[Davidcopperfield]

Hello

Basing on this http://g-tedproductions.blogspot.com/2007/01/revisiting-trail-and-head-angle-for.html
and
http://www.kreuzotter.de/deutsch/lenk.htm
We are able to calculate ground contact patch for a given fork offset and wheels size.
Let us see the steeper the HA is and the more offset we have the smaller the contact patch is.
The slacker the HA and less offset we have the more contact patch we get.
The steeper the HA is the less offset we need to get the same contact patch with slacker HA and longer offset.
Thus slack angles + long offset will improve the handling - less contact patch, but longer wheel base - the whole bicycle is long and unpractical in tight switchbacks.
Until now everything is intuitive and common. Here comes an example of Spider 29" which is reported with 73' HA (or any other 29" bike in the future) to be ready to pitch you over the bars more likely than other with slacker HA. Of course we get the steereability of 26er, but DH section must be tackled with your butt going farther behind the saddle.

- My question is what would happen with Spider 29" DH inappropriateness if we get Reba with 43mm or 51mm offset instead of 38mm. Will that much offset make it harder to endo on DH sections or the bike will get too twichy and will be swayed from side to side like 26er on uphills (the front wheel fiddling)
If we got Reba with 35, 32, 30mm offset, accordingly this would make more contact patch but how would it affect DH section on our mentioned Spider 29"?

 

[Law]

We are able to calculate ground contact patch for a given fork offset and wheels size.
Let us see the steeper the HA is and the more offset we have the smaller the contact patch is.
The slacker the HA and less offset we have the more contact patch we get.
The steeper the HA is the less offset we need to get the same contact patch with slacker HA and longer offset.
Thus slack angles + long offset will improve the handling - less contact patch, but longer wheel base - the whole bicycle is long and unpractical in tight switchbacks.

When I read this I just shook my head. You really don't know what you are talking about. I for sure don't know what you are talking about. "Contact Patch" does not change depending upon the head angle and offset as far as I can figure.

You might be better off putting you thoughts into something more productive, like looking for porn on the web or something.

 

[Singlespeedpunk]

The contact patch size stays the same no matter the head angle / off-set, it is purely dependant on wheel diameter and tyre width.

The part that changes is the centre of the contact patch in relation to the steering axis...kind of like the "trail" on a shopping trolly wheel that keeps it pointing the direction of travel when you push the trolly forward.

The bigger the trail (slack HA or small off-set) gives a more stable steering. A steeper HA and/or larger off-set on the forks moves the contact patch nearer the steering axis making the steering handling "faster" and more "twitchy"

Hope this helps,

Alex

 

[ferday]

You might be better off putting you thoughts into something more productive, like looking for porn on the web or something.

or maybe actually buying and riding a real 29er, which as far as i can tell he still hasn't.

this guy is too dumb to even consider as a troll...

 

[RoundRockTJ]

Contact patch will not change. Maybe you used the wrong words. The larger fork offset will increase the wheelbase which will help in the downhill sections of course. Also, on DH sections the 29" wheels hold more centrifical force adding to its stability. So it will still "FEEL" and "BE" more stable that a 26er. A change in fork offset will not be that big of a deal for the Spider. Not sure what long wheelbase quick turning bikes are out there.

But from what I can get, fork offset is all about feel. And if you know how to ride then it would not matter. Getting ready to fly over the bars is a little much. If anything, the stability will increase.

 

[Cloxxki]

IMO the centrifical force only really holds true for something when the wheels are not touvhing the trail. Different attack angle of various wheel size deflect differently when coming in contact with trail objects, I think THAT's what does the trick for 29" wen going DH.

Longer doesn't have to be badder. A really short front end for instance will have you endo the moment you feather the front brake when going down anything steep, or having the rear roll over a little marble on the trail.

 

[donkey]

I'm honestly a little surprised that this guy hasn't been silenced/banned yet. Holy crap.

 

[GlowBoy]

It's obvious (good God, I hope) that you're not really talking about contact patch, because that has absolutely nothing to do with the geometry of the bike. Are you talking about trail? It almost sounds like it, although even then the original post doesn't quite make sense.

It's possible the original post could turn into a question that made sense. David, do you want to clarify what you're talking about (obviously not regarding contact patch, again I hope) and try asking again?

 

[SyT]

.

It's possible the original post could turn into a question that made sense. David, do you want to clarify what you're talking about (obviously not regarding contact patch, again I hope) and try asking again?

For the love of all that is holy, stop casting the chum. Can't keep the boy outa the nets as it is.....

 

[Keholio]

Several things can effect tire contact patch size and make it bigger.
Moving from 38mm to 51mm tire can make the contact patch bigger.
Moving from 51 to 38 PSI in the tires makes the contact patch bigger
Increasing rider weight by 51 Kgs makes the contact patch bigger (my normal winter weight gain after cross season ends.)
But, moving from 38 to 51 mm offset has no effect on tire contact patch.
Agree with Cloxxki short front end is a contributing factor for endo tendancies of the bike. From my experience I can say other contributing factors are:
Higher center of gravity
fork brake dive
poor brake modulation
having a passanger ride on the handlebars down big drops
poor line choice
wrong sized bike
riding in an altered state
Chain suck
getting in over my head on tech terrain
my favorite in Fruita, "look, no hands"

However, this said, My Karate Monkey which is very short on the wheelbase is a very capable descender for what it is. The bike handles steep technical terrain very well, not as well as a DH rig but it still gets the job done.

 

[English Bob]

Increasing rider weight by 51 Kgs makes the contact patch bigger (my normal winter weight gain after cross season ends.)

You put on 51kg over the winter? Are you Jan Ulrich by any chance?:eekster:

 

[Keholio]

No Jan here,
Likewise I can be found in those long dark winter months perfecting my donut eating skills. My wife says she can hardly tell . . .

 

[IBIKEAZ]

So if I had a bike that was already pretty long in the wheelbase, more trail makes it even longer? and that would be better for downhills but worse for climbing, faster handling?

 

[GeoKrpan]

The new fork offset moves the top of the fork legs forward but the bottom of the legs stay in the same place. In other words, it steepens the fork legs.
It does not change the wheelbase and I can't see how it could affect the contact patch.
The effect of the new offset is the reduction of trail.
Before the new offset 29ers resorted to steeper HTAs to reduce trail.
Now that the new offset is here you will see bike makers slackening the HTAs.
A case in point is the Monocog Flight 29er versus the Monocog 29er.
The Flight has a 71 degree HTA, the Monocog a 72 degree HTA.
The design of the Flight took into account the advent of the new offset.
The Monocog is a "first wave" 29er, designed when the new offset was not on the horizon.
Trail is trail, it doesn't matter how you achieve it, whether you use steeper HTAs or more fork offset.
The advantage of slacker HTAs with more offset is less toe overlap, a possible problem in the smaller frame sizes. I have no problem on my size large frame with a 72 degree HTA and the old fork offset and size 12 feet.
I have the impression that putting a fork with the new offset on a first wave 29er with a HTA of 72 degrees or greater is NOT a good idea. I think it would make the front end too steep. Not good on steep technical downhill. It might be OK with a rigid fork but I think it would be a disaster with a suspension fork.
Some people would like it but I think most people would hate it.
A Spider 29er with it's 73 degree HTA is already too steep with the old fork offset. It would be ridiculously steep with a fork with the new offset.
Ironically, early 29er makers who, perhaps naively, spec'd a HTA of 71 degrees or less will be OK with the new offset. Those are the bikes that gave 29ers the unkillable reputation for steering slowly.
Fortunately, it looks as though fork manufacturers are going to be offering forks in both the new and the old offsets, at least for the time being. The old offset will probably be phased out.
If you have a "first wave" 29er with a HTA of 72 degrees or more and you like it the way it is make sure you buy a fork with the old offset.

 

[GrumpyOne]

Most of what you said is accurate except for the first two sentences. More rake = the dropouts further away from the center line of the steerer.
So as you would say, the tops stay in same place but the dropouts move forward. Making the fork legs slacker in angle.

As to what DC is talking about, I have no idea.


jw

The new fork offset moves the top of the fork legs forward but the bottom of the legs stay in the same place. In other words, it steepens the fork legs.

 

[IBIKEAZ]

So with my new frame (HTA 70.7) getting a WB fork with 43mm offset and large travel 135mm (slackening the HTA more), would be a good thing I think.

 

[Guitar Ted]

Hold on there a minute!

The new fork offset moves the top of the fork legs forward but the bottom of the legs stay in the same place. In other words, it steepens the fork legs.
It does not change the wheelbase and I can't see how it could affect the contact patch.

This is erroneous. The off set is achieved in the crown in most fork designs, but it can also be acheived by "off setting" the drop outs from the steering axis. Check out an old first generation Manitou fork for a good example of what I'm talking about. So, to correct the statement here that you made, the axle is off set from the steering axis to achieve whatever specified off set is desired. How you manage to do this can be by off setting the crown, the drop outs, or a combination of the two. As you can see, this certainly affects the wheelbase.

The effect of the new offset is the reduction of trail.
Before the new offset 29ers resorted to steeper HTAs to reduce trail.

Yes, this is correct.

Now that the new offset is here you will see bike makers slackening the HTAs.
A case in point is the Monocog Flight 29er versus the Monocog 29er.
The Flight has a 71 degree HTA, the Monocog a 72 degree HTA.
The design of the Flight took into account the advent of the new offset.
The Monocog is a "first wave" 29er, designed when the new offset was not on the horizon.
Trail is trail, it doesn't matter how you achieve it, whether you use steeper HTAs or more fork offset.

Yes, trail is trail, but you can achieve it with a combination of factors.

The advantage of slacker HTAs with more offset is less toe overlap, a possible problem in the smaller frame sizes. I have no problem on my size large frame with a 72 degree HTA and the old fork offset and size 12 feet.

You are right. Toe overlap is a problem with smaller sizes. This new crop of longer offset forks will allow for even smaller sized 29"ers with suspension forks to be produced without toe overlap problems.

I have the impression that putting a fork with the new offset on a first wave 29er with a HTA of 72 degrees or greater is NOT a good idea. I think it would make the front end too steep. Not good on steep technical downhill. It might be OK with a rigid fork but I think it would be a disaster with a suspension fork.
Some people would like it but I think most people would hate it.

Maybe. Possibly, but some folks might really dig it. Witness the three or four different offsets for road racing bikes. To each their own.

A Spider 29er with it's 73 degree HTA is already too steep with the old fork offset. It would be ridiculously steep with a fork with the new offset.

"Rediculously quick", perhaps, but steepness, (in referance to head angle) should remain the same, given the assumption that we are working with the same axle to crown measurements across the board. (Axle to crown can affect head tube angle and thus trail)

Ironically, early 29er makers who, perhaps naively, spec'd a HTA of 71 degrees or less will be OK with the new offset. Those are the bikes that gave 29ers the unkillable reputation for steering slowly.
Fortunately, it looks as though fork manufacturers are going to be offering forks in both the new and the old offsets, at least for the time being. The old offset will probably be phased out.
If you have a "first wave" 29er with a HTA of 72 degrees or more and you like it the way it is make sure you buy a fork with the old offset.

Yes, for the time being you will be able to "tune" your handling to your liking with different offsets in suspension forks, just like we have been able to do with rigid forks for a few years now.

 

[Guitar Ted]

What is a "good thing"?

So with my new frame (HTA 70.7) getting a WB fork with 43mm offset and large travel 135mm (slackening the HTA more), would be a good thing I think.

It depends on your desired result.

It sounds as though your handling would remain very similar to what you have now with the changes you detail. If that's "a good thing", then you will be happy, I guess.

 

[rockcrusher]

so for reference sake:

What is this "new offset" and what was the "old offset"? (sorry for the questions but I am new to this stuff...again)

and less trail is a good thing(?) How so?

 

[Guitar Ted]

so for reference sake:

What is this "new offset" and what was the "old offset"? (sorry for the questions but I am new to this stuff...again)

"Old off set" was determined from 26"ers "ideal" XC offset of 38mm-40mm. The suspension companies had to make a huge investment in tooling and manufacturing to set up the new off sets, so they waited until this crazy 29"er thing looked like money before they invested. The "old" off sets on 29"er forks was a band aid to get 29"er suspension out on the market to satisfy the needs of the early adopters of the format. Although the Marz forks on early Fishers were 43mm, if I'm not mistaken.

and less trail is a good thing(?) How so?

Going from 26 inch wheels to 29 inch wheels automatically increases the trail measurement, so the first 29"ers adapted by increasing the head angle a bit to compensate for sluggish handling. This was only mildly acceptable to most single track fiends, since 29"ers were still not as snappy as their 26"er cousins in the handling department.

We have since seen some bikes go to a steeper head angle (73 degrees ) to get that 26"er feel and now with the new off sets we will gain back more quickness since a lower trail figure increases the "quickness" of the front end, while a higher trail figure usually is said to be "slow" handling, or more stable.

If you use the analogy of the shopping cart wheel, you can see that if the wheel "trails" the steering axis by a greater distance, the cart will be harder to turn, while if the distance is shorter to the point that the wheel is barely behind the steering axis the cart wheel wobbles nervously and the cart becomes more unstable, harder to hold in a straight line, than before. Somewhere in the middle is a happy medium.