Rake/Trail vs Offset

[93 VFR750 RC36]

I'm looking at options to put an R1 front end on. I know I can use 929/954 triples but the offset is different (30mm vs 40mm) and everyone says this increases the rake/trail.

I thought offset was the distance from the head bearing centre line to the line running between the two fork centre lines. However, if that was the case, offset would have precisely no effect on rake or trail, merely increase/decrease wheelbase a tad.

The geometry would change as you went from steering centred to either lock but I would have thought less offset would give more response, not less.

Can someone please tell me where I'm going wrong? Google is just confirming my assumptions and adding to the confusion.

Stew

[Baileyrock]

You are correct that it will Not change Rake, but the shorter distance between the center lines of the stem and tubes will change the Trail.

Rake would only change if you raised or lowered the front or rear of the bike.

Member BLS has a program that calculates the effects of a Trail change.

BR

[coopduc]

The way I see it, all else remaining the same, increasing offset will decrease trail and decreasing offset will increase trail

[rdguy]

The way I see it, all else remaining the same, increasing offset will decrease trail and decreasing offset will increase trail

Ayup...

http://tonyfoale.com/Articles/RakeEx/RakeEx.htm

Glenn

[V4 Rosso]

You are correct that it will Not change Rake,

With increased offset and axle to crown length remaining constant, rake will decrease slightly. Hard to predict what it does for trail without doing some (simple) math.

[SEBSPEED]

You are correct that it will Not change Rake,

With increased offset and axle to crown length remaining constant, rake will decrease slightly. Hard to predict what it does for trail without doing some (simple) math.

Correct, more offset + same fork length actually lowers the front of the bike thereby decreasing the rake angle.

You'd be surprised to know how just a few mm can affect a bikes handling.

[93 VFR750 RC36]

Right, got it.

Putting R1 forks in 929 triples will reduce the offset to 30mm from 40mm. Set at the same fork length, the rake and trail will both change. However, if the fork length is set to retain the stock rake, the trail will increase to just shy of 10mm (Cos26 x 10 = 8.98mm).

Seeing as I'm trying to improve the performance and handling of my bike, and not wanting a Choppvfr, I don't want the trail to increase.

I'm currently running the stock forks (well, almost stock with F3 internals) raised 6mm in the clamps. This will have reduced rake a smidgin (technical description) and trail a tadge (again . . . ).

Has anyone done a survey of 50mm triple clamp offsets? It would seem that all R1, Gixxer and CBR stuff is all 30mm or less but I may be mistaken.

I guess I'm looking at custom triples or 41/43mm forks to maintain stock geometry.

Just out of curiousity, what suspension software is available out there. I know of the Tony Foale stuff, but is there any recommended starter software (not too expensive or shareware) just to start off?

Stew

[JamieDaugherty]

Has anyone done a survey of 50mm triple clamp offsets? It would seem that all R1, Gixxer and CBR stuff is all 30mm or less but I may be mistaken.

I guess I'm looking at custom triples or 41/43mm forks to maintain stock geometry.

30mm offset is a good number. I wouldn't get your head too bent around one particular number. Swapping to a different set of forks is almost certainly going to net you a good deal of extra performance. If you have any handling issues, which I highly doubt you will, then you need to adjust the geometry to correct them.

I find a lot of people who look at the numbers for a long time and try to acheive some magical combination. There are some guidelines, but at the end of the day you need to put it together and see how it rides. I can understand if you are concerned that you'll end up with an ill-handling beast after this swap. I really don't think you'll find that to be the case.

[vfrcapn]

I think if you crunch the actual number you'll see the trail increase to be closer to 15mm. You can adjust the f/r height to take a couple degrees out of the rake and bring the trail net increase to about 1/3 of that. It looks like you can calculate this yourself and don't need a software package. I'd suggest a spreadsheet, set up the formula and play with the numbers.

I agree there is some art to this but still think it's more science than art. A member here crafted some custom triples with original offset for just this conversion.

[JamieDaugherty]

Entering in the data (25.3° rake, 30mm triple clamp offset) into my spreadsheet I get 109.7mm of trail. That's really not bad. I try to shoot for 102-105mm for most riders, but anything in the 98-110mm is fine. Assuming a 1458mm wheelbase, an addition of 25mm of rear ride height would bring the trail down to 103.7mm for that setup. If you go back to the stock VFR offset of 40mm the numbers get ugly.

My suggestion is the same as above - go with the CBR triples with 30mm of offset and you'll be just fine!

[vfrcapn]

Entering in the data (25.3° rake, 30mm triple clamp offset) into my spreadsheet I get 109.7mm of trail. That's really not bad. I try to shoot for 102-105mm for most riders, but anything in the 98-110mm is fine. Assuming a 1458mm wheelbase, an addition of 25mm of rear ride height would bring the trail down to 103.7mm for that setup. If you go back to the stock VFR offset of 40mm the numbers get ugly.

My suggestion is the same as above - go with the CBR triples with 30mm of offset and you'll be just fine!

Your calc sounds right on. But stock VFR offset (43.4mm) based on Honda's published numbers puts trail at 94.3mm. That seems like a big increase and one I definitely noticed after my conversion.

[JamieDaugherty]

Your calc sounds right on. But stock VFR offset (43.4mm) based on Honda's published numbers puts trail at 94.3mm. That seems like a big increase and one I definitely noticed after my conversion.

You are right that is a significant change...... for the good! 94mm of trail is fairly low, certainly outside the desirable range. Once again, that's not to say a bike with 94mm of trail won't handle well. There are many things to consider with geometry setup. From what I'm seeing here, going to a set of triple clamps with 30mm of offset will only make the VFR handle better.

[Andy Griff]

Gents, an interesting thread !

Currently looking at doing an RC51 front end conversion on my 97 VFR750 based track bike (837cc)

Using the RC51 triple clamps I loose 10mm of fork offset - as the VFR uses 40mm offset vs 30mm on the RC51

Initially I thought that this might sharpen up the handling. After all, the RC51 is a pure sports bike... the VFR's extra offset must give stability at the detrement of agility, its a tourer right ?

I have since decided that this is not the case... as decreasing the offset increases the trail, and therefore increases stability. (self centre affect)

The VFR750 seems to be the only road bike to use 40mm offset - (Yamaha's VMax does but also, that’s more of a cruiser)

Honda obviously did something right when they designed this bike as it was praised from day one.

But, I'm confused... If more offset sharpens handling, then why does the VFR have 10 to 15mm more than the average sports bike ?

I love the handling of my bike - all I've done is raised the rear ride height slightly and dropped the forks through the yokes 10mm - and using a 16" front wheel.

I really want to use the RC51 forks which I have in my garage, but don't want to upset the neutral balance of the bike.

Unsure weather to try a set of std RC51 triples (which I would have to buy from eBay), or just fork out on a set of custom triples to retain the 40mm offset (and empty my wallet).

Any thoughts, comments, suggestions welcome !

Regards,

Andy

[Sparrow75]

Honda obviously did something right when they designed this bike as it was praised from day one.

But, I'm confused... If more offset sharpens handling, then why does the VFR have 10 to 15mm more than the average sports bike ?

I love the handling of my bike - all I've done is raised the rear ride height slightly and dropped the forks through the yokes 10mm - and using a 16" front wheel.

Hey Andy, good questions.

I am still learning more about this myself. But I would suspect that not all sport bikes, or tourers or cruisers use the same steerer angle. Therefore bikes with slightly different offset, steering angle, wheel size, etc could be set up to have the exact same trail...although not the same rate of trail change over the suspension range during compression.

Am I out to lunch with that generalization? If trail is reduced, could it be extremely unstable if the fork compresses enough to allow the trail to approach 0 mm? If understand this correctly, poor rear suspension could affect trail during sudden or hard braking and make the bike unstable, also.

Arman

[steve27bha]

Right, got it.

Putting R1 forks in 929 triples will reduce the offset to 30mm from 40mm. Set at the same fork length, the rake and trail will both change. However, if the fork length is set to retain the stock rake, the trail will increase to just shy of 10mm (Cos26 x 10 = 8.98mm).

....

Stew

Not quite. You have the cos function upside down, and the change in trail is given by 10 / cos26 = 10 / 0.899 = 11.13

That's an extra 2.14mm.

[Henkies]

On 15-9-2010 at 3:20 AM, vfrcapn said:

Your calc sounds right on. But stock VFR offset (43.4mm) based on Honda's published numbers puts trail at 94.3mm. That seems like a big increase and one I definitely noticed after my conversion.

Old topic perhaps, but where did you find this offset? 

[JZH]

Must have been Honda's PR people.  I have personally measured many different Honda triple clamps and I would say that figure is approximately 3.4mm off!

 

Ciao,

[vfrcapn]

On 1/30/2018 at 2:05 PM, Henkies said:

Old topic perhaps, but where did you find this offset? 

I can't even find my bike keys from this morning, much less a figure from eight years ago.

 

:)