Fork angle different to steering head angle

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brent j
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Fork angle different to steering head angle

Postby brent j » Sun Feb 02, 2020 6:48 pm

Something I noticed while putting my RL together today, the top and bottom triple clamps appear to have different offsets. The fork legs are on a different angle to the steering head. The rake angle is not affected but having the forks on a flatter angle effectively increases the offset of the axle and so reduces trail. I can only assume that as the suspension compresses the rake angle gets steeper but the offset reduces and slightly increases the trail. Theoretically giving an increase in trail and stability.
Yamaha TY250's (twin shock bikes) are set up the same way with the forks on a different angle to the steering head



David Lahey
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Re: Fork angle different to steering head angle

Postby David Lahey » Sat Mar 21, 2020 12:41 pm

There are a couple of trials bikes from the 1970s that have the tubes parallel with the steering stem but most have angular offset.
There has been a move in recent years in Twinshock and P65 circles for people to fit custom-made triple clamps that have no angular offset.
It's an interesting field for experimentation.
Another thing that is being experimented with on some old trials bikes is the total trail. It's another good thing for people who like trying out things to see what it does. I remember in the early 2000s I had a test ride on a TL125 special that had a Honda MR175 or MT125 complete front end. The steering on that thing in flat skaty turns was the best I had ever felt on a twinshock at the time and got me interested in such things. The trail with an MT125/MR175 front end is a fair bit more that the standard TL125. Not long after that I started hearing about people fitting late model Alpina triples on their Sherpa Ts, which increases the trail. When I finally test rode one I was fascinated with how different the whole bike felt to ride.


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brent j
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Re: Fork angle different to steering head angle

Postby brent j » Sat Apr 11, 2020 8:23 pm

The (probably) SP250 forks on my RL have less offset than RL forks so my trail will be increased from 76mm to 80mm for the same steering head angle. It will be interesting to see what the difference in handling is although I don't have anything to compare it too.



David Lahey
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Re: Fork angle different to steering head angle

Postby David Lahey » Sat Apr 11, 2020 9:28 pm

Your bike is sounding better and better. I like the idea of a little bit more trail than standard on an RL front end


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brent j
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Re: Fork angle different to steering head angle

Postby brent j » Sun Apr 12, 2020 11:52 am

The biggest problem with my RL is that I want to ride it NOW! but it's still a few months off. I hope to start on the engine soon.

I'm quite used to bikes with minimal trail. As a youngster I tried the MX hero bit but quickly found I was too cautious. I got into trials and rode for years in NZ and up here in Darwin. My current ride is a KTM Freeride and when I get on a "normal" bike it feels like a chopper.



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Re: Fork angle different to steering head angle

Postby David Lahey » Sun Apr 12, 2020 2:46 pm

It's amazing how different a slight change of rake or trail can make a bike feel. A couple of years ago, Greg and I had a test ride on a Sherpa T that had been fitted with Alpina triple clamps (which I think means something like 20mm more trail) and I was amazed how completely different it felt in the front end. I had trouble getting my head around just how different it was. Some things about it were better than standard and some things were not but it was the scale of the difference that stunned me.
I rode one of Dan Gollagher's fabulous TL125 specials as it was in the 2000s and was amazed at how good the steering was. The front end was the whole front end from an MR175 or MT125 which are trail bikes but (probably because the rake was less on the TL125 special) it felt perfect for trials.


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brent j
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Re: Fork angle different to steering head angle

Postby brent j » Sun Apr 12, 2020 9:04 pm

Trail has more influence on handling than rake.
I spent a few years building XT/TT500 Yamahas for VMX. I did a lot of swapping around with different Yamaha and Suzuki fork legs and triple clamps. I even bored a set of Honda triples to take a Yamaha steering tube.
Yamaha and Suzuki both have 60mm total offset, Yamaha are 25mm in the triples and 35 in the forks, Suzuki are 30 &30. Honda were about 21.5 in the triples.
By reducing trail the steering becomes lighter and if you just corner faster and faster the front will eventually slide first.
Increase trail and the opposite happens.
I tried every combination and a "neutral" setting was when both wheels would drift together,

Have a look online, Tony Foale did some experiments with a BMW 750. He ran 30, 15 and 0 degree steering head angles but always kept the trail the same by reversing fork legs and triple clamps. There was NO DIFFERENCE in stability at any angle The steeper the steering head the less the bike would run off on a side slope the forks would bind under braking.

I see a lot of drag bikes that are raked out in the belief that increasing the rake will increase stability. Increasing the rake increases the TRAIL and that's what increases the stability.

I really do enjoy experimenting and learning about these sorts of things.



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Re: Fork angle different to steering head angle

Postby David Lahey » Mon Apr 13, 2020 10:19 am

I love hearing about people experimenting with their bikes. It sure is great fun.
Another thing I thought of regarding the feel of a trials bike when a change is made to the front end is that when the trail is changed by changing the offset or by changing the head angle or by changing both, the wheelbase and front/rear weight distribution are also changed and these are very important on a trials bike. I suspect that on the Sherpa T I rode with the Alpina triples, some of the difference in the feel of the bike was due to the rearward change in front axle position shortening the wheelbase and the pegs being further forwards relative to the wheelbase, changing the front/rear balance.
My latest experimenting has been with a KT250. It's apart at the moment waiting for me to paint the frame and rebuild the motor after making and testing a series of frame alterations. The end result is a standard KT wheelbase with KT front end but with lighter steering.


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Re: Fork angle different to steering head angle

Postby JC1 » Wed Apr 22, 2020 1:28 pm

Brent, steering geometry is a fascinating topic.

We had a lengthy discussion on such matters (rake, trail, weight distribution etc) on here a few years ago, under the thread "Steer Clear" with relevance to trials bikes (mostly twinshocks), some of it specific to RLs

viewtopic.php?f=16&t=9348

Might be worth a read


"Men are never more likely to settle a matter rightly than when they can discuss it freely"


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