Fork offset & trail - true effects on handling... discuss.

[potheadpixi]

Calling all PB chassis / suspension boffins / racers...

I've been looking at doing an RC51 USD fork conversion on my VFR750FV based track bike. (See VFR837R Project in project bike section)

The VFR uses an unusually large fork offset of 40mm. Most sports bikes have an offset of around 20 - 30mm max.

I initially though that the greater offset was to do with the fact that the VFR is not a true sports bike - Less offset must therefore give a more racey setup - GSXR's RSV's CBR's etc. all run much less offset after all.

However, after researching the relationship between fork offset, trail and their consequential effects on handling I came to the following conclusions...

More fork offset = Less trail... Which in turn leads to sharper handling at the expense of high speed stability (less caster / reduced self centring effect on steering)

Less fork offset = More trail... Which in turn leads to greater stability at the expense of steering response (more caster / increased self centring effect / more steering input required by rider)

The RC51 runs an offset of 30mm (10mm less than the VFR) and so in theory, bolting a RC51 front end onto my viffer will actually make the handling LESS racey. (Assume ride height / tyre circumference remains unchanged).

I don't understand this... How can every bike more sporty than a VFR run LESS fork offset ? Is there something within the chassis / swingarm design which affects this ?

Before I go on, can anyone explain to me why all sports bikes use less offset than my VFR, when geometrical theory states that this will sacrifice responsiveness for stability.

I'd be particularly interested to hear from any engineers or racers who have had expierience setting up bikes with adjustable yoke offset (not rake) and measurably felt the difference in these offset settings.

Cheers for now,

PHP.

[spacemonkey]

Calling all PB chassis / suspension boffins / racers...

I've been looking at doing an RC51 USD fork conversion on my VFR750FV based track bike. (See VFR837R Project in project bike section)

The VFR uses an unusually large fork offset of 40mm. Most sports bikes have an offset of around 20 - 30mm max.

I initially though that the greater offset was to do with the fact that the VFR is not a true sports bike - Less offset must therefore give a more racey setup - GSXR's RSV's CBR's etc. all run much less offset after all.

However, after researching the relationship between fork offset, trail and their consequential effects on handling I came to the following conclusions...

More fork offset = Less trail... Which in turn leads to sharper handling at the expense of high speed stability (less caster / reduced self centring effect on steering)

Less fork offset = More trail... Which in turn leads to greater stability at the expense of steering response (more caster / increased self centring effect / more steering input required by rider)

The RC51 runs an offset of 30mm (10mm less than the VFR) and so in theory, bolting a RC51 front end onto my viffer will actually make the handling LESS racey. (Assume ride height / tyre circumference remains unchanged).

I don't understand this... How can every bike more sporty than a VFR run LESS fork offset ? Is there something within the chassis / swingarm design which affects this ?

Before I go on, can anyone explain to me why all sports bikes use less offset than my VFR, when geometrical theory states that this will sacrifice responsiveness for stability.

I'd be particularly interested to hear from any engineers or racers who have had expierience setting up bikes with adjustable yoke offset (not rake) and measurably felt the difference in these offset settings.

Cheers for now,

PHP.

Do a drawing, it will all become obvious then.... Buy Tony Foale's book. He explains a lot of this and the drawings are all in there...

Fork offset is terribly important in iteself. It is simply used to obtain the desired trail as there are limits on what you can do with head angle when using telescopic forks and the physical practicality of where you hang the steering tackle.

[will#224]

Interesting, the sxv450 i used to race (tootle round on) had adjustable offset clamps. The offset preferred was 14mm, it made the bike more stable in turns and the turn in was better. Some shortened the swingarm too.

[lorenzo]

I'sd say that you looking at offset and trail in isolation to fork angle is probably not an especially good way of doing things. At a guess the trail is introduced to bikes with steep angles to counter act some of the twitchiness.

[potheadpixi]

I'sd say that you looking at offset and trail in isolation to fork angle is probably not an especially good way of doing things. At a guess the trail is introduced to bikes with steep angles to counter act some of the twitchiness.

You may well be right there Lorenzo.

The VFR might use a very different headset angle to the full on sports bikes.

Anyone have any figures ?

PP

[speedy sie]

4+6=10

[lorenzo]

You may well be right there Lorenzo.

The VFR might use a very different headset angle to the full on sports bikes.

Anyone have any figures ?

PP

Got no figures, but my basic maths tells me that it gets really complicated when you consider that you'll be putting forks on that are a bit shorter than stock, which'll tilt the bike forward and then increase the headstock angle. Then take into account that forks aren't a fixed length and my brain starts to hurt.

To be honest, what I did on my vfr was to ignore things like maths, get a set of 954 blade forks and yokes (might've been 929 yokes, whichever the stepped one is), then spring them to suit and make sure I got the sag right. From there on it felt pretty good, although the damping was a smudge softer than I'd like ideally, which could be corrected pretty easily.

[2moto]

You are correct in that most modern sports bikes run an offset of 25-30 mm. As already pointed out by others, you can't look at offset in isolation. It is the offset combined with head angle that determines the trail.

For example, your VFR has an offset of 40 mm and a head angle of 27.5 deg. This results in a ground trail of 109.25 mm. This is pretty conservative but about right for a VFR type bike.

In contrast, the RC51 has an offset of 30 mm and a head angle of 23.5 deg, resulting in a ground trail of 96.21 mm, which is pretty much in the ball park for modern sportsbikes.

If you change your VFR to RC51 yokes, you would end up with a trail of 120 mm, which will make it pretty slow steering, provided you don't change the front and rear ride heights. To get back to the original trail of 109.25 mm, using the RC51 yokes, you'd need to change the head angle to 25.7 deg. This maybe possible given the RC51 forks are shorter, but will probably also require raising the rear. I have no idea what other effects this would have on handling.

By the way, you can download a basic trail/rake calculator for free from Tony's website: Steering geometry calculator.

This is a common problem when trying to put a different front end on bikes. Unless it's the same model, it's rarely as simple as bolting on parts if you want to have at least halfway decent handling. Best thing to do is to use a front end from a newer model, or get your existing yokes machined to suit the RC51 forks. This may then require different wheel bearings, spacers, etc.

[deviant]

What he said ^. Head angle (frame), offset (yokes) and position of the spindle axis relative to the fork centreline (forks) will all have an effect.

And of course the whole lot will change depending where in the suspension travel you are.

Easiest way of dealing with it is probably to draw it to scale:

http://www.google.co.uk/search?source=ig&a...free+2D+cad+pro

[porter_jamie]

or, you could say the reason most bikes have a head angle of about 23 odd degrees is because if it didnt, the wheel would hit the engine, and the reason they have some offset is to give a bit more clearance for the forks to the frame.

IMO, within limits, the actual figures dont really make much difference and is a rider preference thing. Some like a razor sharp bike that is on the verge of slapping the whole time, and some would consider that dangerous.

It is what it does to the whole bike, particularly weight distribution that really matters. Changing the ride height each end alters the weight distribution, but also happens to change the trail a bit which can alter how it feels. Jacking the bike up at the back can help stop it running wide, I don't think that changing the trail in isolation (if it was possible) would stop a bike running wide for example, it is probably mroe weight on the front.

Changing the Rake will change the amount of camber change over 'toe' or steering angle, which might alter something, but i dont think it really matters, and there's better things to worry about like proper tyres.

thats what i think i know, after some years of messing around, but i absolutely don't pretend to know what i'm talking about.

i would say if it is a road bike, it matters even less.

edit just read the op, and no i have not messed around with offset on a bike, so all the above is of no use then really...doh.

edit some more - just read your project bike link - very nice, and i want one. also since you look serious, then yes, you probably should worry about the offset.

[Mark/Foggy]

Just to add more parameters for you to think about, tyre profiles. Oh yeah and we're talking front and rear, where a lot of steering input comes from. Skiny rear and a big round, safe feeling sports tour tyre like on the VFR needs the longer trail to keep it rolling into and then picking up nicely as you come out. Shorten the trail too much on this combo could lead to the front tucking as the trail/self centering is combating the tyres contact patch from rotating about the steering angle.

Fit a big tyre at the back and as you roll onto the outer edge it's moving weight onto the front at a harder rate. Combined with the triangular section of a sports tyre the bike will drop on it's side quicker and as a consequence you don't need the larger trail figure to get the turn started with the counter steer. As the weight shifts rearward again the additional grip from a large rear contact patch means that the weight will shift quicker than the other tyre set up requiring less self centering by the front.

Yada Yada Yada

[redmarque]

If you really want to start messing around. Input the geometry into a copy of Tony Foale's Full bike setup software.

Extremely useful to see what changes you make would be. http://www.tonyfoale.com/

Other good reading would also be John Bradley's The Racing Motorcycle a Technical guide for Constructors Vol 1 http://www.amazon.co.uk/Racing-Motor-Cycle...s/dp/0951292927

[potheadpixi]

Thanks for your well informed replies guys.

All food for thought and much appreciated... I knew this was the best place to ask such a question !

Spacemonkey. I've never read Tony Foales book (or heard of him, until now). I'll try and find a copy. Thanks.

I did read John Robinsons chassis book, very interesting, but it didn't give me the answers I was looking for.

Lorenzo. I'm currently running clip ons below the std VFR yoke and plan to do the same with the RC51 forks too... I don't think fork length will be too much of an issue for me.

I tried a set of CBR954 forks, but they where way too short at 724mm.

The RC51 forks are 755mm in length (I think) which is considerable longer than many other sports bike forks. They are also more firmly sprung, although a trip to Maxton will be in order to optimise them for my bike.

A custom (drooped) top yolk will also help ride height if required. I'm aiming to end up with exactly the same ride height as I currently have if at all possible.

3upracing. Thanks for the facts and figures... I never knew the VFR's head angle was 27.5 deg. Nice to know these facts !

I'll have a good look at Tony’s website too. Cheers for that.

Mark. Thanks for your very valid point referring to tyres. The wheel / tyre combinations are not std on my bike.

It seems I may have been missing the bigger picture.

The most important thing for me is to improve the handling, and not spoil it by fitting the RC51 forks.

Current chassis mods are limited to a 180 section rear tyre, 16" blade front wheel, road / track tyres, rear ride height raised approx 10mm and forks raised through std yokes by 10mm.

With these mods the bike is much sharper than a std VFR750. It turns well, but remains stable and predictable - even though it has fair bit more power than std.

The front gives a little shake now and than when I get on the gas hard (which a std VFR750 would never do) but I'm happy with this setup.

Ultimately, I know that there is room for improvement - this old V4 was designed as a sports tourer after all.

If I go for the conversion, I'll almost certainly have a set of custom triple clamps made up to keep the offset at 40mm, and use an sp1 or sp2 front wheel. Tyre choice is getting difficult in 16".

Harris do a set of adjustable clamps for the RC51, these have adjustment up to 40mm offset and should fit the viffer according to my research.

Thanks again.

Any more comments or advice more than welcome !

PHP.

[potheadpixi]

Just tried to download Tony Foales demo software but it's in MS DOS and my Mac doesn't like it !

Looks like I'll have to hijack the misses laptop.