Apparently I have a Low Trail Fork!

[Tom Norton]

This morning I installed a set of 650b wheels ,with 38mm tires , on my 1984 Miyata 610. I then took the measurements to check trail. I came up with 32mm.

[Justin Hughes]

I know it's pedantic, but it gets said around here all the time and I think it's worth pointing out that forks do not have trail values. Trail values are determined by the frame (HTA), fork and wheel/tire together. 

You're saying that your stock 1984 Miyata (original fork) has 32mm trail with 68-584? Sounds unlikely as that would be out of line for the sport touring and racing bikes of the time. 

Justin

[Steve Palincsar]

On 10/09/2017 12:44 PM, Justin Hughes wrote:
> I know it's pedantic, but it gets said around here all the time and I
> think it's worth pointing out that forks do not have trail values.
> Trail values are determined by the frame (HTA), fork and wheel/tire
> together.
>
> You're saying that your stock 1984 Miyata (original fork) has 32mm
> trail with 68-584? Sounds unlikely as that would be out of line for
> the sport touring and racing bikes of the time.
>
>

Come to that, a stock 1984 Miyata sport touring bicycle fitting a 68mm
wide tire seems unlikely as well, don't you think?

--
Steve Palincsar
Alexandria, Virginia
USA

[Justin Hughes]

I guess you're joking, but if not it should be clear my post featured a typo. 

[Theodor Rzad]

Hi Tom

I'm getting before/after trail figures of 55mm (27 x 1 1/8" per spec) and 51mm w/ 650b x 38. How did you calculate it?

Regardless, I'm sure it'll be great as a 650b conversion as long as you can work our brake reach.

[Tom Norton]

I used a straight edge along the centerline of the headtube all the way to the floor. I marked the floor with a pencil mark. Then I used a framing square and lined it up with the centerline of the axle and marked the floor. I then measured between the 2 points.

[Theodor Rzad]

Your method should work assuming your bike's vertical axis is exactly normal to the floor plane (or you measure both sides and average them). I looked up the geometry online and calculated using yojimg's online tool. 

Does your 610 have the original fork? A shorter than stock one could drop the trail figure by steepening the head angle.

On Mon, Oct 9, 2017 at 11:47 AM, Tom Norton <tnort...@gmail.com> wrote:

I used a straight edge along the centerline of the headtube all the way to the floor. I marked the floor with a pencil mark. Then I used a framing square and lined it up with the centerline of the axle and marked the floor. I then measured between the 2 points.

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[Tom Norton]

I believe it is the original fork. The bike was in a plumb or vertical position when I took the measurements. I won't be able to remeasure until next week as the 650b bikes are in the truck as we are going touring for a few days.

[Justin Hughes]

2" (50mm) was pretty standard sport tourers then. What did the catalog list as fork offset? 

Justin

On Monday, October 9, 2017 at 2:56:03 PM UTC-4, Theodor Rzad wrote:

Your method should work assuming your bike's vertical axis is exactly normal to the floor plane (or you measure both sides and average them). I looked up the geometry online and calculated using yojimg's online tool. 

Does your 610 have the original fork? A shorter than stock one could drop the trail figure by steepening the head angle.

On Mon, Oct 9, 2017 at 11:47 AM, Tom Norton <tnort...@gmail.com> wrote:

I used a straight edge along the centerline of the headtube all the way to the floor. I marked the floor with a pencil mark. Then I used a framing square and lined it up with the centerline of the axle and marked the floor. I then measured between the 2 points.

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[Theodor Rzad]

1984 catalog lists 55mm rake for the "Grand Touring" geometry.

1984 Miyata Catalog

[David Parsons]

On Monday, October 9, 2017 at 9:44:52 AM UTC-7, Justin Hughes wrote:

I know it's pedantic, but it gets said around here all the time and I think it's worth pointing out that forks do not have trail values. Trail values are determined by the frame (HTA), fork and wheel/tire together. 

You're saying that your stock 1984 Miyata (original fork) has 32mm trail with 68-584? Sounds unlikely as that would be out of line for the sport touring and racing bikes of the time. 

1984 is kind of late for it, but there were some 27"-wheeled machines made with low trail forks.   I've a Tange fork from that era kicking around that's got about 65mm of rake, and a horrible gaspipe frame that's got about 70mm of rake (both skip finds; the HGF exists merely for brazing practice, but the Tange fork briefly made one of my 650b machines low trail before I put a carbon fork onto the thing.)

[Brad]

A couple of thoughts.
Moving a wheel forward in a horizontal rear dropout also moves it slightly down.
That can change head tube angle. Steeper will result in less flop, (aka low trail).
Two of three data points in the Frame Geometry Project data for Miyata 610 in that time frame show 73º head angles.
They also show 5.5 cm fork rake.  Some forks may have been raked a little more at the factory.  It isn't a perfect art, it just needs to be a balanced set.
A lower bottom stack height on the headset would also change the head angle toward steeper.
With 650B (ISO 584) x 38 tires a 73 head angle and 55mm of rake  calculates to 45 mm of trail, and a flop factor of 12 uisng JimG's trail calculator.
That is the high edge of low trail. 
Surely a fun ride and capable of handling a front bag without undo flopping around. 
You probably can't walk it down the street holding just the seat and keeping a straight line, though.

On Monday, October 9, 2017 at 12:23:54 PM UTC-4, Tom Norton wrote:

[Paul Jackson]

How did the chainstay clearance look?  Do you think you could fit a 42mm tire?

I'm considering a conversion of my Six Fifteen.

On Monday, October 9, 2017 at 12:23:54 PM UTC-4, Tom Norton wrote:

[mitch....@gmail.com]

On Monday, October 9, 2017 at 10:23:54 AM UTC-6, Tom Norton wrote:

This morning I installed a set of 650b wheels ,with 38mm tires , on my 1984 Miyata 610. I then took the measurements to check trail. I came up with 32mm.

That would require a fork with 73mm of rake/offset, assuming the correct catalog head angle of 72 deg.

--Mitch 

[Tom Norton]

Paul, I did not look at it close enough to see if a 42mm would fit. When I get back from this short bike trip I will do a remeasure.

[Steven Garen]

I have an '85 610 that I converted to 700c. I have schwalbe little bens 700x38 in there with a couple mil of clearance on either side at the chainstays. I even squeezed sks fenders over them. So I wouldn't be surprised if you could fit 650x42

[Joe Broach]

On Mon, Oct 9, 2017 at 12:51 PM, Theodor Rzad <tsr...@gmail.com> wrote:

1984 catalog lists 55mm rake for the "Grand Touring" geometry.

1984 Miyata Catalog

​Yeah, on the smaller sizes, with a 72 deg HTA. The taller ones were 73 HTA/50 rake (https://docs.google.com/spreadsheets/d/1PApsm_0cOWH1Ol3TLOTj4bE_w0w-EhWMM9F4AuE7E8c/edit#gid=0).

I'd be surprised if a Miyata were that off-spec, but you never know!

Best,

joe b

pdx or​

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