly. You can see the principle here: http://geoffjames.blogspot.co.nz/201...-handling.html . I've simplified it a bit since then and it still works perfectly. I've also attached a write-up I did for a fellow Street Triple owner as he wanted a starting point to check and correct his own alignment. In the photos, you can see that I've substituted a tape measure for the front rig shown in the blog.

I mainly did it because I'm an anal mechanical engineer . I don't know whether it's of much interest to you but hope it might answer a few questions. 15mm offset is bigger than I'd want to put up with.

Cheers,

Geoff

[fixer]

Thanks for the feedback. In this case, the wheeltrack offset is so substantial that it's patently obvious even if you just lie on the ground behind the bike and eyeball it. I'm sure that even the fanciest of frame alignment machinery would show something similar. I've tried both a straightedge and the string method with the same result - at the front the gap between the rh side of the tyre and the string is 0mm and on the lh side, the gap is 30mm.

I suspect it might be harder with a shaft-drive BMW, but in principle, with a bit of spacer reshuffling and some machining of the rear wheel, i think it would be possible to get the wheels of the Scrambler to line up properly and for the sprockets to line up correctly at the same time.

However, moving the rear wheel over by 15mm, is not something I would do lightly. Presumably Triumph built it that way for a reason. Though I am almost completely stumped as to what that reason might be. I had a quick glance at a Triumph Thruxton in the bike shop and the wheels did not appear to be anything like as offset as mine, if they were offset at all. I can only assume that the offset of the rear wheel in some way helps to move the centre of gravity of the bike to counteract the weight of the HUGE but very pretty exhaust system on the right hand side of the bike.

To be honest, other than when I take my hands of the bars, I can't detect any strangeness in the way the bike turns in. Though it does seem to be a little harder than I would expect to ride in a straight line at a slow walking pace. And, lets face it, this is not a GSX-R. It has Trailwing tyres and makes 59hp. It's never going near a track and probably won't even be pushed that hard on the road.

But I would love it if someone could explain to me the reason(s) why a manufacturer might build a bike this way. And there must come a point where the offset is so great that an imbalance in the steering of the bike would become obvious to the rider. I would have thought?

[Paul in NZ]

My suggestion is to stop looking at the wheels... You cant see them from the seat anyway BUT if you are worried look at another scrambler...

[Blackbird]

All good questions but I suspect that you're not going to get uniform definitive answers from different manufacturers. I suspect that one of the larger contributors to misalignment on a new bike is a combination of manufacturing tolerances and assembly. From memory, my Street Triple had an offset of closer to 10mm when I first got it. Judicious uneven movement of the rear adjusters and moving one fork fractionally in the clamps brought it much closer. This might well be your best course of action if it really bothers you. However, if you're not generally a hard charger in the twisties, you probably won't notice anyway so doing nothing might be your best option.

I rode a pre-unit construction Tiger 100 when I was a student. The frame used to flex horribly and I never checked wheel alignment either. Rode like a complete twat everywhere and when I fell off, it was inevitably because I was riding like a dick. What I'm trying to say is that the impact of misalignment for normal road riding is pretty small in the scheme of things you can do to keep yourself safe

[HenryDorsetCase]

why would anyone give you a hard time about buying a Scrambler. I really liked mine. I even kept the Ohlins shocks I got for it in case I got another one. Or a Bonnie.

I never noticed any misalignment in mine but I never looked for it either. I did clean a lot of shit out from the cover over the front sprocket. It is a real trap for grit and chain lube and shit - FYI

[Yow Ling]

before lasers came out people used string, i suppose you could use red string for the laser beam look

[HenryDorsetCase]

or bolt something to the front sprocket (which I thought was genius) and measure the offsets that way:

http://www.kiwibiker.co.nz/forums/sh...xtensive-clean

[Grumph]

This thing has wire spoke wheels. yes ?

If so, first check the front is central in the forks. This is more important than the rear...

The out of line problem could be as simple as a badly built wheel. If you're dead sure you want it moved over - and there's room tyre/chain line - take the bare wheel to a good wheel builder and ask for the rim to be moved over the appropriate distance.
It won't be cheap as some spokes will probably have to be replaced - but it's easier than making up spacers and machining the hub.

[mossy1200]

They used to make k100 brick race bikes with larger back wheels and offset of up to 20mm due to the shaft drive.
Apparently was successful.

[AllanB]

Read some of the big block Hogs are offset significanlly due to the primarly drive/belt alignment.


Weird shit.


Appears the main alignment issue on a m-cycle is not so much the plane parellel to each other although offset but a twist within the factory set plane.

[nodrog:Me]

ffs its how its built.

[koba]

We had a local bucket with about a 20mm offset, this was corrected and then it went faster on the track.
Not saying it was certain cause and effect but the owner at the time was well experienced and he seemed to like it a lot better when it was in line.

[fixer]

Thanks guys.

This conversation, while not overly revealing, has at least made me relax a little. It looks like "don't worry, be happy" is the general consensus, and no one can really explain why a bike would be built that way.

An email from Triumph read as follows: "Several of our models have an offset between the wheels, this is not unusual. Running the wheels with an amount of offset improves the overall stability of the bike without any noticeable effect on handling."

Not exactly enlightening, but it is at least authoritative. Though how an amount of offset would improve the overall stability is a bit of a mystery to me.

So basically, from what Triumph tells me, you can't use the good old string method to check the wheel alignment on my bike unless you know that the wheel centres are offset (I have asked them to confirm that 15mm is the correct offset). The user manual tells me to rely on the marks on the chain adjusters. Of course, the unreliability of marks on chain adjusters is one of the few things that I thought pretty much everyone agreed on. Check with Mr Google.

Thanks for all your contributions ... even "ffs its how its built".

And I can't subtract ... I've been riding for 43 years, not 33 years.